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Salt Tolerance of Landscape Plants for Reclaimed Water Irrigation

The Issue

Water is a limited natural resource for most of the arid and semi-arid regions of the United States. Despite this, rapid population growth and development are occurring in these areas, especially California. Many municipal water providers are faced with the need to reduce demand for freshwater supplies while protecting against drought and cutting down on wastewater discharges into sensitive bays and estuaries. Agencies encourage the use of reclaimed or recycled water from wastewater treatment facilities for appropriate non-potable uses, including urban landscape irrigation.

In 2000, 19.5 percent of recycled water in California was used for landscape irrigation, saving enough fresh water to supply 300,000 homes. An important caveat to the use of reclaimed water for landscape irrigation is that after most of the water treatment processes, sodium chloride is the most detrimental chemical compound remaining in the recycled water. Little information is available on the tolerance of common landscape plant species to the levels of salts found in reclaimed waters. This basic information is needed by landscape managers to ensure the maintenance of healthy landscapes, given the reality of increased use of reclaimed water for irrigation.

What Has ANR Done?

Professor Lin Wu led a team of researchers in establishing the tolerance of nearly 40 landscape tree and shrub species to the levels of sodium chloride commonly found in reclaimed or recycled water. Common California landscape species were grown in the field and in greenhouse containers. Plants were irrigated regularly with water containing salt levels slightly above those found in most reclaimed waters, applied both by drip and sprinkler irrigation systems. Control plants were irrigated with potable water.

Plant heights and canopy diameters were measured at the beginning and after six weeks of salt treatments. Visual symptoms such as chlorosis and leaf burn were recorded. Plant species were placed into three salt-tolerance categories for how well they grew compared to the control plants (low, less than 50 percent growth; moderate, 50-90 percent growth; and high, greater than 90 percent growth).

Species showing high tolerance to reclaimed water included Japanese boxwood, oleander, juniper, dwarf olive, Mexican piñon pine and California fan palm. Abelia, butterfly bush, Chinese hackberry, trumpet vine, marguerite, ginkgo and Chinese pistache exhibited low tolerance to irrigation with recycled water.

The Payoff

Salt Tolerance Plant List is Landscape-Planning Tool

Wu’s research has been summarized in a widely published list of the tolerance of various landscape plant species to reclaimed water irrigation (http://ucce.ucdavis.edu/freeform/slosson/documents/1998-19992094.pdf). The green industry can use the list in designing landscapes that thrive under reclaimed water irrigation.

Demonstration gardens of landscape species irrigated with reclaimed water have been planted in San Jose and by the Marin County Water District. So far, no negative impacts on either the plants or the environment have been reported. Researchers recommend infrequent, heavy irrigation with reclaimed water rather than frequent, light watering, and sprinkler irrigation at night or in the early morning, not on hot, dry, windy days.


Supporting Unit: Department of Plant Sciences

Loren Oki, Associate CE Specialist, Landscape Horticulture
Department of Plant Sciences
UC Davis