Keep Plants Green with Gray Water

In times of drought, most home gardeners must ration their water use. Sometimes this means watering vegetables and favorite flowers while watching lawns and other plants wither. But across the country some intrepid gardeners are foregoing the tap and turning to another source: gray water.

Gray water is commonly defined as any household wastewater except for toilet water, which is called black water. In arid communities with annual water shortages, such as California, Florida and the Southwest, gray water systems have been in use since the 1980s.

There are still some parts of the country where using gray water is actually illegal. The main concern is the potential for adverse health effects. Gray water can contain harmful microorganisms, which is why gardeners should follow some common-sense practices, such as not using it on plants that will be eaten.

Using gray water reduces your potable water use—and your water bill—because you're not using tap water on your plants. And because the wastewater you're using on your garden isn't getting pumped back to the city's wastewater treatment system, you're also saving energy. Gray water also contains soap residue, which adds phosphorous and potassium to your soil, reducing the need for fertilizers. And don't forget: being able to keep your trees and shrubs alive during city water bans will safeguard your investment in landscaping.

Author and water conservationist Robert Kourik calls gray water "one of the few crimes that's good for society." Kourik is a strong proponent of gray water and of water conservation at all times, not just during a drought. He points out that using gray water during a hose ban might raise some eyebrows, because your gardens will be green and thriving while your neighbors' yards are wilting. "Fight against hose bans," he says. "Work to have your water department set up a tiered system of water rates. Or, if someone uses potable water extravagantly, even at the most costly rate, social peer pressure may put the cork on wasted fresh water."

Some gardeners have been known to collect their own gray water through decidedly low-tech means, like keeping 5-gallon buckets on hand in which to dump dirty dishwater. Others have jury-rigged their washing machine outputs, using hoses that dump directly into a flower garden or lawn area.

If you decide to collect gray water from your home, be sure not to store it for more than 24 hours. After that, bacteria can multiply rapidly, turning your gray water black. Use drip irrigation or hand watering and avoid spraying gray water, which can spread bacteria through the air. Though some studies have shown gray water adds no more harmful bacteria to soil than regular water, it's still a good idea to err on the side of caution. For this reason, it's best not to use gray water on any edible plants, including all vegetables.

• Avoid recycling water that has been chemically softened, because the dissolved salts can harm soil and plants.
• Don't use wastewater that contains boron (from cleaners such as borax), because it can be toxic to plants.
• Avoid using dishwater used to clean cooking surfaces that touched raw meat.
• Avoid laundry water if washing soiled diapers or greasy rags. (If you've set up a hose to drain directly from your washer into your garden, you'll want to have a way to divert this back into the sewer system during these times.)
• Avoid gray water entirely if someone in your household has an infectious disease.
• Make sure your soil drains well. You don't want gray water to pool on the surface.
• Call your local building or health department to find out whether gray water use is legal in your town or county.

• The Greywater Guerrillas: Lots of good background information about gray water, along with links to articles and other web sites on the topic.
• Reduce Toxins Now: From drought-prone Austrailia. A good page on gray water, with infomration about chemicals in laundry detergents.

For more articles on improving the world through gardening, see the Garden Activist archive page.