This Guide consists of two parts, each formerly a separate publication:

Part 1—Estimating the Irrigation Water Needs of Landscape Plantings in California: The Landscape Coefficient Method

  • L.R. Costello, University of California Cooperative Extension
  • N.P. Matheny, HortScience, Inc., Pleasanton, CA
  • J.R. Clark, HortScience Inc., Pleasanton, CA

Part 2—WUCOLS III (Water Use Classification of Landscape Species)

  • L.R. Costello, University of California Cooperative Extension
  • K.S. Jones, University of California Cooperative Extension

Part 1 describes a method for calculating landscape water needs, while Part 2 gives evaluations of water needs for individual species. Used together, they provide the information needed to estimate irrigation water needs of landscape plantings.

Part 1 is a revision of Estimating Water Requirements of Landscape Plants: The Landscape Coefficient Method, 1991 (University of California ANR Leaflet No. 21493). Information presented in the original publication has been updated and expanded.

Part 2 represents the work of many individuals and was initiated and supported by the California Department of Water Resources. This third revision (WUCOLS III) includes many species not previously evaluated, as well as an update and reorganization of support information.

These two publications are companion documents and are intended to be used together.

First-time readers are encouraged to carefully review both parts of this Guide before making estimates of landscape water needs.


The Landscape Coefficient Method (LCM) describes a method of estimating irrigation needs of landscape plantings in California. It is intended as a guide for landscape professionals. It includes information that is based on research and on field experience (observation). Readers are advised that LCM calculations give estimates of water needs, not exact values, and adjustments to irrigation amounts may be needed in the field.

L. R. Costello, Environmental Horticulture Advisor University of California Cooperative Extension
N. P. Matheny, Horticultural Consultant HortScience, Inc.
J. R. Clark, Horticultural Consultant HortScience, Inc.


Part 1 leads you through the concepts, terms, and formulas needed to estimate irrigation water needs. You will learn:

  • the key formulas needed for calculations,
  • the principal concepts that serve as a basis for calculations,
  • how to use the methods in the field,
  • how to use estimates in irrigation planning and management,
  • where to find important numbers in reference tables, and
  • considerations for special landscape situations.


After providing background information on estimating water needs for agricultural crops and turf in Chapter 1, landscape needs are addressed in Chapter 2. The landscape coefficient, a key factor in the formula for estimating landscape water requirements, is introduced in Chapter 2. Subsequent chapters give examples of how to calculate and use the landscape coefficient. Chapter 5 addresses irrigation efficiency and gives examples of how it is used to determine total water needs. As a way of “putting it all together,” a worksheet which summarizes the process is provided in Chapter 6. Special topics are discussed in Chapters 7 and 8. The appendices provide further information.


All landscape professionals involved in the planning, installation, and maintenance of irrigated landscapes should find this information of value. This includes architects, planners, contractors, park managers, gardeners, consultants, water suppliers, auditors, and students.


Estimates of landscape water needs are important for at least three reasons:

  1. Water Conservation. Water is a limited natural resource. Efficient water use in urban landscapes contributes substantially to the conservation of this resource. Water use efficiency can be achieved by supplying only the amount of water sufficient to meet plant needs.
  2. Economics. Water costs continue to increase. By applying only that amount of water needed by landscapes, and avoiding excess use, money can be saved.
  3. Landscape Quality. The potential for plant injury caused by water deficits or excess can be minimized by identifying and meeting plant needs.

Getting Started

First-time readers are encouraged to review the en- tire Guide prior to making water needs estimates. Field examples and a practice worksheet in Chapter 6 show how to use the information presented in previous chapters. Be sure to review the appendices; they contain important numbers for calculations.

Formulas and Numbers

Formulas and numbers are needed to calculate irrigation water requirements. Fortunately, the calculations needed here are simple and straightforward. They require only a basic understanding of mathematics. Once you have reviewed the examples and made some calculations on your own, you should have no difficulty. A worksheet with all the formulas and sample calculations is included in Chapter 6.

In agriculture, irrigation water requirements are well established for many crops. In urban landscapes, irrigation requirements have been determined for turfgrasses, but not for most landscape species. This chapter discusses the method used to estimate water requirements for agricultural crops and turfgrasses. Chapter 2 adapts this method for application to landscape plantings.

Water requirements for agricultural crops and turfgrasses have been established in laboratory and field studies by measuring plant water loss (evapotranspiration). The total amount of water lost during a specific period of time gives an estimate of the amount needed to be replaced by irrigation. Since growers and turf managers are not equipped to measure plant water loss in the field, a formula was developed which allows water loss to be calculated. This formula (referred to as the ETc formula) is written as follows:
ETc = Kc x ETo
Crop Evapotranspiration =
Crop Coefficient x Reference Evapotranspiration

This formula states that water loss from a crop (crop evapotranspiration, ETc) equals the amount of water that evaporates from a 4- to 7-inch tall cool sea- son grass growing in an open-field condition (reference evapotranspiration, ETo) multiplied by a factor determined for the crop (crop coefficient, Kc).

Reference evapotranspiration (ETo) is estimated from a Class A evaporation pan or from a specialized weather station. Normal year (historical) average values for many locations in California are found in Appendix A. Current daily ETo values are available from the California Irrigation Management Information System (CIMIS) and can be accessed via the Internet (www.cimis.water.ca.gov) or by contacting the California Department of Water Resources (see Appendix D).

The crop coefficient (Kc) is determined from field research. Water loss from a crop is measured over an extended period of time. Water loss and estimated reference evapotranspiration are used to calculate Kc as follows:
Kc = ETc ETo

As seen in the above equation, the crop coefficient (Kc) is simply the fraction of water lost from the crop relative to reference evapotranspiration. Typically, crop water loss is less than reference evapotranspiration and, therefore, the crop coefficient is less than 1.0. For example, if water loss from corn was measured to be 4 inches in a month, and reference evapotranspiration for the same month was 8 inches, then the crop coefficient would be 0.5. Crop coefficients have been established for many crops and for turfgrasses. A sample of values is given in Table 1.

Table 1—Crop Coefficients for Various Crops and Turfgrasses Kc values for agricultural crops typically change during the seasons: low values are for early season (March/April) or late season (September/October) and high values for midseason (May/June/July).

Kc values
Low High
Deciduous orchard* 0.50 0.97
Deciduous orchard with cover crop** 0.98 1.27
Grape 0.06 0.8
Olive 0.58 0.8
Pistachio 0.04 1.12
Citrus 0.65 year-round
Turfgrass - Cool season species 0.8 year-round
Turfgrass -Warm season species 0.6 year-round
Source:,UC Leaflet Nos. 21427 and 21428 (see,references),
  • Deciduous orchard includes apples, cherries, and,walnuts
    • When an active cover crop is present, Kc may increase by 25 to 80%.

In summary, an estimate of crop evapotranspiration is made from reference evapotranspiration and crop coefficient values. Estimates can be made for any location where reference evapotranspiration data exists and for any crop (or turfgrass) that has a crop coefficient.

Example: A grape grower in Monterey County wants to estimate how much water the vineyard may lose in the month of July. Using the ETc formula, two numbers are needed: reference evapotranspiration (ETo) for July in Monterey and the crop coefficient (Kc) for grapes. July ETo for Monterey can be found in Appendix A, and the Kc for grapes is listed in Table 1 (above). With the two values, the following computation is made:
ETo = 0.18 inches per day x 31 days = 5.58 inches (average for July in Monterey)
Kc = 0.8 (midseason value for grapes)

ETc = Kc x ETo
ETc = 0.8 x 5.58 = 4.46 inches

The grower has estimated that 4.46 inches of water will be lost from the vineyard (via evapotranspiration) in the month of July. By using this ETc estimate, the grower can calculate irrigation water requirements for the vineyard. (For an estimate of the total amount of water to apply, see Chapter 5).

The ETc formula is the key formula for estimating water loss from crops and turfgrasses. A version of this formula will be used in Chapter 2 to estimate water loss for landscape plantings. It is recommended that you become familiar with the ETc formula before continuing.

Two formulas are used to estimate water needs for landscape plantings:

  • the landscape evapotranspiration formula and
  • the landscape coefficient formula.

Both formulas are introduced here and then used in subsequent chapters to estimate water needs. The landscape coefficient was developed specifically for estimating landscape water needs and is the principal focus of Chapter 2.

The method used for estimating water needs for land- scape plantings is basically the same as that used for crops and turfgrasses. The ETc formula discussed in Chapter 1 is simply modified for application to landscapes. One key change, however, has been made: instead of using the crop coefficient (Kc), a landscape coefficient (KL) has been substituted.

The Landscape Evapotranspiration Formula

Water needs of landscape plantings can be estimated using the landscape evapotranspiration formula:
ETL = KL x ETo
Landscape Evapotranspiration = Landscape Coefficient x Reference Evapotranspiration

This formula (called the ETL formula) states that water needs of a landscape planting (landscape evapotranspiration, ETL) is calculated by multiply- ing the landscape coefficient (KL) and the reference evapotranspiration (ETo).

As mentioned above, the ETL formula is basically the same as the ETc formula from Chapter 1, except that a landscape coefficient (KL) has been substituted for the crop coefficient (Kc). This change is necessary because of important differences which exist between crop or turfgrass systems and land- scape plantings (see “Why a Landscape Coefficient”).

The following is an example of a simple calculation using the landscape coefficient in the landscape evapotranspiration (ETL) formula.

Example: A landscape architect wants to estimate water loss for the month of August from a large groundcover area being considered for a new commercial office park in Fresno. The architect looked up the reference evapotranspiration for August in Fresno (Appendix A) and found it to be 7.1 inches. The architect assigned a landscape coefficient value of 0.2. Using this information and the landscape evapotranspiration formula (ETL formula), the architect makes the following calculations: KL = 0.2
ETo = 7.1 inches for August in Fresno

ETL = KL x ETo
ETL = 0.2 x 7.1 = 1.42 inches

The architect estimates that the groundcover will need 1.4 inches in the month of August. (This is not the total amount of irrigation water needed, however, as irrigation efficiency needs to be considered. This topic is addressed in Chapter 5.)

In this example, a landscape coefficient was as- signed. In actual practice, KL needs to be calculated. The formula needed to calculate KL is the heart of the landscape coefficient method and is the subject of the next discussion.

The Landscape Coefficient Formula

As the name implies, the landscape coefficient was derived specifically to estimate water loss from land- scape plantings. It has the same function as the crop coefficient, but is not determined in the same way. Landscape coefficients are calculated from three factors: species, density, and microclimate. These factors are used in the landscape coefficient formula as follows:
KL = ks x kd x kmc
Landscape Coefficient = species factor x density factor x microclimate factor

This formula (called the KL formula) states that the landscape coefficient is the product of a species factor multiplied by a density factor and a microclimate factor. By assigning numeric values to each factor, a value for KL can be determined. The land- scape coefficient is then used in the ETL formula, just as the crop coefficient is used in the ETc formula.

Why a Landscape Coefficient?

Crop coefficients are used for agricultural crops and turfgrasses, so why not for landscape plantings? There are three key reasons why landscape coefficients are needed instead.

  1. Unlike a crop or turfgrass, landscape plantings are typically composed of more than one species. Collections of species are commonly irrigated within a single irrigation zone, and the different species within the irrigation zone may have widely different water needs. For example, a zone may be composed of hydrangea, rhododendron, alder, juniper, oleander, and olive. These species are commonly regarded as having quite different water needs and the selection of a crop coefficient appropriate for one species may not be appropriate for the other species. Crop coefficients suitable for landscapes need to include some consideration of the mixtures of species which occur in many plantings.
  2. Vegetation density varies considerably in landscapes. Some plantings have many times more leaf area than others. For example, a landscape with trees, shrubs, and groundcover plants closely grouped into a small area will have much more leaf area than one with only widely spaced shrubs in the same-sized area. More leaf area typically means an increase in evapotranspiration (water loss) for the planting. As a result, a dense planting would be expected to lose a greater amount of water than a sparse planting. To produce a reliable estimate of water loss, a coefficient for landscapes needs to account for such variation in vegetation density.
  3. Many landscapes include a range of microclimates, from cool, shaded, protected areas to hot, sunny, windy areas. These variations in climate significantly affect plant water loss. Experiments in Seattle, Washington, found that a planting in a paved area can have 50% greater water loss than a planting of the same species in a park setting. Other studies in California found that plants in shaded areas lost 50% less water than plants of the same species in an open field condition. This variation in water loss caused by microclimate needs to be accounted for in a coefficient used for landscape plantings.

Collectively, these factors make landscape plantings quite different from agricultural crops and turfgrasses, and they need to be taken into account when making water loss estimates for landscapes. The landscape coefficient was developed specifically to account for these differences.

ET Rates and Plant Water Needs
Soil water availability plays a major role in controlling the rate of water loss from plants (ET rate). Many plants will lose water at a maximum rate as long as it is available. For example, some desert species have been found to maintain ET rates equivalent to temperate zone species when water is available. When soil moisture levels decrease, however, ET rates in desert species decline rapidly.

In landscape management, it is not the objective to supply all the water needed to maintain maximum ET rates. Rather, it is the intent to supply only a sufficient amount of water to maintain health, appearance and reasonable growth. Maxi- mum ET rates are not required to do this.

The ETL formula calculates the amount of water needed for health, appearance and growth, not the maximum amount that can be lost via evapotranspiration.

The Landscape Coefficient Factors: Species, Density, and Microclimate

Three factors are used to determine the landscape coefficient:

  • Species
  • Density
  • Microclimate

These factors are key elements of the landscape co- efficient method and need to be understood fully before KL and ETL calculations are made. As well as describing each factor, the following sections give information on how to assign values to each.

Species Factor (ks)

The species factor (ks) is used to account for differences in species’ water needs. In established landscapes, certain species are known to require relatively large amounts of water to maintain health and appearance (e.g., cherry, birch, alder, hydrangea, rhododendron), while others are known to need very little water (e.g., olive, oleander, hopseed, juniper). This range in water needs is accounted for in the species factor.

Species factors range from 0.1 to 0.9 and are divided into four categories: Very low < 0.1
Low 0.1 - 0.3
Moderate 0.4 - 0.6
High 0.7 - 0.9

These species factor ranges apply regardless of vegetation type (tree, shrub, groundcover, vine, or herbaceous) and are based on water use studies for landscape species (Table 2) and applicable data from agricultural crops (Table 1).

An evaluation of plant water needs (based on field observations) has been completed for over 1,800 species. These values are presented in Part 2 (WUCOLS III). Species factor values can be found by looking up the species under consideration, and selecting an appropriate value from the category range. The following is an example of using the WUCOLS list to select an appropriate ks value.

Example: A landscape manager in Pasadena is attempting to determine the water requirements of a large planting of Algerian ivy. In using the ETL formula, the manager realizes a value for the species factor (ks) is needed in order to calculate the landscape coefficient (KL). Using the WUCOLS list (Part 2), the manager looks up

Algerian ivy (Hedera canariensis) and finds it classified as “moderate” for the Pasadena area, which means that the value ranges from 0.4 to 0.6. Based on previous experience irrigating this species, a low range value of 0.4 for ks is chosen and entered in the KL formula. (If the manager had little or no experience with the species, a middle range value of 0.5 would be selected.)

Although the above example is straightforward, the assignment of species factors to plantings can be difficult. Refer to “Assigning Species Factors to Plantings” for guidance in making ks assignments.

Table 2—Irrigation Needs of Well-Established Landscape Species Determined from Field Research
Values are given as the minimum fraction of reference evapotranspiration needed to maintain acceptable appearance, health, and reasonable growth for the species. See Appendix D for complete references.

Plant Species Fraction of ETo
Potentilla tabernaemontani 0.5 - 0.75
Sedum acre 0.25
Cerastium tomentosum 0.25
Liquidambar styraciflua 0.2
Quercus ilex 0.2
Ficus microcarpa nitida 0.2
Hedera helix ‘Neddlepoint’ 0.2
Drosanthemum hispidum 0.2
Gazania hybrida 0.25-0.50
Vinca major 0.3
Baccharis pilularis 0.2
Reference: Staats and Klett; Hartin, et al; Pittenger, et al

Assigning Species Factors to Plantings

  1. For single-species plantings— When only one species occurs in the irrigation zone, use the ks value assigned in the WUCOLS list. For example, coyote brush is assigned to the “low” category and has a ks value from 0.1 to 0.3.
  2. For multiple-species plantings—
    1. When species have similar water needs: In well-planned hydrozones where species of similar water requirements are used, the selection of a ks value is straightforward: simply select the category to which all species are assigned and choose the appropriate value. For example, if all the species are in the moderate category, then a value from 0.4 to 0.6 is selected.
    2. When species water needs are not similar: In cases where species with different water needs are planted in the same irrigation zone, then the species in the highest water-need category determine the ks value. This assignment is required if all plants are to be retained without water stress injury. For example, if species in low, moderate, and high categories are planted in the same irrigation zone, then to avoid water stress injury to species in the high category, a ks value from 1.7to 0.9 would need to be selected. Unfortunately, this means that species in the moderate and low categories will receive more water than needed, which may result in injury.
    1. Considering that plantings with mixed water needs are not water-efficient in most cases and the incidence of plant injury may increase, some management options are worth considering:
    • If only a small number or percentage of plants are in the high category, then the replacement of such plants with species with lower water needs would allow for the selection of a ks in a lower range.
    • If all plants are to be retained, but a level of appearance somewhat less than optimal is acceptable, then a ks value from a lower range may be selected. For example, in the case where plants in the low, moderate, and high categories are in the same irrigation zone, a ks value from the moderate range may be selected with the understanding that some injury to species in the high category may result.
    • In cases where all plants are to be retained and no water stress injury is acceptable, then supplemental irrigation for species in the high category should be considered. Again using the case where species in low, moderate, and high categories are planted in the same irrigation zone, a ks value from the moderate range may be selected for the planting, provided additional water is supplied to individual plants with higher water needs. This approach requires an adjustment to the irrigation system whereby additional sprinklers or emitters are used to deliver supplemental water to species with higher water requirements.
    1. For species in the “very low” category—
    It is important to remember that certain species can maintain health and appearance without irrigation after they become established. Such species are grouped in the “very low” category and are assigned a ks of less than 0.1. Essentially this classification means that species in this group do not need to be irrigated unless winter rainfall is abnormally low. Accordingly, if no irrigation is supplied, then there is no need to calculate a landscape coefficient and a ks value is not assigned. In low rainfall years, some irrigation may be needed, however, and a ks value of 0.1 should be sufficient to maintain health and appearance in these species.

Density Factor (kd)

The density factor is used in the landscape coefficient formula to account for differences in vegetation density among landscape plantings. Vegetation density is used here to refer to the collective leaf area of all plants in the landscape. Differences in vegetation density, or leaf area, lead to differences in water loss.

The density factor ranges in value from 0.5 to 1.3. This range is separated into three categories:
Low 0.5 - 0.9
Average 1.0
High 1.1 - 1.3

Immature and sparsely planted landscapes typically have less leaf area than mature or densely planted landscapes, and thus lose less water. These plantings are assigned a kd value in the low category. Plantings with mixtures of vegetation types (trees, shrubs, and groundcovers) typically have greater collective leaf areas than plantings with a single vegetation type, and thus will lose more water. These plantings are assigned a density factor value in the high category. Plantings which are full but are predominantly of one vegetation type, are assigned to the average category.

Example: The grounds manager of a college campus in San Diego wants to determine the landscape coefficient for a planting consisting of gazania groundcover and a few widely-spaced escallonia shrubs. Since the plants cover the ground surface completely, the planting is considered to be full. Based on these vegetation density characteristics (i.e., full and predominantly of one vegetation type), the manager determines that this is an average density planting and assigns a kd value of 1.0.

Although this example might infer that the selection of the density factor is fairly simple, it can be difficult to determine. Vegetation density varies considerably and assigning density factors can be confusing. Many cases exist where plant spacing and distribution is not uniform and where a mixture of vegetation types exist.

Unfortunately, a standardized system of evaluating vegetation density for landscapes does not exist. Nonetheless, limited information from agricultural systems (principally orchards) can be applied to landscapes. The following sections describe two terms, canopy cover and vegetation tiers, which when applied to landscape plantings provide some guidance in assessing vegetation density.

Canopy Cover

Canopy cover is defined as the percentage of ground surface within a planting which is shaded by the plant canopy (or, simply, percent ground shading). A planting with full canopy cover will shade 100% of the ground surface, while a 50% canopy cover will cast a shadow on 50% of the ground area. The higher the canopy cover the greater the density of vegetation on a surface area basis.

Most mature landscape plantings have a complete canopy cover, i.e., the trees, shrubs, and groundcovers shade 100% of the ground surface. New plantings, immature plantings, and widely-spaced plantings are examples of cases where the canopy cover is less than 100%.

Orchard data gives an indication of how canopy cover affects water loss. Studies show that water loss from orchards does not increase as canopy cover increases from 70% to 100%. Below 70% cover, however, orchard water loss declines.

Applying this information to landscapes, plantings of trees with a canopy cover of 70% to 100% constitutes a complete canopy cover condition, and would be considered as average for density factor assessments. A tree planting with less than 70% canopy cover would be in the low category.

For plantings of shrubs and groundcovers, a canopy cover of 90% to 100% constitutes complete cover. This represents an average condition for density fac- tor assessments, while less than 90% cover would be in the low category.

Vegetation Tiers

Canopy cover gives an assessment of vegetation density on an area basis, i.e., the percent ground area covered by vegetation describes the closeness or sparse- ness of plants in a planting. Another dimension needs to be considered for landscapes: the vertical dimension. Landscapes are frequently composed of plants of various heights: tall trees, low ground- covers, and shrubs somewhere in between. Due to the typical growth form of each vegetation type, “tiers” of vegetation result.

When combinations of these vegetation types occur in a planting they add a height element which will have an affect on water loss. In orchard plantings, for example, field research has shown that the addition of a cover crop increases evapotranspiration from 25% to 80% above a bare soil condition. In other words, adding a groundcover-like planting beneath orchard trees results in a substantial increase in water loss.

In landscapes, groundcovers and/or shrubs planted in the understory of trees are likely to have a similar effect on water loss as found in orchard settings. Additionally, by adding trees to a groundcover plant- ing or shrubs to a tree-groundcover planting, an in- crease in water loss would be expected.

In most cases, the presence of vegetation tiers in landscapes constitutes a high density condition. For example, a planting with two or three tiers and complete canopy cover would be considered to be in the high kd category.

Plantings with multiple tiers which do not have a complete canopy cover, however, may not constitute a high density condition. A new planting with trees, shrubs, and groundcovers, for example, has three vegetation tiers but canopy density is low. Although three tiers are present, this planting would be classified as low density.

Assigning Density Factor Values

Canopy cover and vegetation tiers are used to assess vegetation density for density factor assignments. Since it is very difficult to account for all the variation in vegetation density which occurs in landscapes, the following assignments are made simply as a guide to making reasonable assessments.

Average Density: kd = 1.0
Plantings of one vegetation type: for trees, canopy cover of 70% to 100% constitutes an average condition. For shrubs or groundcovers, a canopy cover of 90% to 100% is considered to be an average condition.

Plantings of more than one vegetation type: for mixed vegetation types, an average density condition occurs when one vegetation type is predominant while another type occurs occasionally in the planting, and canopy cover for the predominant vegetation type is within the average density specifications outlined above. For example, a mature groundcover planting (greater than 90% canopy cover) which contains trees and/or shrubs that are widely spaced would be considered to be average density. Additionally, a grove of trees (greater than 70% canopy cover) which contains shrubs and/or groundcover plants which are widely spaced would constitute an average condition.

Low Density: kd = 0.5 - 0.9
Low density plantings are characterized largely by canopy covers less than those specified for the average density condition. For instance, a tree planting with less than 70% canopy cover would be assigned a kd value less than 1.0. The precise value assigned (between 0.5 and 0.9) would be based on the canopy cover assessment: a lower kd value for a thinner canopy cover.

For shrubs and groundcovers, canopy cover less than 90% constitutes a density less than average and a kd value less than 1.0 would be assigned.

Plantings with mixed vegetation types generally have greater canopy covers than those of a single type. For instance, a groundcover planting with canopy cover of 50% constitutes a low density condition and a kd of 0.7 might be assigned. If an occasional tree occurs in the planting, then the principal effect is one of increasing canopy cover, and an upward adjustment in kd to 0.8 or 0.9 would be warranted.

High Density: kd = 1.1 - 1.3
When canopy cover is full for any vegetation type, then increases in density result from increases in the number of plants of other vegetation types. For example, by adding trees to a mature groundcover planting (groundcover canopy cover = 100%), an increase in vegetation density occurs. The addition of shrubs to the planting further increases the density. This mix of vegetation types creates a layering or tiering of vegetation which represents potential increases in water loss. Upward adjustments of kd can be made to account for vegetation tiering. The highest density condition, where all three vegetation types occur in substantial numbers in a planting, would be assigned a kd of 1.3. In plantings where lesser degrees of vegetation tier- ing occurs (e.g., a two-tiered planting), then a kd value of 1.1 or 1.2 is appropriate.

Microclimate Factor (kmc)

Microclimates exist in every landscape and need to be considered in estimates of plant water loss. Features typical of urban landscapes (such as buildings and paving) influence temperature, wind speed, light intensity and humidity. These features vary considerably among landscapes, resulting in differences in microclimate. To account for these differences, a microclimate factor (kmc) is used.

The microclimate factor ranges from 0.5 to 1.4, and is divided into three categories:
Low 0.5 - 0.9
Average 1.0
High 1.1 - 1.4

The microclimate factor is relatively easy to set. An “average” microclimate condition is equivalent to reference evapotranspiration conditions, i.e.., an open-field setting without extraordinary winds or heat inputs atypical for the location. This microclimate is not substantially affected by nearby buildings, structures, pavements, slopes, or reflective surfaces. For example, plantings in a well-vegetated park which are not exposed to winds atypical of the area, would be assigned to the average microclimate category.

In a “high” microclimate condition, site features in- crease evaporative conditions. Plantings surrounded by heat-absorbing surfaces, reflective surfaces, or exposed to particularly windy conditions would be assigned high values. For example, plantings in street medians, parking lots, next to southwest-facing walls of a building, or in “wind tunnel” areas would be assigned to the high category.

“Low” microclimate conditions are as common as high microclimate conditions. Plantings that are shaded for a substantial part of the day or are protected from winds typical to the area would be assigned low values. These include the north side of buildings, courtyards, under building overhangs, and on the north side of slopes.

The high and low microclimate categories have ranges of values. For example, the low category ranges from 0.5 to 0.9. The specific value assigned within a category will depend on an assessment of the degree to which the microclimate will affect plant water loss. For example, trees in a parking lot which are exposed to constant winds (atypical for the general area) will be assigned a higher value in the high category than if the location was not windy. Conversely, a courtyard planting in afternoon shade and protected from winds will be assigned a kmc value in the low category, but less than that for a planting without afternoon shading.

Example: An irrigation consultant is estimating landscape water requirements for a large residential development. The buildings, parking lots, walkways, and open areas at the site create substantially different microclimates within plantings. Starting with the open areas, he determines that conditions are quite similar to reference ET measurement sites and as- signs them to the average category (kmc = 1.0). Trees in the parking lot are exposed to heat from the asphalt pavement and reflected light from cars and are assigned to the high category. Since the parking lot is not exposed to extraordinary winds, however, he chooses a midrange value of 1.2. Shrub and groundcover plantings on the northeast side of buildings are shaded for most of the day and are assigned to the low category. Being protected from winds typical of the area as well, they are given a kmc value of 0.6, in the lower end of the range.

Assigning Microclimate Factor Values

Average Microclimate: kmc = 1.0
Site conditions equivalent to those used for reference ET measurements represent an average microclimate. Reference ET is measured in an open-field setting which is not exposed to extraordinary winds or heat inputs from nearby buildings, structures, or vehicles. Plantings in similar conditions would be considered to be in an average microclimate. Plantings in park settings are most typically assigned to this category. Although some hardscape may exist, vegetation dominates the landscape. Large plantings of groundcover, groves of trees, and mixtures of shrubs, turf, and trees in relatively open areas represent examples of an average microclimate condition. Small parks with adjacent buildings, extensive hardscapes, or exposed to extraordinary winds would not be included in the average category.

Low Microclimate: kmc = 0.5 - 0.9
Sites which are shaded or protected from winds typical to the area are considered to be in the low microclimate category (Costello et al. 1996). Features of the site modify the microclimate such that evaporative conditions are less than those found in the average microclimate. Plantings located on the north side or northeast side of buildings, shaded by over- head structures, or within courtyard settings are typically assigned a kmc value in the low range. Plantings protected from winds by buildings, structures, or other vegetation also would be assigned to the low category. The specific value assigned for the microclimate factor will depend on the specific site conditions. For example, a planting in a courtyard which is shaded most of the day and protected from winds may be assigned a value of 0.6, while a similar planting which is located on the northeast side of a building may be assigned a value of 0.8.

High Microclimate: kmc = 1.1 - 1.4
Sites which are exposed to direct winds atypical for the area, heat inputs from nearby sources, and/or reflected light would be considered to be in the high microclimate category. These features of the site increase evaporative conditions above those found in an average microclimate condition. Plantings located in medians, parking lots, or adjacent to south or southwest facing walls which are exposed to higher canopy temperatures than those found in a well-vegetated setting would be in the high category. Plantings in wind tunnel locations and those receiving reflected light from nearby windows, cars, or other reflective surfaces are also in high microclimate conditions. The specific value assigned will depend on the specific conditions. For example, a shrub planting located next to a southwest facing wall may be assigned a kmc value of 1.2, while a similar planting next to a southwest wall which is composed of reflective glass and is exposed to extraordinary winds may be assigned a value of 1.4.

Table 3— Summary Table Values for Landscape Coefficient Factors

High Moderate Low Very Low
Species Factor* (ks) 0.7-0.9 0.4-0.6 0.1-0.3 <0.1
Density (kd) 1.1-1.3 1.0 0.5-0.9
Microclimate (kmc) 1.1-1.4 1.0 0.5-0.9
* Species factor values may change during the year, particularly for deciduous species. See Table 1 for seasonal changes in crop coefficients for agricultural crops.

The landscape coefficient formula was introduced in Chapter 2, and the three factors which determine its value were discussed. Now these factors are used to calculate values for the landscape coefficient. A series of field cases show the range of values that can be determined for KL. In Chapter 4, calculations using the landscape coefficient in the ETL formula are presented.

Using the information presented in Chapter 2, values for the landscape coefficient can be calculated. The following cases show how the landscape coefficient is used for a variety of species, density, and microclimate conditions. Species factor values will be taken from the WUCOLS list, while density and microclimate values are based on the planting and site conditions described. For quick reference, the following table gives values for each factor.

Landscape Coefficient Factors

High Moderate/Average Low Very Low
Species 0.7-0.9 0.4-0.6 0.1-0.3 <0.1
Density 1.1-1.3 1.0 0.5-0.9
Microclimate 1.1-1.4 1.0 0.5-0.9

Case 1—A large, mature planting of star jasmine in a park in San Jose. It is in full sun and has little wind exposure.
ks = 0.5
kd = 1.0
kmc = 1.0
KL = 0.5 x 1.0 x 1.0 = 0.5
Analysis: Star jasmine is classified as moderate in the WUCOLS list (moderate range = 0.4 to 0.6) and a midrange ks value of 0.5 is assigned. Since the planting is mature it will be considered full (i.e., canopy cover = 100%), and being of one vegetation type, it is classified as an average density and kd is 1.0. The microclimate is similar to reference evapo- transpiration conditions (full sun, open area, no ex- traordinary winds) and, therefore, is classified as average and kmc is 1.0.

Case 2—A mixed planting of dwarf coyote brush, Pfitzer juniper, oleander, purple hopseed, and olive in an office park in Los Angeles. The planting is full, exposed to sun all day, but not to extraordinary winds.
ks = 0.2
kd = 1.2
kmc = 1.0
KL = 0.2 x 1.2 x 1.0 = 0.24
Analysis: All species are classified as low in the WUCOLS list and are assigned a midrange value of 0.2. Canopy cover is 100%, and since all three vegetation types occur, this is classified as a high density planting and a kd value of 1.2 is assigned. The microclimate is average and a value of 1.0 is assigned.

Case 3—A mature planting of rockrose, star jasmine, and dichondra in an amusement park in Sacramento. The planting is in full sun and atypical winds are infrequent.
ks = 0.8
kd = 1.0
kmc = 1.0
KL = 0.8 x 1.0 x 1.0 = 0.8
Analysis: Species in this planting are in three different WUCOLS categories: low (rockrose), moderate (star jasmine), and high (dichondra). To maintain the dichondra in good condition, a ks value of 1.7 is needed. This means, however, that both the rockrose and star jasmine will receive more water than they need. Obviously this is not a water-efficient planting. Both the density and microclimate conditions are average and were assigned values of 1.0.

Case 4—A widely-spaced planting of camellia on a university campus in San Francisco. Canopy cover of the planting is 40% to 50%. A 4-inch mulch covers the ground throughout the planting. It is in full sun and no extraordinary winds occur.
ks = 0.5
kd = 0.5
kmc = 1.0
KL = 0.5 x 0.5 x 1.0 = 0.25
Analysis: Camellia is classified as moderate in the WUCOLS list and is assigned a midrange value of 0.5. This is a low density planting of a single species and a kd value of 0.5 is assigned. The microclimate is average and given a value of 1.0.

Case 5—A planting of leatherleaf mahonia and Burford holly in an office park in Pasadena. The planting is full, but shaded in the afternoon by an adjacent building. The building also blocks afternoon winds typical for the area.
ks = 0.5
kd = 1.0
kmc = 0.6
KL = 0.5 x 1.0 x 0.6 = 0.30
Analysis: Both species are classified as moderate in the WUCOLS list and are assigned a midrange value of 0.5. The canopy cover is full and since only one vegetation type occurs, it is classified as average density. Since the building shades the planting and protects it from wind, the microclimate is low and a kmc value of 0.6 is assigned.

Case 6—A mixed planting of sweetgum, Rhaphiolepis sp., Wheeler's dwarf pittosporum, Raywood ash, and English ivy at a zoo in San Diego. The planting is mature (canopy cover is 100%), in full sun, and ex- posed to continual strong winds not typical for the area (i.e., windier than the reference ET location).
ks = 0.5
kd = 1.2
kmc = 1.3
KL = 0.5 x 1.2 x 1.3 = 0.78
Analysis: All species in this planting are classified as moderate in the WUCOLS list and are assigned a midrange value of 0.5. Since the canopy cover is 100% and all three vegetation types occur, this is a high density planting and a kd of 1.2 is assigned. Since the site is atypically windy for the area, the microclimate is classified as high and a kmc of 1.3 is assigned.

Case 7—A new planting of rockrose, manzanita, pink melaleuca, and bushy yate along a freeway in Monterey County. All plants are 5-gallon container stock, planted in full sun, and are not exposed to extraordinary winds. Canopy cover is 20 to 30%. A 4-inch layer of mulch covers the ground throughout the planting.
ks = 0.2
kd = 0.5
kmc = 1.0
KL = 0.2 x 0.5 x 1.0 = 0.1
Analysis: All species in this planting are classified as low in the WUCOLS list and a midrange value of 0.2 is given. Since this is a new planting and canopy cover is not full, it is placed in a low density category and assigned a kd value of 0.5. The microclimate is average and assigned a value of 1.0. (See Chapter 8 for information on irrigating new plantings.)

These field examples should provide an understanding of how values for each of the landscape coefficient factors are assigned and used. In addition, an appreciation for the diversity of species, differences in vegetation density, and variation in microclimates which exist in landscapes should be realized. In many cases, there will be a different landscape co- efficient for each irrigation zone.

For discussions of the following special planting cases, refer to Chapter 8:

  • New Plantings
  • Trees in Turf
  • Individual Specimens
  • Vines
  • Herbaceous Plants

The landscape coefficient and reference evapotranspiration now are used to estimate landscape evapotranspiration for the plantings described in Chapter 3. This chapter completes the process used to produce estimates of landscape water loss. Subsequent chapters discuss how to use estimates of ETL to calculate total irrigation water needs and how to apply this information in landscape management programs.

In Chapter 3, seven landscape planting cases were described and used for landscape coefficient calculations. These cases will be used here to calculate landscape evapotranspiration with the ETL formula. The ETL formula was described in Chapter 2 and is presented here for quick reference:
ETL = KL x ETo
Landscape Evapotranspiration = Landscape Coefficient x Reference Evapotranspiration

For each case, reference evapotranspiration (ETo) values will be taken from Appendix A. All are normal year average values for the month of July for the respective locations.
Case 1— KL = 0.5: ETo for San Jose = 7.44 inches ETL = 0.5 x 7.44 = 3.72 inches
Case 2— KL = 0.24: ETo for Los Angeles = 6.5 inches ETL = 0.24 x 6.5 = 1.56 inches
Case 3— KL = 0.8: ETo for Sacramento = 8.6 inches ETL = 0.8 x 8.6 = 6.88 inches
Case 4— KL = 0.25: ETo for San Francisco = 4.9 inches ETL = 0.25 x 4.9 = 1.22 inches
Case 5— KL = 0.30: ETo for Pasadena = 7.4 inches ETL = 0.30 x 7.4 = 2.22 inches
Case 6— KL = 0.78: ETo for San Diego = 5.8 inches ETL = 0.78 x 5.8 = 4.59 inches
Case 7— KL = 0.1: ETo for Monterey = 5.5 inches ETL = 0.1 x 5.5 = 0.55 inches
These calculations show that landscape irrigation water needs vary substantially. Estimates range from 0.55 inches to 6.88 inches—more than a 12-fold difference.

The two factors used to determine ETL, the land- scape coefficient and reference evapotranspiration, are solely responsible for producing these differences in water loss estimates. For plantings in the same location (i.e., where the same ETo values will be used), the differences will arise solely from the landscape coefficient. To produce useful estimates of water loss, therefore, it is important to carefully determine the value of KL.

Even though the ETL formula has given an estimate of water loss from a landscape, the total amount of irrigation water needed has not been determined. The total amount is calculated from two factors: ETL and irrigation efficiency. The following chapter dis- cusses irrigation efficiency and shows how it is used to determine the total amount of water to apply.

The first four chapters have described the process for estimating plant water needs. To calculate the total amount of water to apply, irrigation efficiency needs to be addressed. This chapter introduces the formula for calculating total water needs and discusses the irrigation efficiency factor. How this information applies to irrigation management is discussed in Chapter 6.

The ETL formula calculates the amount of irrigation water needed to meet the needs of plants. This is not the total amount of water needed to apply, however. Since every irrigation system is inefficient to some degree, the landscape will require water in excess of that estimated by ETL. In this chapter, irrigation efficiency will be discussed and then used to calculate the total amount of water to apply.

Irrigation Efficiency

Efficiency can be defined as the beneficial use of applied water (by plants). The following formula is used to calculate irrigation efficiency:
Irrigation Efficiency (%) = Beneficially Used Water x 100

                                            Total Water Applied

An efficiency of 100% would mean that all applied water was used by the planting. This rarely occurs. Consequently, irrigation efficiency is less than 100% in virtually all cases and additional water should be applied to account for efficiency losses.

A determination of irrigation efficiency (IE) for landscape plantings is challenging. As yet, a standard method has not been established. The approach used for turf irrigation, distribution uniformity (DU), is not appropriate for most landscape plantings.

Three approaches are considered here: calculation, estimation, and goal setting. Each method has significant limitations, and are presented here only as possible options to consider.


To calculate irrigation efficiency, values for ETL and TWA are needed. In landscapes, beneficially used water is the equivalent of ETL (the amount of water estimated to be needed by a planting). This is calculated as described in Chapter 4. The total water applied can be determined by operating an irrigation system for a scheduled cycle and measuring the total water used (usually read from a water meter). The following example shows a typical calculation:
ETL = 4 inches (calculated using the ETL formula); TWA = 5 inches (measured)

IE = ETL x 100 = 80%


In the above example, the system has an 80% efficiency, or 8 out of every 10 gallons of applied water is used beneficially by the planting. Two gallons are lost, perhaps to runoff, evaporation, leakage, or wind spray. To supply 8 gallons of water means that a total of 10 gallons needs to be applied.

This approach has limited application for two reasons:

  1. it requires a water meter to measure the amount of water applied, and
  2. it may include efficiency losses associated with poor scheduling.

It assumes that applied water is close to optimum for the landscape plants and the system operating capabilities. It may be, however, that inefficiencies are linked to the operating schedule. For example, the irrigation duration may be too long for the planting.


In cases where the total water applied cannot be measured, then irrigation efficiency may be esti- mated. Estimates are based on an assessment of the design and performance of the irrigation system. A system which is well designed and operated can have an efficiency range of 80% to 90%. Poorly designed and operated systems may have efficiencies of less than 50%. A representative range of efficiencies for landscape systems is proposed here to be from 65% to 90%.

Estimating is a subjective process where two assessments of the same system can vary widely. The utility of an estimate will be related to the knowledge and experience of the estimator.

Goal Setting

Irrigation efficiency values may also be based on a design and/or management goal. For instance, a new landscape may be designed to achieve an irrigation efficiency of 90%. Or, an existing landscape may be managed to achieve an irrigation efficiency of 85%. Both values represent efficiency goals. These efficiency values are then used to estimate the total water needed to achieve the goal. This approach is useful for water budgeting purposes, but does not provide a useful estimate of actual system performance.

All three of these methods are highly approximate. Until a standard method of measuring landscape irrigation efficiency is determined, however, they pro- vide some guidance.

Total Water Applied

Regardless of the method used to determine irrigation efficiency, the total amount of water needed for a landscape planting is calculated using the following formula:


Total Water Applied = Landscape Evapotranspiration

                                          Irrigation Efficiency

The following are examples of calculations using irrigation efficiency and landscape evapotranspiration to determine the total water to apply. The first three cases presented in Chapters 3 and 4 will be used. An irrigation efficiency value of 70% is as- signed for each case.

Case 1— ETL = 3.72 inches; IE = 70% or 0.7
TWA = 3.72 = 5.31 inches


(see Case 1 in Chapter 4)
Case 2— ETL = 1.56 inches; IE = 70% or 0.7
TWA = 1.56 = 2.22 inches


Case 3— ETL = 6.88 inches; IE = 70% or 0.7
TWA = 6.88 = 9.8 inches


It is clear from these calculations that irrigation efficiency plays a very large role in determining the total amount of water to apply. Water added to account for efficiency losses ranges from 0.67 inches to 2.88 inches.

If the efficiency of the system is greater or less than 70%, the total water applied will vary accordingly. The magnitude of this effect can be seen in the following calculations where IE values from 30% to 90% are used. The ETL value from the first sample calculation (3.72 inches) is used in each case. @ 30% IE, TWA = 3.72 = 12.4 inches


@ 60% IE, TWA = 3.72 = 6.2 inches


@ 90% IE, TWA = 3.72 = 4.1 inches


These calculations indicate that for the same land- scape plants, at the same location, and under identical environmental conditions, the total amount of water applied varies from 4.1 inches to 12.4 inches, due solely to irrigation efficiency differences. Clearly, the IE factor needs to be addressed very carefully when planning

Chapters 1 through 5 have introduced a number of formulas and numbers that are used to estimate landscape water needs. This chapter puts all the equations together to show the calculation process. Subsequent chapters discuss considerations for applying estimates and special planting situations.

Three steps are needed to estimate irrigation water needs of a planting:

  1. calculate the landscape coefficient,
  2. calculate landscape evapotranspiration, and
  3. calculate the total water applied.

These steps are combined in a worksheet format on the following page. After the worksheet, an example is given to show how it is used, followed by a discussion of converting units from inches of water to gallons.

Converting Inches to Gallons

Landscape evapotranspiration (ETL) and total water applied (TWA) values have been given in units of inches. Frequently, it is of interest to know how many gallons of water are needed. Inches of water can be converted to gallons by using: 1) a conversion factor, and 2) a measure of the area to be irrigated.

  1. The conversion factor, 0.62, can be used to convert inches-of-water-per-square-foot to gallons. A volume that is one-foot long, one-foot wide, and one-inch deep contains 0.62 gallons of water. This means that there are 0.62 gallons of water in a square-foot-inch. (There are 325,851 gallons in an acre-foot of water.)
  2. The area to be irrigated needs to be measured. To use the conversion factor, units of square- feet are required.

With the area and the conversion factor, gallons of water can be calculated using the following formula:
Estimated water in gallons = estimated water in inches x area (square feet) x 0.62

Example: It was determined that 2.11 inches of water was needed for a groundcover planting. Let’s say the planting covers 5,000 square feet.

To convert inches to gallons:
Gallons = 2.11 inches x 5,000 sq. ft. x 0.62 = 6,541

It is estimated that 6,541 gallons of water are needed to maintain the 5,000 square feet of groundcover.


Worksheet Example

A landscape manager in San Bernardino is interested in estimating water requirements for a large planting of African daisy (Osteospermum fruticosum) for the month of July. The planting is in an open area and is not exposed to extraordinary winds for the area. The manager estimates that irrigation efficiency is 70% and, using the work-sheet, follows the three steps (see below).


The landscape manager has estimated that the groundcover will need 2.11 inches of water for the month of July. Using this estimate, the manager can develop an irrigation schedule. Other factors may need to be considered before deciding if this estimate is appropriate for the planting. Chapter 7 addresses these considerations.

Before water needs estimates are used for landscape planning and management purposes, a few points need to be considered. In Chapter 7, five special topics which are relevant to using estimates are addressed. The following chapter discusses some special planting situations.

The previous chapters have described how to estimate irrigation water needs for landscape plantings. These estimates can be used in landscape planning and management to:

  • develop water budgets for planned or existing landscapes,
  • assist in the design of landscapes to meet irrigation goals,
  • assist in designing and managing effective hydrozones,
  • help in the determination of irrigation system efficiency (i.e., along with measurements of total water use), and
  • serve as an auditing tool by providing assessments of the amount of water landscapes need compared to that actually being used.

When using landscape water estimates for these purposes, however, a few considerations are important to note. These are discussed briefly under the following special topics headings.

Field Adjustments

The landscape coefficient method provides estimates of water needs, not exact values. Consequently, adjustments likely are needed in the field. If plants are showing signs of water stress, then an upward adjustment will be needed. Conversely, when it appears that too much water is being applied, then a downward adjustment is warranted. It is strongly recommended that when irrigation water estimates are implemented in the field that they be followed by careful monitoring.

Irrigation Schedules

An estimate of water needs is the first step in developing an irrigation schedule. Irrigation frequency, duration, and cycles also need to be determined to create a schedule. These are determined from the soil infiltration rate, rooting depth, sprinkler application rate, allowable depletion amounts, and soil water holding capacity. Each of these factors needs to be evaluated to determine how frequently to irrigate, how long to irrigate at any one time, and how many irrigation cycles are needed.

Soil Evaporation

Water loss may occur from the soil as well as from plants. This is most common when ground shading is less than 100% and a mulch is not present. The rate of evaporative water loss from soils depends on soil wetness, texture, structure, and density. When soil evaporation contributes to landscape water losses, water estimates should be increased by 10% to 20%. With sufficient mulching, however, bare soil surfaces will not be a source of water loss.

Salts and Leaching Fractions

When soil salt concentrations are sufficiently high to cause plant injury, the application of water in excess of that needed to meet plant needs is necessary. This process is called “leaching” and the percentage of applied water used to move salts below the root zone is called the “leaching fraction”. For example, if 100 gallons of water is applied, and 25 gallons percolated below the root zone to remove salts, this would be a 25% leaching fraction. The leaching fraction needed for a landscape will depend on soil salt concentrations, tolerable levels, depth of the root zone, and soil physical properties. To determine an appropriate leaching fraction, it is recommended that managers consult with a qualified soil laboratory. The leaching fraction will add water to that needed for plants (ETL), and the total water applied (TWA) will increase.

Reclaimed Water

The use of reclaimed water in landscape irrigation is becoming more common. Reclaimed water varies in quality, however, depending on the source and treatment process. Some reclaimed water is of high quality with little potential to injure plants. In other cases, reclaimed water may be of low quality, containing injurious levels of salts or specific elements. When irrigating with reclaimed water, planners and managers will need to assess and monitor water quality. Some upward adjustments in water estimates may be needed to reduce plant injury potential with low quality water. Consult a qualified laboratory when making such adjustments.

Although the application of the landscape coefficient method has been described for many landscape cases, there are some special planting situations that require further consideration. These cases are de- scribed in Chapter 8. This concludes the process of making water needs estimates for landscape plantings. Remember, the appendices contain important reference information to use in calculations.

New plantings, trees in turf, individual plants, vines, and herbaceous plants represent special cases which require further consideration in making water needs estimates. All are common elements of landscapes.

New Plantings In terms of irrigation water needs, the key differences between new and mature plantings are in density factor assignments and irrigation efficiency. Typically, canopy cover is substantially less in a new planting and the lowest kd value, 0.5, is appropriate. Irrigation efficiency is also typically low for new plantings.

A landscape coefficient (KL) calculation for a new planting was made in “Using the Landscape Coefficient Formula” (Chapter 3, example 7). In the ex- ample, a kd value of 0.5 was used which produced a KL of 0.1 (ks = 0.2, kmc = 1.0).

Based on experience, it may be thought that irrigating a new planting at one tenth of reference evapotranspiration is insufficient. Generally, landscape managers believe that new plantings need even more water than mature plantings. When irrigation efficiency (IE) is considered, however, the amount of water needed increases substantially. Indeed, it is because of very low efficiencies when irrigating new plantings that the total amount of water is much greater than that needed solely for the plants.

A sample calculation helps to show the role of irri- gation efficiency in new planting irrigation. Using example 7, ETL = 0.1 for a new planting in Monterey County in July. The total amount of water needed is calculated using the TWA formula: TWA = ETL


Selecting an irrigation efficiency of 10%,
TWA = 0.1 = 1.0 inch


Ten times more water needs to be applied than that actually needed for the plants. This is based on a 10% irrigation efficiency for a new planting which is sprinkler irrigated. An IE of 10% is reasonable because most of the root mass of new plantings is confined to the rootball, with available water consisting of only that held in the rootball and, in some cases, a small volume of adjacent soil. Sprinklers deliver water to the entire planted area, not just the rootballs, so much of the water falls outside the usable area.

For instance, in a planting area of 100 sq. ft., only 10 sq. ft. may be occupied by rootball. Thus, if water is distributed uniformly, only 10% of the water applied falls in the root zone, which produces a 10% irrigation efficiency.

Irrigation efficiencies for some new plantings may be even less than 10%. If a planting is sparse and root zone occupies less than 10% of the irrigated area, and/or some of the water that lands on the rootball is lost to evaporation, percolation, or run- off, then IE may be less than 10%.

As roots develop into the adjacent soil, however, irrigation efficiency increases rapidly. For instance, if after one year, roots have developed into the adjacent soil to the point that half the planting area has some root mass, then water landing on half the area potentially may be absorbed by plants. In this case, irrigation efficiency has increased 5-fold to 50% (assuming no loss from runoff, evaporation, etc.).

It should be recognized that sprinkler irrigation of new plantings (i.e., of container grown plants) is not efficient. Other methods should be considered for water conservation purposes. Drip systems deliver water directly to rootballs and, therefore, have higher efficiency. Potentially, hand watering is also more water efficient than sprinkler irrigation, provided it is done carefully.

As root development increases into the adjacent soil, sprinkler irrigation efficiency increases, while drip irrigation efficiency may actually decrease if emit- ters are not moved or supplemented to supply the larger root zone. Dual systems of both drip emit- ters and sprinklers may have the greatest potential for maximizing efficiency for new and developing plantings: the drip system being used for the new planting and the sprinklers employed once the root system has developed.

Trees in Turf

The water needs of most tree species planted in turf are generally met by the relatively high water needs of turf. Turf crop coefficients range from 0.6 (warm season species) to 0.8 (cool season species). This range is sufficient to satisfy the needs of all trees in the moderate, low, and very low WUCOLS categories. Trees in the high category may need supplemental water, particularly if they are planted in warm season turf. Trees in cool season turf are not likely to need supplemental water.

Aside from meeting total water needs, some other factors need to be considered regarding trees in turf:

  1. Species Selection. Not all tree species can be expected to perform well in turf. Species in the low and very low WUCOLS categories may be injured or killed by turf irrigation. Many species are adapted to dry summer conditions (e.g., oak species) and frequent irrigations associated with turf may result in root injury, typically from disease or poor aeration. Species selection is very important. When specifying trees in turf, species should be limited largely to those classified as “high” on the WUCOLS list. Species from the “moderate” category may be used in some cases, but there will be a greater potential for injury.
  2. New Turf Around Established Trees. When new turf (and associated irrigation) is installed around established trees, precautions are needed to avoid injury to the trees. This is particularly the case for trees that were not formerly irrigated. By supplying water to the root zone of established trees the potential for injury from disease or poor aeration increases substantially. Certain species (e.g., oaks) are more sensitive to such changes than other species. The root crown area is particularly sensitive and needs

special consideration. To help ensure the survival of both the turf and trees in this situation, it is recommended that a certified arborist be consulted.

  1. Drought Years. In times when the water supply for turf becomes restricted (e.g., drought years), the water needs of trees in turf may not be met. During previous droughts in California, many trees in turf areas were severely injured or killed when water was withheld from turf. Frequently, the turf recovers when irrigation resumes, but the trees do not. It is very important to provide water directly to trees during such times.
  2. Newly-Planted Trees. Water supplied to meet turf needs is often not sufficient for newly planted trees in turf. Although turf irrigation is likely sufficient for most species once established, newly planted trees have special requirements. In most cases after planting, the roots of new trees are confined to the rootball, or a relatively small volume of soil. Much of the water supplied in turf irrigation (typically via sprinklers) does not rewet the rootball sufficiently. It is only the water that lands on the rootball that can be absorbed, and in most cases this is not adequate to meet the needs of the tree. As a result, many trees are very slow to develop in turf, and some are injured or killed. Supple- mental water (delivered manually or by drip systems) are strongly recommended for trees in turf.

In addition to special water needs, newly planted trees in turf also may be inhibited biologically by the turf. This is an effect known as “allelopathy,” where one plant inhibits the development of another by the release of phytotoxic materials from its roots. Turf species are recognized as having allelopathic effects on young trees and, therefore, an area (2 ft. radius) around newly planted trees should be kept turf-free. Ideally mulch is applied to the soil surface in the turf-free zone to reduce evaporation and minimize the potential for mower or trimmer injury.

  1. Shallow Rooting and Windthrow. Turf irrigation typically supplies water to the surface 3 to 6 inches of soil, the active root zone for most turf species. Consequently, turf irrigations are relatively shallow and frequent (i.e., when com- pared to tree irrigation depths of 1 to 3 ft.). As a result, tree roots in turf areas tend to develop close to the soil surface. There has been some concern regarding the potential for reduced anchorage associated with shallow root systems of trees in turf. It is thought that large trees may have a higher potential for windthrow. Although this occurrence has been observed, there is no documentation to show that the potential for tree windthrow is higher in turf than elsewhere. Nevertheless, it is generally held that deep irrigations for trees in turf are beneficial. They not only increase the potential for root development deeper in the soil profile, but they also increase the size of the soil volume from which roots can extract water.

Individual Plants

To this point, the landscape coefficient method has been used to estimate water needs of plantings (i.e., groups of plants). It also can be used to estimate water needs of individual plants. The three factors (species, density, and microclimate) are used to determine a landscape coefficient as before. A few considerations apply for individual plants, however, and they are discussed for shrubs and trees separately.


ks: Species factor values are found in the WUCOLS list.
kd: For most shrubs, an average density factor of 1.0 will be appropriate. For very large shrubs, an upward adjustment to 1.1 may be warranted.
kmc: In most cases, the microclimate factor would be assigned as discussed in Chapter 2.


ks: Species factor values are found in the WUCOLS list.
kd: For small trees (< 15 feet tall), an average density factor of 1.0 would be appropriate. For larger trees, an upward adjustment to 1.1 or 1.2 accounts for the increase in leaf area found in many canopies.
kmc: In most cases, the microclimate factor would be assigned as discussed in Chapter 2. For large trees, however, an upward adjustment to 1.2 or 1.3 to account for wind flow through the canopy may be appropriate.

Example: The urban forester for the city of Modesto is interested in estimating water needs for a large Modesto ash tree located in a downtown city plaza for the month of July.

First, the forester needs to assign values for each of the landscape coefficient factors. In the WUCOLS list Fraxinus velutina ‘Modesto’ is classified as “moderate” with a ks value of 0.4. Since this is a large, dense tree, the forester uses a density factor value of 1.1. The microclimate in the plaza warrants a “high” microclimate factor value. In addition, the forester wants to adjust for wind flow through the canopy since no trees or buildings are nearby to attenuate the wind. The forester selects a kmc value of 1.5. Using these values, a calculation of the landscape coefficient can be made.
KL = ks x kd x kmc
KL = 0.4 x 1.1 x 1.5 = 0.66

With the landscape coefficient calculated, the land- scape evapotranspiration formula is used to calculate ETL:
ETL = KL x ETo KL = 0.66
ETo = 8.0 inches (for July in Modesto) ETL = 0.66 x 8.0 inches = 5.28 inches

The urban forester has estimated that the tree needs 5.28 inches of water for the month of July to maintain good appearance, health, and growth. A further adjustment to this value is needed to account for irrigation efficiency (see Chapter 5).

An alternative method for estimating water loss from an individual tree is described in Lindsey and Bassuk (1991). This method uses leaf area index (LAI) to account for density differences in tree canopies.


Vines occur in many landscapes and need to be considered in water loss estimates. Vines can contribute substantial leaf area to a planting whether they canopy cover. Upward adjustments in kd are likely needed when vines are present. These can range from small increases (0.1) to large (0.3) depending on the amount of vegetation (leaf area) added.


Estimates of water needs for plantings of annual species can be made using the landscape coefficient formula. As for woody plantings, values for KL and ETo are needed. ETo values are obtained as described previously, while KL needs to be calculated from the three factors, ks, kd, and kmc. The microclimate factor, kmc, is determined as before, and kd will range from 0.5 to 1.0 depending on the fullness of the plantings. The species factor, ks, is more difficult to determine as many species are not included in the WUCOLS list. Generally, the water requirements of annual plants are relatively high and a ks range of 0.4 to 0.8 is suggested for most species. By assigning values for ks, kd, and kmc, the land- scape coefficient, KL, can be calculated and an estimate of water needs (ETL) is determined.

1999 Edition

L. R. Costello University of California Cooperative Extension
K.S. Jones University of California Cooperative Extension

Project Leader: L. R. Costello, Environmental Horticulture Advisor University of California Cooperative Extension San Mateo and San Francisco Counties
Project Coordinator: K. S. Jones, Horticulture Associate, University of California Cooperative Extension San Mateo and San Francisco Counties

North-Central Coast: Barrie D. Coate, Horticultural Consultant—Barrie D. Coate & Associates, Los Gatos, 1992, 1994,1999*; Laurence R. Costello, Horticulture Advisor—UC Cooperative Extension, San Mateo and San Francisco Counties 1992, 1994, 1999; Katherine S. Jones, Horticulture Associate—UC Cooperative Extension, San Mateo and San Francisco Counties 1992, 1994, 1999; James MacNair, Horticultural Consultant—MacNair & Associates, Glen Ellen, 1992, 1994, 1999; Nelda Matheny, Horticultural Consultant—Hort Science, Inc., Pleasanton 1992 John Meserve, Horticultural Consultant—Santa Rosa, 1999; Tony Norris, Parks and Landscaping Superintendant—City of Richmond, 1999 Linda Novy, President & C.E.O.—Gardener's Guild, Inc., San Rafael, 1992; Richard Sealana, Land Management Consultant—Sealana Associates, Fremont, 1992, 1994, 1999 Dan Sheehy, Area Manager—Cagwin & Dorward, Inc., Novato 1992, 1994, 1999; M. Nevin Smith—Suncrest Nurseries, Watsonville, 1999

Central Valley: Fred Allen, Landscape Architect—City of Modesto, 1992; Ralph Carhart, Landscape Architect—CalTrans, Division of Maintenance, Sacramento 1992 Ann Chandler, Owner—Cornflower Farms Nursery, Elk Grove 1992; Pam Elam Geisel, Farm Advisor—UC Cooperative Extension, Fresno County,1992, 1994, 1999 Larry Fitzgerald, former Container Production Manager—Oki Nursery, Sacramento, 1992; Gary Hickman, Farm Advisor—UC Cooperative Extension, San Joaquin County, 1992, 1999 Martin Hildebrandt—Scenic Nursery, Modesto, 1999; Ed Perry, Farm Advisor—UC Cooperative Extension, Stanislaus County, 1992, 1994, 1999 Lance Walheim, Horticultural Consultant—Exeter, 1992

South Coastal: Randy Baldwin, General Manager—San Marcos Growers, Santa Barbara, 1992, 1994 Jeff Cope—City of Santa Barbara, 1999; Cynthia Drake, Horticultural Consultant—San Diego, 1999; Scott Molentin, Landscape Architect—Estrada Land Planning, San Diego, 1992, 1994, 1995 Wesley A. Humphrey, Horticultural Consultant—Fallbrook, 1992, 1994, 1999; Frederick M. Lang, Landscape Architect—South Laguna,1992 Lynn Ocone, Garden Writer—Sunset Magazine, Los Angeles 1992; Rick Mosbaugh, President—Statice Landscape Inc., Los Angeles 1992, 1994; Ray Sodomka, Owner—Turk Hesselund Nursery, Montecito, 1992, 1994, 1999; Tom Larson—Integrated Urban Forestry, Laguna Hills, 1992, 1994, 1999

South Inland: Mike Evans, Partner—Tree of Life Nursery, San Juan Capistrano 1992, 1994 Michael MacCaskey, Garden Writer—Sunset Magazine, Los Angeles, 1992; L. K. Smith, Landscape Architect—Newbury Park, 1992, 1994, 1995; Kenneth K. Kammeyer, Landscape Architect—Kammeyer & Associates, Corona ,1992, 1994, 1999

High & Low Desert: Ronald L. Baetz, Administrative Services Officer—Desert Water Agency, Palm Springs, 1992 Jerry Clark, Landscape Architect—City of Palm Desert, 1999; William Deady, Horticultural Consultant—Morongo Valley, 1992, 1994; David Harbison, Water Management Specialist—Coachella Valley Water District, Coachella, 1992, 1994, 1999 Eric Johnson, Desert Landscape Consultant—Palm Desert, 1992, 1994; Bob Perry, Landscape Architect—Claremont ,1992, 1994, 1999; Ruth Watling, Horticultural Consultant—Mountain Center, 1999

  • Year(s) of participation on WUCOLS Committee


1992: B. Coate, R. Sealana, R. Carhart, G. Hickman, J. MacNair, K. Jones, D. Sheehy, N. Matheny, L. Walheim, P. Geisel, E. Perry, F. Allen, A. Chandler, L. Novy, L. Fitzgerald, L. Costello, F. Lang, K. Smith, T. Larson, R. Perry, L. Ocone, L. Costello, W. Humphrey, S. Molentin, R. Sodomka, K. Smith, M. MacCaskey, M. Evans, R. Baldwin, R. Perry, E. Johnson, W. Deady, K. Jones, R. Baetz, D. Harbison

1998: R. Sealana, K. Jones, L. Costello, J. MacNair, J. Meserve, M. Hildebrandt, G. Hickman, N. Smith, P. Geisel, E. Perry, B. Coate, T. Norris, D. Sheehy, R. Watling, K. Jones, W. Humphrey, C. Drake, R. Perry, J. Cope, K. Smith, D. Harbison, R. Sodomka, J. Clark, L. Costello, S. Molentin, K. Kammeyer


Water conservation is an essential consideration in the design and management of California landscapes. Effective strategies that increase water use efficiency need to be identified and implemented. One key strategy to increase efficiency is that of matching water supply to plant needs. By supply- ing only the amount of water needed to maintain landscape health and appearance, unnecessary applications that exceed plant needs can be avoided. To do so, however, requires some knowledge of species needs.

This Guide provides irrigation water needs evaluations for over 1,900 species used in California landscapes. It is based on the observations and field experience of 41 knowledgeable landscape horticulturists in California (see list of Regional Committees). It was developed to provide guidance in the selection and maintenance of plants based on irrigation water needs. Specifically, it can be used to:

  • assist landscape architects, designers, and planners in selecting plants for water efficient landscapes,
  • assist landscape managers in evaluating water needs of existing plantings and in creating irrigation schedules that match species needs,
  • provide options for landscape managers who wish to create hydrozones, i.e., to change species composition to reduce wide variations in water needs within plantings, and
  • provide a basis for estimating water needs for new landscapes.

The project was initiated and funded by the Water Use Efficiency Office of the California Department of Water Resources. Work was directed by the University of California Cooperative Extension (San Francisco and San Mateo County office). The first edition of the Guide was completed in 1992. A second edition was published in 1994, and this third edition was completed in 1999. In each edition, additional species evaluations have been included. The third edition was funded by the U.S. Bureau of Reclamation.

Getting Started

If you are using the Guide for the first time, we suggest you begin by reading the following sections on “Categories of Water Needs”, “Standard Conditions”, “Plant Types”, and “Regions”. These sections contain background information which is needed to use the Guide effectively.

If you have used the Guide before, and are familiar with the terms and the evaluation process, proceed directly to “Species Evaluations,” page 62. Be advised, however, that new information has been introduced in WUCOLS III.

The following will help you locate information on important topics.

What does High, Moderate, Low and Very Low mean? See “Categories of Water Needs,” page 52.
What are Standard Conditions? See “Standard Conditions,” page 53.
What is meant by Plant Types ?See “Plant Types,” page 55.
What is meant by Regions?See “Regions,” page 56.
How do I calculate the right amount of irrigation water to apply? See “Part 1” of this guide.
Is there more to know? See “Other Important Information About the Guide,” page 59 and “Appendix B, Invasive Species,” page 143.

Categories of Water Needs

The key question addressed by WUCOLS committee members was the following:

In order to be maintained in good condition, in the region of California being considered, and under the standard conditions outlined, does the species need high, moderate, low, or very low amounts of irrigation water?

This question served as the starting point for the evaluation process. After defining the terms “Regions” and “Standard Conditions” (see following sections), species were evaluated as needing High, Moderate, Low, and Very Low amounts of irrigation water. Expressed as a percentage of reference evapotranspiration (ETo), these categories were quantitatively defined as follows:
High (H) = 70 - 90% ETo
Moderate (M) = 40 - 60% ETo
Low (L) = 10 - 30% ETo
Very Low (VL) = <10% ETo

Water needs categories assigned for each species were determined by consensus of the committee. Assignments were made for each of six regions. When disagreements occurred, the higher water need category was assigned. For example, if some evaluators thought the species needed a “moderate” ranking, while others thought “low” was appropriate, then the “moderate” assignment was used.

Species assigned to the Very Low (VL) category were considered to need little or no irrigation during years of average rainfall.

If the committee did not have experience growing the species in the region, a question mark (?) was assigned. This does not imply that a species should not be tried.

If the species was considered inappropriate for the region, a forward slash ( / ) was assigned.

Using ETo percentages, calculations of irrigation water requirements can be made. For example, a species assigned to the moderate (M) category is evaluated as needing between 40% and 60% of reference evapotranspiration to be maintained in good condition. Say, for the month of July, ETo is 6 inches, then the species needs between 2.4 inches and 3.6 inches of irrigation water for the month. For more information on calculating water requirements for landscapes, see Part 1.

The following examples show how Categories of Water Needs are used.

Evaluations for Acer macrophyllum:

  • Regions 1 and 3.....M (moderate)…....irrigate at 40-60% of ETo
  • Regions 2 and 4.......H (high).…………irrigate at 70-90% of ETo
  • Regions 5 and 6...... / (not appropriate)

Evaluations for Acacia smallii:

  • Regions 1, 2 and 5…. / (not appropriate)
  • Region 3.........…....VL (very low)…. little or no irrigation needed
  • Regions 4 and 6.….L (low).……........irrigate at 10-30% of ETo

Evaluations for Zexmenia hispida:

  • Regions 1, 2, 3, 4, 5 and 6……? committee members did not know species water needs


  1. Reference evapotranspiration (ETo) is defined in “Standard Conditions.”
  2. Cases where there are question marks in several regions usually indicate plants that are new to the nursery trade in California. Consult horticultural literature for more information about species water needs.

It is helpful to look at all the evaluations for each species, (i.e., for all six regions) to get a general assessment of species needs. If there is variation among regions for a species, looking at all evaluations for the species can help you select an irrigation level at the high or low end of the category's range.

Standard Conditions

The following conditions were applied to all species evaluations.

Established Plants

Species irrigation water needs are assessed for plants that have become “established” in the landscape. “Established” meaning that substantial root development has occurred in the landscape soil adjacent to the rootball. The landscape soil becomes the principal source of water for established plants rather than the rootball soil. The time for establishment varies among species and with soil conditions, but generally occurs by the second or third year after planting. After establishment, roots of trees, shrubs, groundcovers, etc., become intertwined in the soil, creating a common rootzone.

Reference Evapotranspiration Conditions (ETo)

ETo is defined as water loss from a large field of 4-to-7-inch-tall, cool-season grass that is not water stressed. Although ETo can be measured directly, it is usually calculated from weather data. Daily ETo information for many regions of the state is avail- able through the California Irrigation Management Information System (CIMIS). Evaluations are made for site conditions equivalent to those used for ETo measurements, i.e., full sun, no extraordinary winds, no shading from nearby structures or plants, and no heat inputs from nearby sources such as buildings, pavements, or reflective surfaces. As an exception, shade-requiring species (e.g., Japanese aucuba) are evaluated for shade conditions. Shade species are considered to be those plants which when exposed to full sun for some part of the day will show vis- ible injury. Since species vary in their shade re- quirements (for example, all day versus afternoon shade), any species requiring some shade to avoid injury (in the region) is evaluated for shade.

See “Appendix D, Additional Resources,” for in- formation on how to obtain CIMIS data.

Good Quality

Plant performance can vary substantially depend- ing on the amount of water supplied. Small amounts may simply prevent the dehydration of plant tissues, but appearance is likely to be affected. Increasing amounts may improve appearance (leaf color, canopy density or fullness), but may not be enough to promote growth. More water may be sufficient to maintain good appearance and support typical (average) growth for the species (and flower or fruit production if desired). Still more water may result in excessive growth; while more water may cause decline (typically from root disease) in certain species. Since both appearance and some growth (not excessive) are important in most landscapes, evaluations were made to provide sufficient water for the species to be maintained as such, i.e., in good condition. This is somewhat difficult to evaluate precisely for some species, however, so whenever a question was raised as to whether a species required a greater or lesser amount of water to maintain good quality, the higher evaluation (more water) was assigned.

Groundwater Not Available

Although some species of plants develop root systems deep enough to extract groundwater (e.g., Quercus lobata), groundwater is not available in all planting sites. A species capable of extracting groundwater may not be able to do so because the water is simply not available. Therefore, evalua- tions are made for conditions where the only sources of water were rainfall and irrigation. In areas where groundwater is available and a species is known to utilize ground water, then adjustments in irrigation scheduling should be made for that species (or group of species).

Plants Must Be Irrigatable

In some cases the soil surface may be sealed around plants (particularly trees) by pavements or other surface barriers. This inhibits the infiltration of water into the rootzone. In other cases the soil volume capable of holding water may be so small and may dry so rapidly that it may be difficult to maintain available water in the rootzone. In either case, the amount of water identified as being needed to maintain good quality may not be sufficient simply be- cause the plant is not “irrigatable.” Evaluations made here assume as a standard condition that the species can be irrigated, i.e., the water applied can enter and be held in the rootzone sufficiently long for uptake.

Plant Types

The species list includes over 1,900 species of land- scape plants which are identified by botanical and common names. The plants are listed alphabetically according to botanical names. An index of com- mon names follows the species list.

Each plant falls into one or more of the following vegetation types: Trees, Shrubs, Groundcovers, Vines, Perennials (includes ferns, grasses, and bulbs) and Biennials. Plant types are entered on the list for each plant under “Type” as:

  • T......... Tree
  • S......... Shrub
  • V......... Vine
  • Gc...... Groundcover
  • P......... Perennial
  • Bi….... Biennial

Cultivars, with some exceptions, are not mentioned. It is presumed that most cultivars will have the same water requirements as the species. Examples of exceptions include the following:

  1. Nandina domestica the cultivar ‘Purpurea’ was included because it was thought to require more water than the species in three regions,
  2. Lonicera japonica ‘Halliana’ was included because the cultivar was thought to be more common than the species,
  3. Illicium floridanum ‘Alba’ was included because it was the only example of the species listed.


Turfgrasses were not evaluated by the committee. For your convenience, several turf species are listed in the “Species Evaluations” section. Water use requirements listed are from University of California Publication 21491, Turfgrass Evapotranspiration Map, Central Coast of California. This publication also contains other important information regarding turfgrass irrigation such as regional ET variability, correcting for rainfall, dew, and fog and calculating sprinkler run times.


Since there are substantially different climate zones[1] in California, species are evaluated for six regions which represent different climatic conditions.

Region 1: North-Central Coastal (California Climate Zones 14, 15, 16, and 17) (CIMIS ETo Zones 1, 2, 3, 4, 6 and 8)[2] Region 2: Central Valley (California Climate Zones 8, 9 and 14), (CIMIS ETo Zones 12, 14, 15, and 16) Region 3: South Coastal (California Climate Zones 22, 23 and 24), (CIMIS ETo Zones 1, 2, 4 and 6) Region 4: South Inland Valleys and Foothills (California Cli- mate Zones 18, 19, 20 and 21), (CIMIS ETo Zone 9) Region 5: High and Intermediate Desert (California Climate Zone 11), (CIMIS ETo Zones 14 and 17) Region 6: Low Desert (California Climate Zone13), (CIMIS ETo Zone 18)

Notes on Regions

Within each region there is some variability in cli- mate patterns among the cities listed. For example, some cities may be considerably warmer than others during the summer months, yet they are within the same region. This variability can only be reduced by increasing the number of regions, which would cause the list to become enlarged and somewhat more complicated.

For certain locations (considered atypical for the region), it may be useful to consider evaluations from another region that more closely characterizes the location of interest. For example, if a city in Region 1 has a climate more closely characterized by Region 2, then Region 2 species evaluations should be considered for that location. Such assessments will need to be based on the judgement of the user.

If a city is not listed and is located in California Climate Zone 14 which overlaps regions 1 and 2, it will be necessary to decide if the city is more similar in climate to Petaluma (coastal influence) or Sacramento Valley.

If a city is located in a California Climate Zone which was not evaluated (zones 1, 2, 3, and 7— mainly high elevation, cold winter areas) an estimate may be made by looking at all the evaluations for the species in question. Hardiness is typically the major factor in determining if a species is appropriate or not.

The main difference between the California high and intermediate desert regions is that the high desert is colder in the winter; as the elevation increases so does the frequency of temperatures below freezing.

As a result, species which are listed as appropriate for the low desert and inappropriate for the high desert may be marginally hardy and appropriate to try in the intermediate desert.

Some Cities that Characterize Each Region Region 1 Region 2 Region 3 Region 4 Region 5 Region 6 North-Central Coastal Central Valley South Coastal South Inland Valley Intermediate & High Desert Low Desert "Concord Cupertino Healdsburg Livermore Los Altos Hills Napa Novato Oakland Petaluma Salinas San Francisco San Jose San Luis Obispo Santa Cruz Santa Rosa " "Auburn Bakersfield Chico Coalinga Fresno Los Banos Marysville Merced Modesto Red Bluff Redding Roseville Sacramento Stockton Tracy Visalia " "Anaheim Camarillo Fallbrook Fullerton Irvine Laguna Beach La Mesa Long Beach Los Angeles Mission Viejo Oxnard Santa Ana Santa Barbara San Diego San Juan Capistrano Santa Monica Ventura Vista Whittier " "Altadena Azuza Chino Corona Covina El Monte Escondido Hemet Ojai Pasadena Perris Pomona Ramona Riverside San Bernardino San Fernando Santa Paula Sun City Thousand Oaks Van Nuys " "Apple Valley Barstow Bishop Boulder City China Lake Gorman Independence Joshua Tree Lancaster Lone Pine Mojave Olancha Palmdale Pear Blossom Tehachapi Victorville " "Borrego Springs Blythe Brawley Coachella Desert Center Desert Hot Springs Death Valley El Centro Indian Wells Indio Jacumba Needles Palm Desert Palm Springs Rancho Mirage Thermal "

Other Important Information About the Guide

Variation in Regional Evaluations

Variation in species evaluations among regions occurs in many cases. Two patterns of variation are found:

  1. where the variation ranges from less water needed in cooler climates to more in warmer ones, and
  2. where less water is required in warmer climates than in cooler ones.

The following examples are typical cases:

Case 1— Laurus nobilis, sweet bay

1 2 3 4 5 6

This is the most common variation. It merely indicates that certain species were thought to require more water in warmer climates.

Case 2—Gleditsia tricanthos, honey locust

1 2 3 4 5 6

A warmer region indicates a lower water requirement than a cooler region. This case reflects differences in observation and experience among regional committees.

Zauchneria spp., California fuchsia

1 2 3 4 5 6
L L VL L / M

This example shows both cases. Sometimes, for certain California natives and other drought tolerant species, there was agreement that the plant would grow with little or no irrigation, but opinions varied as to how well it would perform in a managed land- scape under those conditions.

Drought Stress/Insect Attack Relationships

Although some species perform well with little or no irrigation water, their susceptibility to insect attack and injury may increase with water stress. For example, many Eucalyptus species perform well in non-irrigated conditions in many parts of California. When drought stressed, however, they become susceptible to attack and injury from the Eucalyptus long-horned borer. This is the case as well for Monterey pine (California five-spined engraver beetle) and white alder (Flatheaded borer). For these species, evaluations were made with consideration given to water stress and pest interactions. For ex- ample, although Eucalyptus globulus will perform well in Regions 3 and 4 with little summer water, it was assigned to the “moderate” category to minimize its susceptibility to borer injury.


Most species were evaluated for full sun conditions. Light intensity and duration varies with seasons, microclimates and proximity to the coast. Many species which can be grown in full sun in coastal locations require a measure of shade in inland areas. Others require some shade in all locations. Here, each species was evaluated for the conditions which would produce best appearance and flowering or fruit production for the region. Because of the lack of a standard method for identifying species shade requirements, however, plants needing shade are not noted on the list. Consult horticultural literature for more information on species light requirements.

Winter Irrigation

Although deciduous species are not typically irrigated in the winter months, there may be some need to do so in desert regions. Warm, windy conditions can dehydrate shoots and buds. In addition, some evergreen species may need winter irrigation during drought years or in desert climates.

Summer Deciduous Species

As a drought adaptation, certain species shed their leaves when soil moisture level become low (e.g. California buckeye). Usually, such species do not require irrigation water and are designated Very Low on the list. In cases of low spring rainfall, or when retention of summer leaves is desired, irrigation may be needed.

Special Conditions

Special conditions such as new plantings or a need for rapid growth may require upward adjustments in species water needs.

Revegetation Species

Species selected for revegetation sites should be limited to those which are well adapted to the location and do not require irrigation after establishment. Species used principally for revegetation (i.e., not typically used in irrigated landscape, such as mule fat and poison oak) are not included on the species list.

Invasive Species

Certain species considered invasive both in wildland areas and managed landscapes are available in California nurseries. Their inclusion on this list is not meant to encourage their use, but to alert you that these species can be invasive. For detailed information, see “Invasive Species” (Appendix B).

Using Field Data

Although substantial information exists on the irrigation water needs of agricultural species and turfgrasses, little information is available for woody and herbaceous landscape species. Field studies have quantified the irrigation requirements for six groundcover species (Pittenger, 1990) and three tree species (Hartin, 1991). This information has been used in these evaluations. Considering that over 1,900 tree, shrub, groundcover, vine, and perennial species are available from California nurseries, however, a considerable amount of work still needs to be done before field data alone can be used to deter- mine species water needs.

Limitations of the List

This list is limited in a number of ways:

  1. It is subjective (i.e., it is based largely on field observations rather than scientific data). As such, evaluations are not definitive and may change as more research-based information becomes available.
  2. It is a partial list—not all landscape species are included. It is a large list which includes most plants available from California nurseries, but it does not include all plants. Additions to the list are expected as new species are introduced or less common species are evaluated.
  3. Not all regions of California are included in the evaluations. Extrapolations may be needed from a region evaluated to one that is not.

Species Evaluations

The three plant species listed below are examples of entries on the Species Evaluation List. As a quick reference, a key to symbols is included below. For more information on terms and the evaluation process, see previous sections.

1 2 3 4 5 6
T Ailanthus altissima tree of heaven VL VL L L L L *
S Brugmansia spp. angel’s trumpet M / M H / /
Gc Dodonaea procumbens hopseed L L L  ?  ?  ?
Key to Symbols

H,High M,Moderate L,Low VL,Very Low /,Inappropriate ?,Unknown


1, North Central Coastal 2,Central Valley 3,South Coastal 4,South Inland Valley 5,High and Intermediate Desert 6,Low Desert


T,Tree S,Shrub V,Vine Gc,Groundcover P,Perennial (includes ferns, grasses and bulbs) Bi,Biennial

    • , Greater Statewide Concern
  • ,Lesser Statewide Concern

File:WUCOLS Species Eval List 1999.pdf

File:WUCOLS Common Name Index.pdf

Grass Type Irrigation Requirements
annual bluegrass cool season 80% of Eto
annual ryegrass cool season 80% of Eto
Bermuda grass warm season 60% of Eto
colonial bentgrass cool season 80% of Eto
creeping bentgrass cool season 80% of Eto
hard fescue cool season 80% of Eto
highland bentgrass cool season 80% of Eto
Kentucky bluegrass cool season 80% of Eto
kikuyugrass warm season 60% of Eto
meadow fescue cool season 80% of Eto
perennial ryegrass cool season 80% of Eto
red fescue cool season 80% of Eto
rough-stalked bluegrass cool season 80% of Eto
seashore paspalum warm season 60% of Eto
St. Augstine grass warm season 60% of Eto
tall fescue cool season 80% of Eto
zoysizgrass warm season 60% of Eto
From: University of California ANR publication 24191, Turfgrass Evapotranspiration Map, Central Coast of California.

Appendix A— Reference Evapotranspiration Values for Selected Locations in California

Table 1 gives monthly average values for reference evapotranspiration (ETo) in selected California locations. All values are reported in inches per day.

To calculate inches per month, select a location in the column on the left, then select a month and read the value corresponding to the location. Multiply the column value times the number of days in the month. For example, reference evapotranspiration in Sacramento for the month of August is 7.75 inches (.25 x 31 = 7.75).

The numbers in Appendix A are normal year (historical) averages, derived from several years of data for the month and location. Adjustments to normal year values may be needed to account for:

  1. Variation in actual ETo totals for a month. From year to year the actual amount of evaporation may be substantially different than the historical average. For example, the historical average ETo for August in Sacramento is 7.75 inches. If the summer was particularly cool, however, the actual value may be 25% less than average, or about 5.8 inches. Conversely, the actual amount may be substantially greater during a very hot summer. Adjustments to reflect actual ETo conditions will be appropriate in some years.
  2. Variation in location. Adjustments in ETo may be needed for the location of the landscape planting. The climatic conditions at the ETo measuring site may be substantially different than those at the landscape site. For example, San Francisco does not have a CIMIS station. CIMIS stations closest to San Francisco are in Marin County and San Mateo County. To use data from either Marin or San Mateo for San Francisco, a downward adjustment in ETo would be needed since both locations are considerably warmer than San Francisco. It is important to know where ETo measurements are being taken and then decide whether meaningful differences exist between your location and the measurement location. The assistance of a qualified biometeoroligist is recommended if adjustments for location are needed.

Appendix A—Table 1

Reference Evapotranspiration Rates for Selected Cities*

Daily Average Reference Evapotranspiration by ETo Zone (inches per day)
ETo Zone City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1 Santa Monica 0.03 0.05 0.08 0.11 0.13 0.15 0.15 0.13 0.11 0.08 0.04 0.02
2 Santa Cruz 0.04 0.06 0.1 0.13 0.15 0.17 0.16 0.15 0.13 0.09 0.06 0.04
3 Monterey/Salinas 0.06 0.08 0.12 0.16 0.17 0.19 0.18 0.17 0.14 0.11 0.08 0.06
4 San Diego 0.06 0.08 0.11 0.15 0.17 0.19 0.19 0.18 0.15 0.11 0.08 0.06
5 Santa Rosa 0.03 0.06 0.09 0.14 0.18 0.21 0.21 0.19 0.15 0.1 0.05 0.03
6 Los Angeles 0.06 0.08 0.11 0.16 0.18 0.21 0.21 0.2 0.16 0.12 0.08 0.06
7 Alturas 0.02 0.05 0.08 0.13 0.17 0.21 0.24 0.21 0.16 0.09 0.04 0.02
8 San Jose 0.04 0.06 0.11 0.16 0.2 0.23 0.24 0.21 0.17 0.11 0.06 0.03
9 San Bernardino


0.07 0.1 0.13 0.17 0.19 0.22 0.24 0.22 0.19 0.13 0.09 0.06
10 Paicines 0.03 0.06 0.1 0.15 0.19 0.24 0.26 0.23 0.17 0.1 0.05 0.03
11 Sonora 0.05 0.08 0.1 0.15 0.19 0.24 0.26 0.24 0.19 0.12 0.07 0.05
12 Fresno 0.04 0.07 0.11 0.17 0.22 0.26 0.26 0.23 0.18 0.12 0.06 0.03
13 Quincy 0.04 0.07 0.1 0.16 0.21 0.26 0.29 0.25 0.19 0.12 0.06 0.03
14 Sacramento 0.05 0.08 0.12 0.17 0.22 0.26 0.28 0.25 0.19 0.13 0.07 0.05
15 Bakersfield 0.04 0.08 0.12 0.19 0.24 0.27 0.28 0.25 0.19 0.13 0.07 0.04
16 Hanford 0.05 0.09 0.13 0.19 0.25 0.29 0.3 0.27 0.21 0.14 0.08 0.05
17 Needles 0.06 0.1 0.15 0.2 0.26 0.3 0.32 0.28 0.22 0.14 0.09 0.06
18 Palm Springs 0.08 0.12 0.17 0.23 0.28 0.32 0.31 0.28 0.23 0.16 0.1 0.07
* For comprehensive descriptions of each zone and to locate your region in a zone, see the California Irrigation Management Information System (CIMIS) color map opposite this page.

California Irrigation Management Information System (CIMIS) Reference Evapotranspiration Zones caption

Appendix A—Table 2

Calculations of Species Water Needs for July for Several Locations in California

Listed are normal year ETo values1 for July and three categories of water needs. Select the appropriate location and water need category. Look down the column to find the estimated water need. This was calculated by multiplying ETo x a water need category (low, medium or high). For example, for Los Angeles in July, the normal year ETo = 6.5 inches. For a planting in the medium category, (0.4 - 0.6) the estimated water need ranges from 2.6 to 3.9 inches.

Estimated species water needs (inches per month)2 for JULY
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
4 Novato 5.8 0.6 1.1 1.7 2.3 2.9 3.4 4 4.6 5.2
1, 2 San Francisco 4.6-4.9 0.5 1 1.4 1.9 2.4 2.9 3.4 3.9 4.4
8 Concord 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
8 San Jose 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
3 Monterey 5.5 0.5 1 1.6 2.2 2.7 3.3 3.8 4.4 4.9
6 San Luis Obispo 6.5 0.6 1.3 1.9 2.6 3.2 3.9 4.5 5.2 5.8
14 Auburn 8.6 0.9 1.7 2.5 3.4 4.3 5.1 6 6.8 7.7
14 Sacramento 8.6 0.9 1.7 2.5 3.4 4.3 5.1 6 6.8 7.7
12 Modesto/Stockton 8 0.8 1.6 2.4 3.2 4 4.8 5.6 6.4 7.4
12 Fresno 8 0.8 1.6 2.4 3.2 4 4.8 5.6 6.4 7.4
15 Bakersfield 8.6 0.9 1.7 2.5 3.4 4.3 5.1 6 6.8 7.7
14 Redding 8.6 0.9 1.7 2.5 3.4 4.3 5.1 6 6.8 7.7
4 Santa Barbara 5.8 0.6 1.1 1.7 2.3 2.9 3.4 4 4.6 5.2
4 Ventura 5.8 0.6 1.1 1.7 2.3 2.9 3.4 4 4.6 5.2
6 Los Angeles 6.5 0.6 1.3 1.9 2.6 3.2 3.9 4.5 5.2 5.8
1, 2 Laguna Beach 4.7-4.9 0.5 1 1.4 1.9 2.4 2.9 3.4 3.9 4.4
4 San Diego 5.8 0.6 1.1 1.7 2.3 2.9 3.4 4 4.6 5.2
9 San Fernando 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
9 Pasadena 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
9 Riverside 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
9 Ramona 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
9 San Bernardino 7.4 0.7 1.4 2.2 2.9 3.7 4.4 5.1 5.9 6.6
17 Palmdale 9.9 1 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9
17 Lancaster 9.9 1 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9
17 Victorville 9.9 1 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9
17 Bishop 9.9 1 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9
17 Independence 9.9 1 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9
18 Palm Springs 9.6 1 1.9 2.8 3.8 4.8 5.7 6.7 7.6 8.6
18 Coachella 9.6 1 1.9 2.8 3.8 4.8 5.7 6.7 7.6 8.6
18 Needles 9.6 1 1.9 2.8 3.8 4.8 5.7 6.7 7.6 8.6
18 El Centro 9.6 1 1.9 2.8 3.8 4.8 5.7 6.7 7.6 8.6
1. Normal year values and zones are derived from the California Irrigation Management Information System (CIMIS) Reference Evapotranspiration Map, 1999. 2. Please note; these values are not adjusted for irrigation efficiency.

Appendix B— Invasive Species

Certain species, if grown adjacent to wildland areas, have the ability to “invade” native habitats to the detriment of the native species. Others cause problems in managed landscapes. Species of both types are listed here. It is incumbent on landscape architects, designers, and managers to learn which plants are considered to be invasive, and use appropriate caution in their use.

Invasive species are indicated on the list by , or .

Examples:  Arundo donax Considered an important wildland weed (can displace native species in natural communities in one or more regions).

Acacia decurrens Considered a wildland weed of secondary importance, or is potentially invasive, or is a species which is limited to one region, landscaped areas or roadsides.

 Genista spp. NOT ALL Genista species are considered invasive. Refer to “Notes on Invasive Species” for information about Genista monspessulanus French broom.

Notes on Invasive Species

Acacia baileyana—mainly near habitations
Acacia dealbata—Northern coastal to southern inland regions
Acacia decurrens—Northern coastal
Acacia longifolia—Minor threat along coast
Acacia melanoxylon—Northern coastal and inland to southern coastal
Achillea millefolium—Coastal and inland areas in moist places
Ailanthus altissima—Urban and natural areas around the world
Albezia distachya—Coastal areas
Aptenia cordifolia ‘Red Apple’—Coastal zones, mainly southern
Arctotheca calendula—Northern and southern coastal bluffs, foothills
Arundo donax—All regions in moist areas, seasonal water courses
Atriplex glauca—Southern coastal foothills
Altriplex semibaccata - Coastal to inland areas
Briza media - Grasslands
Carpobrotus edulis —Coastal and inland regional throughout California
Carpobrotus chilensis — Coastal and inland regional throughout California
Centranthus ruber—Coastal, inland and foothill regions throughout California
Cistus ladanifer—coastal sage scrub and chaparral
Coprosma repens—Only coastal
Cordyline australis—Only coastal
Cortaderia sellowana—Coastal regions, dunes, scrub and Monterey pine forest
Cotoneaster pannosus—Disturbed sites, many communities, central and northern coast
Crataegus monogyna—Central and northern coast
Cupressus macrocarpa—Northern coastal
Cytisus canariensis—Foothill regions, northern California and Central Valley
Cytisus racemosus—Foothill regions, northern California and Central Valley
Cytisus scoparius—Coastal scrub, oak woodland
Cytisus striatus—Coastal scrub, oak woodland
Delosperma spp. —Potential threat on coast
Duchesnia indica—Potential threat on coast
Echium candicans (fastuosum)—Coastal
Elaeagnus angustifolia—interior riparian areas
Erica lusitanica—possible threat to wildlands
Eucalyptus camaldulensis—Southern coastal canyons and foothills
Eucalyptus globulus—Coastal canyons and foothills, riparian areas
Eucalyptus pulverulenta—Southern coastal
Ficus carica—Central Valley, south coastal and Channel Islands riparian woodlands
Genista monspessulanus—Coastal scrub, oak woodland
Hedera canariensis—Coastal and inland regions in moist and shady places
Hedera helix—Coastal and inland regions in moist and shady places
Helichrysum petiolare—north coastal scrub
Ilex aquifolium—Coastal forests
Imperata cylindrica, I brasiliensis—on federal noxious weed list
Juncus spp.—potential to naturalize moist areas
Ligustrum lucidum—Mendocino coast
Limonium perezii—Southern coastal beaches and bluffs
Lonicera japonica ‘Halliana’ —Coastal and inland regions; moist, shady places
Lotus corniculatus—Roadside weed
Lupinus arboreus—North coast dunes
Lysimachia nummularia—widely naturalized in other states, not in CA to date
Malephora crocea—south coast bluffs, margins of wetlands
Melaleuca viridifolia (quinqueneveria)—severe problem in Florida wetlands, not in CA to date
Mentha pulegium—invades Santa Rosa Plain (Sonoma County)
Myoporum laetum—Northern and southern coastal foothills
Myosotis spp.—Coastal forests
Nereum oleander—Riparian areas
Oenanthe javanica—potential to naturalize in damp habitats
Olea europaea—Southern coastal and inland foothills
Pennisetum setaceum—All dry climate regions, grasslands, desert canyons
Phalaris aquatica—coastal sites with moist soil
Phyla nodiflora—Wet places, vernal pools
Pinus pinaster—Sparingly naturalized central coast Pinus pinea—Sparingly naturalized central coast
Pinus radiata—Central and northern coastal
Pyracantha spp.—Central coastal
Robinia pseudoacacia—Northern valleys and foothills to southern mountains and foothills
Sapium sebiferum—severe problem in Gulf coast wetlands, bottomland forests, beginning to appear in CA in wetlands in Yolo county and along the American River near Sacramento
Schinus mole—Coastal canyons and foothills statewide
Schinus terebinthifolius—Coastal lowlands, wet places
Spartium junceum—Coastal scrub, oak woodlands
Tamarix chinensis, T gallica, T parviflora, T ramosissima (pendantra)—Coastal through desert riparian areas
Tropaeolum majus —Moist coastal regions
Vinca major—Riparian areas, oak woodland, mostly coastal
Watsonia bulbillifera—North coast
Watsonia marginata—North coast
Zantedeschia aethiopica—Coastal streams

Appendix C— Glossary

Acre-foot: The amount of water which covers an acre (43,560 ft.2) to the depth of one foot (12 inches). One acre-foot equals 325,850 gallons.
CIMIS: California Irrigation Management Information System. A network of weather stations located around the state which collects reference evapotranspiration data. The network is managed by the California Department of Water Resources.
Conversion Factor: (0.62 gallons/ft. 2-inch) Used to convert water volume from inches per unit area to gallons per unit area. There are 0.62 gallons in a square foot-inch.
Crop Coefficient (Kc): Fraction of water lost from the crop relative to reference evapotranspiration.
Crop Evapotranspiration (ETo): Water loss from a crop.
Vegetation Density: An evaluation of vegetation surface area per unit volume taking into consideration factors such as tree canopy cover and tiers of vegetation.
Density Factor (kd): One of three factors used to generate a landscape coefficient. Adjusts the landscape coefficient to ac- count for the effect of vegetation density on water loss from a hydrozone.
ET: Evapotranspiration. The sum of water losses through evaporation (E) from the soil and transpiration (T) from the plant.
ETo: Reference Evapotranspiration. The approximation of water loss from a field of 4-to-7-inch-tall cool-season grass that is not water stressed. ETo is measured at CIMIS weather stations in various locations around the state.
ETL: Estimated water needs of the landscape. Calculated by multiplying the landscape coefficient (KL) by Reference Evapotranspiration (ETo).
Hydrozone: A portion of a landscaped area having plants with similar water needs that are served by one irrigation valve or set of valves with the same schedule.
Irrigation Efficiency: A measure of the portion of the total applied irrigation water beneficially used (primarily to satisfy plant water needs). Losses (non-beneficial water use) include unused runoff and evaporation from wet soil surfaces.
Landscape Coefficient (KL): The functional equivalent of the crop coefficient. Used for estimating water needs from landscape plantings. Landscape coefficient = species factor x microclimate factor x density factor.
Microclimates: Areas having different environmental conditions within a climatic zone.
Microclimate Factor (kmc): One of three factors used to generate a landscape coefficient. Adjusts the landscape coefficient to account for the effect of microclimate on water loss from a hydrozone.
Species Factor (ks): One of three factors used to generate a landscape coefficient. Adjusts the landscape coefficient to account for water loss from a hydrozone due to the plant species composition.
Square foot-inch: The amount of water which covers one square foot of area to the depth of one inch. One square foot-inch equals 0.62 gallons.
TWA: Total water applied. An estimate of the total amount of water to apply to a landscape planting. Calculated by dividing ETL (estimated water needs of the planting) by IE (irrigation efficiency).
WUCOLS: Water Use Classification of Landscape Species. A Guide to the Water Needs of Landscape Plants.

Appendix D— Additional Resources


Cal Poly Irrigation Training and Research Center, Landscape Water Manager, (irrigation management software) California Polytechnic State University, San Luis Obispo, CA.

Clebsch, B., 1997, A Book of Salvias, Sages for Every Garden, Timber Press, Portland, OR.

Coate, B., 1990, Water-Conserving Plants and Landscapes for the Bay Area, East Bay Municipal Utility District, Alamo, CA.

Cornell University Bailey Hortorium Staff, 1976, Hortus Third, MacMillan Publishing Co., Inc.

Costello, L.R., D. Thomas, and J. DeVries, 1996, “Plant water loss in a shaded environment: a pilot study.” J. of Arboriculture 22(2):106-108.

Evans, M. and J. Bohn, Tree of Life Wholesale Nursery Catalog 1998, San Juan Capistrano.

Feldman F., and Fogle C. E., 1989, Sunset Waterwise Gardening, Lane Publishing Company, Menlo Park, CA.

Gibeault, V. A., J. L. Meyer, R. Autio, R. Strohman, 1986, “Turfgrass Alternatives With Low Water Needs.” California Agriculture, 40 (7, 8):19-20.

Greenlee, J., 1992, The Encyclopedia of Ornamen- tal Grasses, Rodale Press, Emmaus, PA.

Griffiths, M., 1994, Index of Garden Plants, Royal Horticultural Society, Timber Press, Portland, OR.

Harris, R.W., J. R. Clark, and N. P. Matheny, 1999, Arboriculture: The Integrated Management of Landscape Trees, Shrubs and Vines, 3rd Edition. Prentice Hall, Englewood Cliffs, NJ.

Hartin, J., Meyer, J., 1991 Research conducted at U.C. South Coast Field Station on four landscape tree species. U.C. Cooperative Extension, San Bernardino County. (personal communication).

Hartin, J., Pittenger, D., 1988, Suggested Landscape Trees For the San Bernardino Valley, University of California Cooperative Extension San Bernardino and Riverside Counties.

Johnson, E. and Scott M., 1993, How to Grow the Wildflowers, Ironwood Press, Tucson.

Johnson, E. and Scott M., 1993, The Low Water Flower Gardener, Ironwood Press, Tucson.

Keator, G., 1990, Complete Garden Guide to the Native Perennials of California, Chronicle Books, San Francisco.

Keator, G., 1994, Complete Garden Guide to the Native Shrubs of California, Chronicle Books, San Francisco.

Levitt, D. G., J. R. Simpson, J. L. Tipton, 1995, Water Use of Two Landscape Trees in Tucson Arizona, Journal American Society of Horticultural Science, 120(3):409-416.

Lindsey, P. and N. Bassuk, 1991,“Specifying soil volumes to meet the water needs of mature ur- ban street trees in containers.” J. Arboric. 17 (6):141-149.

Macoboy, S., 1988, What Flower is That? Portland House, New York, NY.

MacNair, J., Estimating Water Use and Irrigation Schedules for Ornamental Landscape, presented at the 1992 Northern California Zeriscape Conference.

Metcalf, L. J., 1987, The Cultivation of New Zealand Trees and Shrubs, Reed Methuen Publishers Ltd., Aukland.

Meyer, J. L. and V. A. Gibeault, 1986, “Turfgrass Performance Under Reduced Irrigation.” California Agriculture, 40 (7, 8):19-20.

Ottesen, C., 1989, Ornamental Grasses, the Am- ber Wave, McGraw Hill, NY.

Perry, R., 1992, Trees and Shrubs for Dry Califor- nia Landscapes, Land Design Publishing, Claremont CA.

Pittenger, D.R, D.R. Hodel and D. A. Shaw, 1990, “Relative water requirements of six groundcover species.” HortScience, 25 (9): 1985. (Abstr.) .

Sachs, R. M., 1991, “Stress-adapted Landscapes Save Water, Escape Drought Injury,” California Agriculture, 45(6):19-21.

Saratoga Horticultural Foundation, 1983, Success List of Water Conserving Plants, San Martin, CA.

Schwankl, L., Hanson, B., Prichard T., 1993, Low- Volume Irrigation: a Handbook for Water Managers, University of California Irrigation Program, University of California, Davis.

Shuler, C., 1993, Low Water Use Plants for California and the Southwest, Fisher Books, Tuc- son.

Smith, M.N., 1997, A Guide to Ornamental Plants for Coastal California with Cultural Notes, Suncrest Nurseries Inc., Watsonville, CA.

Staats, D. and J.E. Klett, 1993, “Evaluation of water conservation potential of non-turf groundcovers versus Kentucky bluegrass.” Colorado State University, Department of Horticulture. Fort Collins, CO. (unpublished).

Sunset Book and Magazine Editors, 1998, Sunset Western Garden Book, Menlo Park, CA.

Turner, R.J., Wasson, E., Ed., 1997, Botanica, My- nah, New York.

Woods, C., 1992, Encyclopedia of Perennials, a Gardener’s Guide, Facts on File, New York.

Vermeulen, N., 1998, Cacti, Rebo Productions, Lisse, The Netherlands.

University of California Publications

2975 Beutel, J., 1977, Saving Water in Home Orchards.

3328 Generalized Plant Climate Map of California, 1988.

2149 Gibeault, V., Meyer, J., Harivandi, A., Henry, M., Cockerham, S., 1991, Managing Turfgrass During Drought.

21333 Furuta, T., 1993, Protea Culture.

21405 Gibeault, V., 1985, Turfgrass Water Conservation.

2976 Harris, R., Coppock, R., 1976, Saving Water in Landscape Irrigation.

4091 McClintock, E., and Leiser, A., 1979, An Annotated Checklist of Woody Ornamental Plants of California, Oregon and Washing- ton.

3276 McClintock, E., Mathias M., and Lewis, L., Ed., 1982, An Annotated Checklist of Ornamental Plants of Coastal Southern California.

21432 Snyder, R., Harivandi, A., 1988, Lawn Wa- tering Requirements Along California’s Central Coast.

21426 Snyder, R., Pruitt, W., and Shaw, D., 1987, Determining Daily Reference Evapotranspiration (ETo).

21491 Snyder, R., Harivandi, A., Lanini, B., 1991, Turfgrass Evaporation Map Central Coast of California.

UC Press

Hickman, James, Ed., 1993, The Jepson Manual, Higher Plants of California, University of Cali- fornia Press, Berkeley.

Mathias, Mildred, Ed., 1982, Flowering Plants in the Landscape, University of California Press, Berkeley.

Other Resources

California Department of Water Resources Office of Water Use Efficiency
901 P Street, P. O.Box 942836, Sacramento, California 94236-0001
(916) 651-9676

California Irrigation Management Information System (CIMIS)
California Department of Water Resources Office of Water Use Efficiency
P. O. Box 942836, Sacramento, California 94236-0001
(916) 651-7030

California Department of Water Resources Information: (800) 272-8869

Species list on the Internet: www.dpla.water.ca.gov/urban/conservation/ landscape/wucols/wucols.html

Integrated Pest Management, www.ipm.ucdavis.edu

University of California Cooperative Extension San Mateo and San Francisco Counties, 625 Miramontes Street, Suite 200 Half Moon Bay, California 94019 (650) 726-9059

UC Cooperative Extension—County Offices (check local phone directory)


Additions to the WUCOLS list can be made. Submit species names to: Irrigation Water Needs Project UCCE
625 Miramontes, Suite 200, Half Moon Bay, California 94019

Submitted names will be sent out for evaluation by committee members and additions will be made periodically.

Copies of this Guide

This Guide is a free publication. Additional copies may be obtained from: Department of Water Resources Bulletins and Reports
P. O. Box 942836, Sacramento, California 94236-0001
(916) 653-1097


  1. California climate zones are described in University of California Publication 3328, Generalized Plant Climate Zones of California and Sunset Western Garden Book.
  2. ETo Zones are described in the California Irrigation Management Information System (CIMIS) Reference Evapotranspiration Map, 1999 (see map on page 141).

Facts about "WUCOLS"
Has end use subjectPlants +
Has end user subjectLandscape professionals + and Resource conservation professionals +
Has general subjectLandscape +
Has introductionPreface This Guide consists of two

This Guide consists of two parts, each formerly a separate publication:

Part 1—Estimating the Irrigation Water Needs of Landscape Plantings in California: The Landscape Coefficient Method

  • L.R. Costello, University of California Cooperative Extension
  • N.P. Matheny, HortScience, Inc., Pleasanton, CA
  • J.R. Clark, HortScience Inc., Pleasanton, CA

Part 2—WUCOLS III (Water Use Classification of Landscape Species)

  • L.R. Costello, University of California Cooperative Extension
  • K.S. Jones, University of California Cooperative Extension

Part 1 describes a method for calculating landscape water needs, while Part 2 gives evaluations of water needs for individual species. Used together, they provide the information needed to estimate irrigation water needs of landscape plantings.

Part 1 is a revision of Estimating Water Requirements of Landscape Plants: The Landscape Coefficient Method, 1991 (University of California ANR Leaflet No. 21493). Information presented in the original publication has been updated and expanded.

Part 2 represents the work of many individuals and was initiated and supported by the California Department of Water Resources. This third revision (WUCOLS III) includes many species not previously evaluated, as well as an update and reorganization of support information.

These two publications are companion documents and are intended to be used together.

First-time readers are encouraged to carefully review both parts of this Guide before making estimates of landscape water needs.
making estimates of landscape water needs. +
Has primary imagewucols00_Cvr.png +
Has source fileWUCOL-Estimating-Landscape-Water-Needs-worksheet.png +, WUCOL-Example-Landscape-Water-Needs-worksheet.png +, WUCOLS Species Eval List 1999.pdf +, WUCOLS Common Name Index.pdf + and WUCOLS-CIMIS-Evapo-Zones.png +
Is publication typeGuidebooks and manuals +