Diane Cary / UC Davis Arboretum

Many currently popular landscape plants require frequent watering in order to perform well in interior California’s summer drought, high temperatures, and hot winds. Horticulturists at UC Davis are studying plant species that have demonstrated tolerance to summer heat and drought. A new project is examining plant species from the UC Davis Arboretum under different irrigation frequencies. Testing is taking place at a campus field site where irrigation can be controlled to reflect varying intensities from practically no water to abundant water.  A second layer of research is being done through Master Gardener volunteers.  Demonstration sites throughout the state are testing these plants, documenting their appearance and performance in the difference climate zones.

Ten species were tested in 2007, and an additional 10 species have been chosen for testing in 2008. The field testing involves 24 of each plant species – six plants on each of four different water treatments. Irrigation frequencies will vary at four different levels ranging between 20 percent and 80 percent of normal plant evapotranspiration. The plants will receive the same amount of water during irrigation, but the number of irrigations will vary. A field weather station will collect and log weather data and open valves at programmed percentages. Height and width measurements will be taken monthly to determine a growth index for each species at each water level. General appearance, flowering time and duration, and any pest problems (or plant death) will be noted to determine whether a plant should be advance to the next phase of testing.

Plants selected from the field trials in Davis will be propagated, grown in containers and distributed to established Master Gardener demonstration gardens in Redding, Livermore, Palo Alto, Grass Valley, Mariposa, Fresno, Point Loma, El Cajon, and Fallbrook. A project coordinator in Davis supplies evaluation sheets and provides training to Master Gardeners carrying out evaluations. Evaluations are conducted seasonally and compiled for analysis and publication. Master Gardeners report on plant features such as general appearance, flowering, and other ornamental features, as well as durability, size, form, and leaf qualities.

Population increases and growth in California will strain the quality of life for communities with limited water supplies that continue to landscape with plants requiring frequent irrigation. A greater availability of plants tested for drought and heat tolerance, coupled with information about their cultural needs, will encourage expanded use of these plants in low water-use landscapes and help conserve this precious resource.

Evaluations by dedicated Master Gardeners will provide information on the attractiveness and usefulness of these plants from the perspective of the gardener. This information will be made available to other gardeners, landscapers and nursery operators through Cooperative Extension and as part of the educational outreach programs of the UC Davis Arboretum and the California Center for Urban Horticulture.