Soapy water can benefit plants, particularly in controlling certain insects, but it's important to ensure that the soap product you use doesn't have additives that are harmful to plants and that you dilute it sufficiently to avoid damage. When using conserved household waste water, avoid contact with edible portions of the plant. Always test a small portion of the plant for tolerance to the chemicals.
Soapy Water Types
The chemical composition of soapy water differs dramatically depending on the kind of soap you use. Commercial insecticidal soap is the safest choice because it's formulated specifically to control pests and minimize injury to plants. Liquid hand soaps and detergent for hand-washing dishes -- not the kind used for dish-washing machines, which typically has high sodium content -- also can be effective and, of course, are less expensive. The risk is that additives, such perfumes, dyes and moisturizers, may be harmful to certain plants. Before using such household soaps, test the diluted mixture on a small portion of the plant and wait a few days to see if there is damage.
Soapy water is effective in killing small, soft-bodied insects, such as spider mites, aphids, white flies, psyllids and mealy bugs. Hard-shelled and larger insects, such as beetle larvae and caterpillars, often are immune to its effects. Soap kills by disrupting insects' cell membranes and it also may remove their protective waxy coating, which causes their bodies to dehydrate. Soapy water only kills insects if you apply it directly to their bodies. Be sure to spray the undersides of leaves where pests often hang out. Repeat the application every four to seven days to control new visitors or hatchlings.
Household Waste Water
You can use water conserved from your shower, hand dish washing and laundry -- sometimes called gray water -- to irrigate your garden, with certain caveats. Gray water may contain bacteria. Do not store it for more than 24 hours and keep it well away from the edible parts of food crops. Do not use gray water on household plants because they don't have sufficient soil to disperse harmful additives. Only use laundry water if your detergent is liquid, low in sodium and free of borax, and rotate where you use it because repeated use in the same area can raise the alkalinity of the soil.
For most plants, a concentration of 2 percent to 3 percent soap is effective. Use 2 teaspoons of detergent per pint or 5 tablespoons per gallon for a 2 percent solution, and 1 tablespoon per pint or 8 tablespoons per gallon for a 3 percent solution. Start with a minimal dosage and test a small area of the plant for effectiveness and damage, then raise the concentration if the insects don't die and the plant has no damage.
Some plants are too delicate for even specially formulated commercial insecticidal soaps. These include sweet peas, tomatoes, hawthorn and some flowering fruit trees, such as plum and cherry. Do not use any type of soapy water on these plants.