Q: Can I use the graywater from my washing machine to water my vegetable garden and fruit trees?

A: There are two key considerations, discussed below, relating to the use of graywater in the garden: what is meant by the term graywater and on what plants to use it.

Graywater Definition

Graywater is untreated, non-disinfected household wastewater that does not include toilet waste. It may be sourced from showers, baths, and washing machines. Untreated water from sources such as kitchen sinks and dishwashers, which may have properties that encourage pathogens, is called dark graywater. Water from toilets and washing machines used to launder diapers or chemically contaminated clothing is called black water. Only graywater should be used in the landscape.

Graywater Usage

Graywater can be safely used to water landscape plants and orchard trees. Because graywater can contain bacteria and viruses that cause illness, it should not be used to grow vegetables if the edible portion may come in contact with the soil. For example, graywater should not be used for growing asparagus, beets, carrots, cucumbers, lettuces and other salad greens, garlic, onions, potatoes, melons, squash, bush beans, radishes, turnips, unstaked tomatoes, or strawberries. Crops not touching the ground like staked tomatoes, peppers, and eggplants should be OK.
https://ccmg.ucanr.edu/files/289340.pdf

Laundry Detergent Considerations

When using laundry rinse water to irrigate plants, your choice of household products will affect the composition of the graywater. The wrong products can adversely affect plants and soils. It is best to avoid using products that contain sodium or sodium compounds, bleach, or boron, as these can result in an alkaline soil condition that suppresses healthy soil biologic activity and is poorly tolerated by many plants.

Although the UC Master Gardener Program of Contra Costa County cannot recommend any products, the FAQ page on Greywater Action's website (https://greywateraction.org/greywater-faq/) lists some products that are plant friendly. These are generally biodegradable, non-toxic, and salt and boron free. The Ecology Center in Berkeley has also evaluated several cleaning products for compatibility with graywater systems. Consult their website at https://ecologycenter.org/factsheets/ for a current list of products. (Click on the link to Greywater-Compatible Cleaning Products.)

Other Considerations

Some key takeaways excerpted from our Marin Master Gardener colleagues are listed below. https://marinmg.ucanr.edu/BASICS/CONSERVE_WATER_-_ENERGY/Graywater/
Do:
Water ornamentals with graywater which is often rich in nutrients.
Check your plants first—Acid-loving plants won't tolerate salts found in high pH graywater and evergreen trees are often more salt-sensitive than deciduous trees.
Alternate graywater irrigation with fresh water to minimize salt build up.
Watch what you put in the wash. Use biodegradable pH balanced, sodium-free, boron-free, chlorine-free products in the washing machine and for bathing.
Apply graywater directly to the ground—don't allow it to be sprayed on plant surfaces.
Use graywater within 24 hours after collecting to minimize bacterial growth.

Don't:
Don't let graywater come in contact with skin.
Don't use on root vegetables such as carrots and onions.
Don't use on lawns unless the graywater is delivered below the surface. If sprayed on the surface, people or pets may come in contact with it.

I hope this helps. Please don't hesitate to contact us with any further questions. Good luck with your tomatoes and fruit trees!

Help Desk of the UC Master Gardeners of Contra Costa County (GD)