Marin Master Gardeners
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Helping Homeowners Save Water in Their Gardens

April 6, 2013
Dave Phelps
Fresh water is so precious. Studies have shown that roughly 30 percent of our water is used outdoors. For the typical residence, that percentage shoots up to more than 50 percent. Of that water, some 20 percent is wasted through inefficient irrigation. For this reason, conserving water in the landscape is essential.

Population growth, climate change, ground water depletion and aging infrastructure have created the perfect storm for water scarcity and soaring water rates. While population growth may be the strongest driver, climate change adds urgency by reducing the snow pack and contributing to saltwater intrusion into our aquifers.

The dire situation Californians face in the future prompted our legislature to pass laws that mandate water conservation in the landscape. It all started with the "20x2020" bills that say we must reduce our per capita water consumption by 20 percent by the year 2020. This set the stage for emergency gray water use bills, as well as the holistic Water-Efficient Landscape Ordinance (WELO). Various municipalities and water agencies improved upon this bill. In Marin, we have Ordinance 421.

If followed, WELO can conserve landscape water in a systematic and comprehensive way. It includes education, soil fertility management and landscape maintenance practices. It also contains stringent standards for turf reduction and irrigation efficiency, as well as promoting rainwater harvesting and the use of reclaimed and gray water.

One requirement is the use of weather-based or ET controllers. ET stands for evapotranspiration, or the loss of water through the combination of evaporation and transpiration — the loss of water through plant leaves. These controllers reprogram the irrigation schedule every day to replace the actual water lost through evapotranspiration. They have proven to show large savings, especially in the fall as day length shortens but daytime temperatures remain high and trick us into applying more water than is needed. While these irrigation controllers are valuable tools, they are only as efficient as the data entered and the state of the irrigation system.

Leaks, overspray, run-off, poor distribution uniformity and unmatched precipitation rates hamper the ability to manage water use. Once these have been addressed, one of the best ways to improve the efficiency of an irrigation system is to make sure plants on each valve or station have the same water needs. In particular, trees should be on a different irrigation zone than other plant types as trees have deep, infrequent water needs. The practice of putting plants with similar water needs onto each irrigation zone is called "hydrozoning."

There has been a lot of work invested in determining the water needs of various garden plants. The University of California Cooperative Extension along with funding from the California Department of Water Resources and input from leading horticulturalists developed the Water Use Classification of Landscape Species, or WUCOLS. This document is the standard by which LEED accreditation, Bay-Friendly Rating, and WELO compliance are measured. It was paid for by our tax dollars and is available for free on the DWR website.

The water use of plants in the WUCOLS document is divided into six regions. This accommodates the water needs plants have if they are grown in different climates. Each species of plant is assigned a rating in each of the six regions. These ratings — high, medium, low and very low — correspond to "crop coefficients" that are used to calculate estimated water use.

A great exercise for any gardener or landscaper is to identify the plants in each irrigation zone or station on his or her irrigation controller and cross-reference them to the WUCOLS list. By planting and transplanting plants by their relative water needs, we can save a lot of water.

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