Water Use Classification of Landscape Species (WUCOLS IV)
Water Use Classification of Landscape Species (WUCOLS IV)
Water Use Classification of Landscape Species (WUCOLS IV)
University of California
Water Use Classification of Landscape Species (WUCOLS IV)

Using WUCOLS Evaluations

WUCOLS Evaluations and Plant Cultural Requirements

Simply because a species is found on the WUCOLS list does not mean that it is recommended for planting. The user has the responsibility to understand the suitability of a species for planting in a particular location. In some cases, a species may be considered to be invasive in the region of interest and should not be planted (see the section “Invasive Species,” below). In other cases, the species has cultural requirements that make it difficult to cultivate in the area, or it may be susceptible to pests that would make it a poor choice for the area. Regardless of the limitation, it is incumbent on the user to be aware of key limitations on the use of species in their region.

Variation in Regional Evaluations

In reviewing the evaluations across regions, some apparent inconsistencies may be seen. For instance, variegated Chinese lantern (Abutilon pictum) is in the category High in the Central Valley, but it is in Moderate in the South Inland region. Why was more water thought to be needed for this species in Sacramento than Riverside? Unlike the South Inland region, the Central Valley experiences summer winds that cause this species to perform poorly if not irrigated at a relatively high level. In another case, Engelmann oak (Quercus engelmannii) is in the category Very Low in the South Coastal region and Low in the North-Central Coastal region. Why might this species need more water in a relatively cooler area? This evaluation was based on the North-Central Coastal regional committee’s collective experience that Engelmann oak needs more water when grown outside of its native range. Although evaluations for many species are similar across regions, it is important to recognize that apparent inconsistencies are often a consequence of differences found in species performance from region to region.

Invasive Species

Clearly, species that are considered to be invasive in a region should not be used. Users are referred to authoritative lists of invasive species, such as can be found in the California Invasive Plant Council (CA IPC) website, http://www.cal-ipc.org. In this update of the WUCOLS list, there is no determination as to whether a species is invasive. Largely, this is because species that are invasive in one region may not be invasive in another region. It is important to designate plants as invasive on a local or regional basis rather than on a statewide basis. If a species has been identified by a credible source as being invasive for a location, it should not be used.

Water Stress and Insect Injury

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 nonirrigated locations in many parts of California. When water stressed, however, they become susceptible to attack and injury by the eucalyptus longhorned borer (Phorocantha semipunctata). This is also the case for Monterey pine (Pinus radiata) and the California fivespined engraver beetle (Ips paraconfusus). For these species, evaluations were made with consideration given to water stress and pest interactions. For example, although Tasmanian blue gum (Eucalyptus globulus) performs well in Regions 3 and 4 with little summer water, it was assigned the category Moderate to minimize susceptibility to borer injury.

Using Species Not Found on the List

Although WUCOLS IV includes the great majority of species available in the wholesale nursery trade in California, it does not include all such plants. If a species is not listed here, it does not mean it cannot be submitted to an agency or organization for compliance with efficient water use in landscaping. The person who submits the species to the agency or organization must provide justification for a water-needs category for the species. This justification could come from a published source or a recognized authority, such as a person who specializes in the cultivation of the species. Alternatively, the species can be submitted for review by WUCOLS regional committees via the WUCOLS website (http://ucanr.org/sites/wucols/).


As noted in the section “Standard Conditions,” most species were evaluated for full sun conditions. Light intensity and duration vary with seasons, microclimates, and proximity to the coast. Many species that 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 that would produce the best appearance (including flowering and/or fruiting) for the region. Consult horticultural literature (see the section “Resources” at the end of this publication) for more information on species light requirements.

Winter Irrigation

Although many species are typically not irrigated during the winter, it may be necessary in the following cases:

  • Sensitive deciduous species will benefit from irrigation in desert regions if warm winter winds dehydrate shoots and buds.
  • During drought years or in desert climates, some evergreen species may need winter irrigation.
  • Certain species planted outside of their natural range, such as coast redwood (Sequoia sempervirens), may need winter irrigation.

Summer-Deciduous Species

Fig. 9. Summer-deciduous species, such as California buckeye, shed leaves when soil moisture levels become low.
Fig. 9. Summer-deciduous species, such as California buckeye, shed leaves when soil moisture levels become low.
As a drought adaptation, certain species shed their leaves when soil moisture levels become low, such as California buckeye (Aesculus californica) (Fig. 9). Usually, such species do not require irrigation and are assigned to the category Very Low. In cases of below average rainfall for the area or where retention of summer leaves is desired, irrigation may be needed.


Fig. 10. Summer-dormant bulbs, such as naked lady, do not need to be irrigated in the summer.
Fig. 10. Summer-dormant bulbs, such as naked lady, do not need to be irrigated in the summer.
Water requirements for bulbs vary not only with species but also with dormancy period. Certain species are winter dormant, while others are summer dormant. Species that are summer dormant, such as naked lady (Amaryllis belladonna), should not be irrigated in the summer. Indeed, irrigation of this and similar species during the summer can lead to infection and decay (Fig. 10). Accordingly, summer-dormant bulbs are generally considered to have very low or low water needs. Conversely, winter-dormant bulbs generally need water in the summer while growing. Keep in mind that even summer-dormant bulbs may need some water in the winter in relatively dry regions, such as the desert. Also, some summer-dormant species, such as giant scilla (Scilla peruviana), can remain actively growing in the summer with irrigation.

Container Plants

WUCOLS evaluations do not apply to container plants (boxes, planters, pots, etc.). Standard conditions used for evaluations specifically exclude container plants and apply only to plants in the ground.

Revegetation Species

Some species on the WUCOLS list, such as California aster(Lessingia filaginifolia), are more commonly used in revegetation projects than in landscapes. For these species, water needs assignments are made for their use in landscapes where irrigation is supplied not simply for survival but also for attractive appearance and good performance.

Turf Substitutes

In Regions 1 through 4, common yarrow (Achillea millefolium) and dymondia (Dymondia margaretae) are evaluated as having low water needs. If these species are used as turf substitutes, however, their water needs may increase to moderate to maintain good performance. Generally, this applies to other species used as turf substitutes.





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