How can gardeners be certain that plants labeled as drought-tolerant are truly drought-tolerant? Many nurseries and garden centers have a section set aside for drought-tolerant plants species and there are many online sources of information on planting drought-tolerant gardens that include plant lists, but are all those plants equally able to survive and thrive on less water than other more thirsty species? And how much water does each species or the many cultivars of drought-tolerant plants need to remain healthy and disease- and pest-free in their climate zones? For many gardeners planting a drought-tolerant garden is a bit chancy.  

UC Master Gardeners now have an excellent source of research-based, very specific and helpful evaluations of newly and recently introduced drought-tolerant plants suitable for the Fresno/Madera climate zone with the revamped and expanded website of the UC Landscape Plant Irrigation Trials.

The website includes a plant index with irrigation recommendations and aesthetic ratings for all the plants evaluated at the UC Davis field site since 2005 and at the South Coast Research and Extension Center in Irvine since 2018. Both sites evaluate landscape plants, grasses, small trees and bushes considered suitable for growing in the WUCOLS* Central Valley region (Sunset climate zones 8, 9 and14 and CIMIS* ETo* zones 12, 14,15 and 16) and the Southern Coastal region (Sunset climate zones 22, 23 and 24 and CIMIS ETo zones 1, 2, 4 and 6). 

For those who are not familiar with UCLPIT, it is a research program begun at UC Davis in 2004 by graduate student Karrie Reed and her advisor Loren Oki, both of whom are still very much involved with the project. The original field trial site at UC Davis was expanded with the second site in Irvine.

Members of the plant/nursery industry, including growers and hybridizers, submit plants for evaluation. The plants to be evaluated were all planted two years previously so that root systems have become established. Evaluations are performed in spring, summer and fall in the field by volunteer UC Master Gardeners, horticulturists, landscape experts, educators, garden writers and others who are invited to participate. The field trails are not open to the general public. Three samples of each plant type receive either high, moderate or low amounts of water during the field trials. The field at UC Davis is in full sun. There is also a 50% shade structure where shade plants are grown. 

The goal of the project is to match the water supply to the plants needs. Measurements of the plants’ evapotranspiration or ETo is weather-based using CIMIS weather stations at UC Davis and the South Coast REC - much the same process as smart weather-based irrigation systems. In this research project the amount of water given to each plant does not change; it remains a fixed amount. The frequency of irrigation changes. The goal is to determine the lowest amount of irrigation that will keep each plant species healthy and vigorous in drought-conditions.

The spring field trials for this year will be held on June 11 at UC Davis and on June 17 in Irvine.  Master Gardeners interested in participating in the field trial evaluations can register online at the website listed above. Check under the heading “Open Houses."

*WUCOL (Water Use Classification of Landscape Species)

*ETo is the evapotranspiration of a reference plant

*CIMIS (California Irrigation Management Station)  

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Written by UC Master Gardener Elinor Teague.