Irrigation and climate zone trials of perennial plants for sustainable landscapes

Unstable water supplies in the Western U.S. have become more uncertain due to increasingly drier winters and rising urban populations, and chemical laden runoff from over-watered urban landscapes threatens the health of waters that receive the runoff. In California, this has led to regulations on landscape planning and installation that require knowledge of plant water requirements and the use of water-conserving plants to balance turf water needs. However, there is very little research-based data on the water use of landscape plant species. To address these issues, 2-year irrigation and climate zone trials were begun in 2005, 2007, 2009, and 2011 on landscape perennials considered by the University of California Davis Arboretum to be water-conserving, pest tolerant, and disease resistant. In each trial, ten species were evaluated on 4 levels of irrigation based on reference evapotranspiration (ET0), the estimated amount of water loss from a reference crop. Six replicates of each species and treatment were planted and established on regular irrigation (80% of ET0) the first year. The following May through October they received an irrigation treatment at 20, 40, 60, or 80% of ET0. Monthly measurements and qualitative evaluations were made to assess differences between treatments. Many of the same plants were also placed in demonstration gardens in different climate zones across California where UC Master Gardeners evaluated their performance. In many cases, there were no significant differences between irrigation treatments in either plant growth or quality, while a few species showed slight differences in one or both areas. The plant water use data being generated in these trials is helping support the necessary transition to sustainable gardens in summer-dry Mediterranean climates.