Reusing Washing Machine Graywater in the Garden

Help From the Contra Costa Master Gardeners' Help Desk

Request: I'd like to reduce the use of tap water to irrigate my garden by recycling water from our washing machine. I understand this recycle/reuse water is called "graywater". Could you provide me some guidance on how to use graywater for my garden?

Response: Thank you for contracting Master Gardeners' Help Desk about using graywater in your garden. It's important that we learn how to use water more efficiently as drought conditions continue. I applaud your desire to use graywater as a tool to optimize our finite water resources. However, the use of graywater does come with some challenges.

Graywater is untreated waste water from clothes washers, showers, bathtubs, bathroom sinks and laundry tubs that is used for outdoor watering. In California, graywater does not include waste water from kitchen sinks, dishwashers, toilets, or laundry water from soiled diapers.

Laundry-to-landscape graywater systems are relatively simple to install and are inexpensive. The hose exiting the clothes washing machine is attached to a valve that separates graywater from water destined for the sewer. One option is to divert graywater through a 1-inch main irrigation line with ½-inch tubing outlets placed throughout the landscape terminating in a valve box set in a mulched basin that surrounds plants being watered. The washing machine pump distributes water directly to the landscape, usually with no filter. A vacuum breaker or backflow prevention device may also be needed. Because these regulations may change at any time, it is important to check with local building enforcement agencies before developing plans for or installing a graywater system. If you decide to install a laundry-to-landscape graywater system, San Francisco Water Power and Sewer has published helpful guidelines, "Graywater Design Manual for Outdoor Irrigation",  http://sfwater.org/modules/showdocument.aspx?documentid=55. Be aware, however, that the regulations in your local jurisdiction will probably be different from San Francisco.

Graywater is also an option for irrigating your ornamentals but it should not be used to water root vegetables or any vegetables whose plant parts come into contact with the soil due to the potential that human pathogens might be present.

For more information on graywater installations, see the East Bay Municipal Utility District website http://www.ebmud.com/sites/default/files/pdfs/Graywater%20Fact%20Sheet.pdf and 
https://www.ebmud.com/water-and-wastewater/water-conservation/watersmart-gardener.

When using graywater to irrigate plants, you should avoid using household products that contain sodium or sodium compounds, bleach or boron, as these can adversely affect plants and soils resulting in an alkaline soils condition not well tolerated by many plants. The UC article "Using Household Wastewater on Plants" http://vric.ucdavis.edu/pdf/fertilization_Householdwastewater.pdf describes the effect on plants of using products which contain boron and chlorine. The Greywater Action website http://greywateraction.org/contentgreywater?friendly?products/ provides a list of products available that are phosphate, sodium, chlorine and boron free. Although we can't recommend any specific brand, Greywater Action considers the following laundry products graywater friendly: Oasis Liquid Laundry Detergent, ECOS liquid detergent, Vaska and Dr. Bronners liquid soap. The Ecology Center in Berkeley also evaluated a number of laundry products for compatibility with graywater systems and the results are listed on their website:  http://ecologycenter.org/factsheets/greywater?cleaning?products/ .

I hope this is helpful. Please feel free to contact us if you have any additional questions.

Thank you for doing your part to conserve water.

Contra Costa Master Gardeners' Help Desk

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Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/