Tree Davis: Trees have a place in a water-conserving landscape

By Ellen Zagory

Experts estimate that up to 50 to 60 percent of annual water use by homeowners goes to landscaping. Large green carpets of lawn contribute significantly to these figures, a landscape ideal that comes to us from lusher, summer-rain climates.

As you ride your bike around Davis, you may notice some people are eschewing the green lawn and the traditional irrigation it requires and installing patios and low-water-use shrubs and perennials instead. If you want to convert your turf to a low-water landscape, you can save on water bills, but be sure to consider existing trees since a sudden reduction in water availability can stress, or even kill, the tree. What to do?

Trees vary in their tolerance of drought. Trees native to regions that normally have rain year-round may not be able to thrive or survive without regular summer moisture. The secret to making these trees able to withstand infrequent irrigation, and reduce water use, is the application of enough irrigation water to penetrate deeply and encourage roots to penetrate deeper into the soil.

According to “Arboriculture, Integrated Management of Trees, Shrubs and Vines” (Fourth Edition), while the majority of tree roots are in the top three feet of soil “surface rooting will be increased if poor irrigation practices keep the soil too wet on the surface and too dry at the lower depths” — conditions fostered by the frequent shallow irrigation often used for turf.

Trees like sawleaf zelkova or Kentucky coffee tree will grow with lawn irrigation but can adapt to less irrigation if encouraged to grow deeper roots by deep irrigation. To get water to penetrate deeply slow application rates are critical to avoid water runoff into streets and drains. Applying water in short intervals, allowing time for absorption between applications but repeating the process multiple times over a day or two also will facilitate water penetrating more deeply and encouraging roots to follow.

If you decide to convert to low-water landscaping, it is important to use trees adaptable to these growing conditions. Tree Davis’ “A Tree Guide for Davis” has added water use to its recommended tree list, a short list based on the city of Davis master tree list.

In the Sacramento area, “Water Use Classification of Landscape Species” estimates that the monthly water use in July for low-water species is between one and three inches. Installation of a flow meter, if you don’t already have one, will help you determine how long you would need to water to maintain tree health.

The movement toward sustainable gardening encourages more people to plant trees that are native to their region. In the UC Davis Arboretum, we grow many California native oaks like the blue oak, coast live oak and interior live oak, species useful in summer-dry landscapes since they are adapted to this type of climate. These species are susceptible to root-crown disease if irrigated in summer, so areas directly below the canopy of the trees are kept dry.

Plantings outside of the canopy area are irrigated every two weeks, however, and it is likely that the trees have roots that take advantage of that water. The native valley oak, a massive deciduous tree that lines the banks of the UCD Arboretum waterway, is more tolerant of summer water and thrives as long as drainage is adequate. The large size of some oak species at maturity, however, may limit their suitability in today’s smaller home landscapes.

Converting to a low-water landscape takes planning and thought. We want to encourage you to save water but at the same time help our urban forest stay healthy and thriving and continue to provide us with all the benefits that trees provide: cleaner air, cooler streets, homes for birds and a lovely place to live.

City of Davis master tree list http://archive.cityofdavis.org/pgs/trees/master.cfm

“A Tree Guide for Davis”: http://www.treedavis.org/node/8

“Water Use Classification of Landscape Species”: http://www.water.ca.gov/wateruseefficiency/docs/wucols00.pdf

— Ellen Zagory is director of public horticulture at the UC Davis Arboretum and a board member of Tree Davis. This column is published monthly.