Gardeners have doused plants with soapy water to kill insects for at least 200 years. But that doesn't necessarily mean it's a good idea. For starters, "soap" can mean a lot of different things, and the soap products available today can vary dramatically from those sold 50 or 100 years ago. So the question remains: Will soapy water kill plants? The answer is yes, if it's the wrong kind of soap or if it's the right soap and it's used improperly or on the wrong type of plant.
Soaps used for insect control on plants range from general-purpose detergents to commercial insecticidal soaps formulated specifically for plant application. Any soap solution can harm plants if not used correctly, and many homemade solutions involving household soaps and detergents can harm plants with any application. For this reason, it's important to avoid any soap product that is not specifically labeled for use on plants. It's also worth nothing that some horticultural experts advise against spraying plants with soapy water of any kind.
Here are some things to consider before you attempt to use soapy water to remove pests from your indoor or outdoor plants.
How Soapy Water Kills Insects
Soapy water can be an effective insecticide for common soft-bodied pests like spider mites, aphids, whiteflies, soft scales, psyllids, earwigs, mealybugs, and thrips. Hard-shelled or larger insects, such as many beetles and caterpillars, aren't usually affected by insecticidal soaps.
Though the precise mechanism isn't certain, experts believe that soap kills insects by disrupting cell membranes or possibly removing their protective wax coatings, causing their bodies to dry out. Soapy water kills insects only when it is sprayed directly onto their bodies; it is not effective to spray plants when no insects are present. Soapy water often must be applied repeatedly, and it washes off with rain, requiring reapplication.
Will Soapy Water Kill Plants?
Commercial insecticidal soap is the safest choice because it's formulated to control pests and minimize injury to plants when used as directed. These products are considered an organic pesticide, which is not true of most common household soaps and detergents. Any soap at high concentrations can burn plant foliage, especially when applied on hot, humid days when the temperature exceeds 90 degrees Fahrenheit.
Some soap products are particularly harmful to plants. Detergents used in dishwashers and clothes washers can be especially harsh. These detergents may contain chemicals like bleach, which will damage leaves, and boron, which can build up to toxic levels in the soil. Some dishwasher detergents also contain water-softening salts that are harmful to plants.
Even liquid dish soaps, like Dawn, Joy, and Palmolive, have grease-cutting power that can strip the waxy cuticle from leaves, drying out the foliage and making the plant more susceptible to disease. Hand soaps and detergents may contain antimicrobials that will kill beneficial microorganisms in the environment.
Don't Spray Beneficial Insects
Be certain that you are spraying pests rather than beneficial insects. Don't spray helpful predators — like ladybugs, lace wings, wasps, and assassin bugs — or pollinators, like bees and butterflies. Consider sacrificing a few parsley plants for the sake of supporting black swallowtail caterpillars. Fortunately, many of these insects are less likely to be harmed by soapy water than your target pests.
Plants That Burn From Soapy Water
Some plants are too delicate even for specially formulated commercial insecticidal soaps. These include sweet peas, some varieties of tomatoes, hawthorns, portulaca, bleeding hearts, and ferns as well as some flowering fruit trees, such as plum and cherry. Do not use any type of soapy water or insecticidal soap on these plants. If you buy insecticidal soap, check the label to see which plants are especially sensitive to that product.
Safely Using Soapy Water
Some general-purpose soaps are labeled for use as insecticide on plants. One example is Dr. Bronner's Pure-Castile Soap. According to its label, this soap should be mixed with water in a ratio of 1 tablespoon of soap liquid per quart (32 ounces) of water to make an insecticidal spray. Here are some tips for using this or any other soapy water solution on plants:
- Use filtered or distilled water for making the solution if your home has very hard water. Over time, hard water can lead to a buildup of minerals on plants.
- Choose unscented soap if you're concerned about soap fragrance or additives affecting the flavor of edible plants.
- Don't spray wilting or drought-stressed plants under any circumstances.
- Apply the spray in the morning or evening when your plants aren't in direct sun.
- Spray the soapy mixture directly on soft-bodied insects, limiting unnecessary spraying of leaves and avoiding tender young foliage. Check the undersides of leaves, where insects often congregate, and spray any you find.
- Rinse off the plants a couple of hours after spraying, if desired, to minimize potential damage.
- Repeat application of the spray as directed until the insects are under control.
Other Uses for Soapy Water
While soapy water isn't always the best choice for spraying plants, there are other ways you can use it as an insecticide in the garden. For example, you can use a bucket of soapy water to kill pests that you handpick from your prized roses or precious tomato plants. This method is very effective for larger insects, like Japanese beetles or cabbage worms. The soap breaks the surface tension on the water, causing the insects to sink and drown.