While Californians are being encouraged to let their lawns die as the state enters its fourth year of drought, staff at Our City Forest is concerned that residents don’t let their trees die as well.

At a March 25 lecture, OCF CEO Rhonda Berry reminded the 50 or so people in attendance that while lush, green lawns aren’t native to California, the trees that shade it are a necessary part of the state’s ecosystem.

And while guest speaker Laura Allen, co-founder of Greywater Action and author of “The Waterwise Home,” agreed, she pointed out that there are ways to keep residential trees and plants healthy while still reducing household water usage.

“There’s no reason you should put clean drinking water on trees,” Allen told the audience at the Environmental Innovation Center.

Instead, Allen’s nonprofit advocates using greywater to nourish plantlife that grows above ground. Californians generate 760 million to 1.5 billion gallons of greywater a day, she said, and reusing it for outdoor watering can cut water usage by 16 to 40 percent.

A typical greywater system diverts a washing machine’s rinse water from the sewage system, so it’s necessary to use soaps that don’t contain salt, boron or chlorine. A valve on the outtake hose can be set to send water into the sewers on those occasions when chlorine bleach and like products are used in the wash.

Since greywater isn’t potable, those using it for outdoor watering should avoid direct contact with it and make sure to avoid pooling or runoff. Since greywater shouldn’t be directly ingested, it should not be used to water root vegetables, Allen said.

DIYers will spend about $150-$250 on parts for a laundry-to-landscape system, Allen said, noting that the Santa Clara Valley Water District is offering a $200 rebate to homeowners who install these systems. Hiring an installer would run about $750 to $2,500, she added.

Other more complex greywater systems not only are more costly but require permits to install, Allen said. These include a branched drain system from sinks and showers and a pump system that stores greywater in a holding tank.

Allen, who lives in Los Angeles, said that while half an inch of rain in LA County potentially translates into 34 billion gallons of water, most of it is lost to urban runoff. By installing a rainwater collection system, she added, homeowners could collect as much as 600 gallons per inch of rain on a 1,000-square-foot roof.

While the slideshow she presented contained an image of the composting toilets at San Francisco’s Crissy Field, Allen told her audience at the EIC that she’d replace it with an image from the San Jose center, where the men’s and women’s restrooms each boast a composting toilet.

The toilets require a pre-flush to coat the bowl with non-stick foam made of water, air and an alcohol-based solution. They use about 2 gallons of water a month, as compared to the average 1.5 gallons per flush of a low-flow toilet.

In addition to community outreach, Allen said, Greywater Action is lobbying state and local governments to ease regulations that make some of these water conservation efforts difficult.

“California’s not quite there with composting toilets,” she added. ‘There’s a lot of work to be done to make all this legal.”

In San Jose, outdoor watering is allowed only before 8 a.m. and after 8 p.m. View the full list of water use rules now in effect at sjenvironment.org/waterconservation.