Marin Master Gardeners
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Drought prompts more people to consider using gray water in gardens.

July 25, 2015
Wendy Irving

We gardeners have been hearing lots of talk about graywater, but what is it exactly and how can we use it to optimize water efficiency in our gardens?

Graywater is untreated wastewater from bathroom tubs, sinks and showers, and your washing machine. On the other hand, untreated water from sources such as kitchen sinks and dishwashers, which may have properties that encourage pathogens, is called dark graywater, and water from toilets and washing machines used to launder diapers or chemically contaminated clothing is called blackwater. Reclaimed water and rainwater are not considered graywater, but are fine resources for landscape irrigation. Reclaimed (sometimes just called “recycled”) water consists of all these water types carefully treated to be reused for both nonpotable and, surprisingly, some potable applications. Now available in Novato — a nonpotable reclaimed water filling station that will allow residents to fill containers with up to as much as 300 gallons. Contact the North Marin Water District for details.

Ordinary graywater is what most Marin gardeners can easily take advantage of now. Do as little as simply siphoning your kid’s bathwater into a container, or collecting cold water in a bucket while you wait for your shower to get warm. Or create a “simple” system, discharging graywater through an irrigation line into the landscape − some basic plumbing skills required, but usually no permits are required. Marin law allows exemption of clothes washer and simple systems from permits. A permit is required for complicated systems.

Simple graywater systems include the “laundry to landscape” type, as well as some systems that use other graywater sources and do not require things like pumps and filters. Check with your city’s building department before proceeding since rules and regulations change.

Why would you want to bother with any of this? Well, recycling graywater to irrigate your garden obviously can reduce your community’s and your own water and energy costs, but it also reduces the amount of precious drinking water disappearing into the landscape. An estimated 30 percent to 50 percent of residential water usage produces graywater — that’s 90 gallons a day for a typical household (2.6 persons). This can go a long way in your garden!

But there are limitations on how you might use graywater in your landscaping, and some of the information available on the do’s and don’ts is conflicting. Much depends on the quality and make-up of your graywater, so some discretion is required.

Feel free to water your ornamentals with graywater, which is often rich in nutrients, but be aware that many acid-loving plants won’t tolerate the salts often found in high pH graywater. Evergreen trees are often more salt-sensitive than deciduous trees, and little is known about the impact of graywater on annual bedding plants. It is wise to alternate graywater irrigation with fresh water to minimize any salt build up. Better yet, use biodegradable, pH balanced and sodium-free, boron-free and chlorine-free products in your washing machine and for bathing.

Apply graywater directly to the ground — don’t allow it to be sprayed on plant surfaces. Don’t use it on your lawn and don’t try to recycle it through your existing irrigation system, drip or otherwise. Avoid runoff. Use graywater within 24 hours after collecting to minimize bacterial growth, but avoid contact with skin regardless.

Research is ongoing, but the UC Cooperative Extension is recommends that, to be safe, you avoid using graywater in your vegetable garden, and definitely do not use it on root vegetables like carrots and onions. The concern is using graywater that might have been contaminated with bacteria or viruses. But others believe that if you are confident of the quality of your graywater, you can use it on your non-root vegetables, being careful not to splash it on any edible parts of your plant.

We have lots of information available to you at the UC Marin Master Gardener Help Desk, email, to help you with your decision-making regarding the use of graywater and the installation of graywater systems.

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