Gray Water in the Vegetable Garden

Can you use dish water on your vegetables? Photo: Kristin Homan, flickr

Question: In these times of water shortage, I had the idea of using my dish water to water the vegetable bed. I wonder if it’s a good idea. I put a bin in my sink and when I wash my hands (we often do these days) or when I rinse my dishes under the tap, the bin fills and then I empty it into my watering can. I also use the water recovered from my dehumidifier. Is it a problem for my vegetables to water them with this water?

Louise Morin

Answer: There’s no problem using dehumidifier water, as it is just H20 (the dehumidifying process removes all minerals). However, whether you should use dish water in the vegetable garden is a much more complex issue.

Dish water along with water from the shower, the kitchen and bathroom sink, dishwashers and washing machines is called gray water. This is compared to white water or clean water (potable water from the tap) and black water (water that contains human fecal matter, notably from the toilet). 

Gray water contains soap, detergents and other chemicals as well as dirt and debris, including human skin cells from hand-washing. In some environmentally friendly homes, especially in arid climates, it is funneled through a separate drainage system, then stored so it can be recycled and reused, although not for drinking and cooking purposes, obviously. If so, it is generally filtered for greater security. Gray water is considered safe for flushing toilets and watering flower beds and lawns, but its use on vegetables remains contentious. 

For one thing, gray water may contain harmful bacteria (Escherichia coli, Klebsiella pneumoniae and Salmonella enterica, etc. are often found in gray water). Plus, their numbers can increase dramatically if gray water is stored rather than used immediately. Within 24 hours, their numbers may be such that gray water should be considered black water.

Also, gray water contains various chemicals, mostly from cleaning products added during the washing process, and some of these can harm plants, notably bleach, sodium and boron. Most of these will be highly diluted and many will break down, but others do accumulate in the soil. So, in general, you can use gray water for watering, but should use it over a large area, not concentrating it in just one spot like a small vegetable garden.

In the Vegetable Garden

Certainly, gray water can be used to water, besides lawns and flowers, fruits that don’t touch the ground, such as apples, raspberries and grapes … but strawberries would be a no-no. Also, fruit-bearing vegetables, like tomatoes, peppers and beans, should be fine. Root vegetables and leaf vegetables, though, could theoretically become contaminated. True enough, rinsing them and cooking them should rid them of bacteria, but handling remains iffy. As a result, health authorities don’t recommend using gray water on root and leaf vegetables … or strawberries!

?Rule of Thumb: Don’t use on vegetables any water you wouldn’t consider putting in your child’s wading pool.

Grandma’s Favorite Insecticide

Back in the old days (pre-1950s), it was common usage to throw water from cleaning dishes onto garden plants to control insects. Of course, if it controlled insects, that might not bode well for human use!

Safe Use of Gray Water

1. Use gray water over the whole yard, not just in a localized area.
2. Avoid using gray water on leaf and root vegetables, doubly so if someone in the house suffers from any pathogenic infection.
3. Use fewer soaps and detergents in washing.
4. Prefer biodegradable soaps and detergents.
5. If possible, don’t store gray water for more than 24 hours.
6. If you do store gray water, use a filtering system to clean it.

If you insist on using gray water on leafy greens and root vegetables (and many people do), make sure you rinse and cook them before serving as well as wash your hands after handling them.