Greywater can either benefit or harm plants, depending on what soaps and detergents you use. Its quality as an irrigation source is directly connected to what you put down the drain. Luckily, it’s easy to choose soaps and other products that are plant-friendly, avoiding the following ingredients:
Salt and sodium compounds. Salts can build up in the soil and inhibit plants’ ability to take up nutrients and water. Minimize and avoid salts.
Boron. This plant microtoxin is damaging even in small amounts. Do not use any products that contain boron, including the laundry additive borax. Because it is nontoxic to people, boron is found in many ecological products.
Chlorine bleach. Bleaches containing chlorine kill microorganisms, including beneficial soil microbes. Hydrogen peroxide bleach can be used instead, or you can turn off your greywater system when using bleach.
Alkaline compounds (optional). Some products raise the level of pH, making the water more basic (or alkaline). This isn’t a problem for most plants, although some types (such as blueberries and azaleas) prefer acidic conditions, and basic water may not suit them. In general, liquid soaps do not increase the pH of the water, whereas bar soaps do. Cleaning products can also be extremely basic (alkaline). If you are using grey-water from a source where only liquid, pH-neutral products are used, greywater can irrigate any plants, including acid-loving varieties. Refer to garden books, extension offices, or local nurseries to determine whether your plants are acid-loving.
Following are some products that have been used successfully for many years in grey-water systems. This list is not exhaustive, and you may find others that are free of boron and very low in salts. Additionally, you can look up the ingredients for personal care products on the Environmental Working Group’s Skin Deep Cosmetics Database.
Washing machine: ECOS, Bio Pac, Oasis, Vaska, Puretergent, FIT Organic, as well as non-detergent options like soap nuts or laundry balls. Powdered detergents are never okay; use only liquid detergents. Watch out for brands like 7th Generation that claim to be greywater-safe but contain boron and salts.
Showers: Aubrey Organics (most types), Everyday Shea, Dr. Bronner’s. In general, typical shampoos and conditioners will not harm your plants. The products are very diluted, liquid (very low in salt), and free of boron.
Cleaning products: Use vinegar-based products, not white powders. Or, turn off the greywater system if you need to do a deep scrub with a salt-based “powder” cleaner.
The amount of salts you can send into your yard without damaging your plants depends on your climate, soil, and plants. If you live in a place with heavy, frequent rainfall, rain will leach salt out of the soil before it can build up to harm plants, so the occasional salty product won’t cause any harm. On the other hand, in places with salty tap water (such as groundwater or Colorado River water) and a dry climate, soils are more prone to salt buildup, so you should take more care to avoid adding salts from greywater. And keep in mind that fertilizers are high in salts and that salt tolerances of plants vary considerably. In arid climates, direct rainwater into greywater basins as well as rainwater basins to flush salts from the soil.
Excerpt from Greywater, Green Landscape, © by Laura Allen, photography by © Michael Dorman, used with permission from Storey Publishing.