I recently moved a number of outdoor plants inside my house for the winter, and all have been doing well for the last few weeks until this week… when I found a colony of tiny pests on the windowsill, on the rim of the pot, and on the stalk of my banana plant.
I had hosed it down, inspected the leaves, and put it in fresh potting soil to prep for overwintering it, but even in the absence of pests to the naked eye, hitchhikers are always a possibility. They lay eggs on the undersides of leaves or hide in the garden soil that was still clinging to the roots.
The aphids seemed to appear overnight, and I needed to get them under control quickly yet naturally — a high concern since the plants were overwintering in our bedrooms. (Those little white specks are nymphs, or young aphids.)
Luckily, when it comes to fast and easy (and cheap!) pest control, organic gardeners know that it takes just two ingredients to make a safe and effective pest spray: liquid soap and water.
Also called insecticidal soap, it's the next step in managing pests when other natural, non-toxic methods (like hand-picking pests off plants, spraying them off with a sharp blast of water, or introducing beneficial insects to the garden) aren't working. Insecticidal soap kills common pests on houseplants and garden plants on contact; you can use the same formula indoors or out.
Commercial versions can readily be found in the gardening aisle of your local home improvement store, but it's a DIY worth doing for its sheer simplicity and low cost. If you have a spray bottle and liquid soap handy, you're already halfway there!
How It Works
Insecticidal soaps exploit the fatty acids in soap to suffocate small, soft-bodied insects and arthropods such as aphids, mealybugs, thrips, whiteflies, spider mites, leaf hoppers, earwigs, and immature scales (crawlers). Upon contact, the fatty acids disrupt the permeability and structure of the insects' cell membranes, dissolving their exoskeletons and fatally dehydrating them.
Contact is the operative word here, as insecticidal soaps only work when sprayed directly on the pests, and are only effective for as long as they remain wet. Dry soap does nothing. If you can't see the pests, you're not likely to get any results with the spray, homemade or not.
What To Use
Essentially, insecticidal soap is a highly refined version of liquid dish soap. But while many recipes may call for dish soap like Dawn, it's important that you don't use Dawn (or the like), as the detergents, fragrances, and dyes in these types of soaps can be harsh on your plants and end up doing more harm than good. (Tip: If it claims to cut grease, steer it clear of your plants.)
I personally like the Dr. Bronner's line of pure-castile liquid soap, which uses fair-trade ingredients and organic oils in its formulations, and is free of additives found in commercial dish soap and hand soap. It's not a detergent like the dish soap you use to wash dishes (which really should be called dish detergent).
Dr. Bronner's baby unscented castile soap is the most versatile for all applications, but you can try their scented versions for a little extra repelling power in the garden. Peppermint is known for deterring aphids, flea beetles, whiteflies, cabbage loopers, and squash bugs. Lavender repels moths, mosquitoes, fleas, and flies, while the strong aroma of eucalyptus is effective against spider mites, scales, aphids, and earwigs.
That, plus plain old tap water, is all you need for homemade insecticidal soap. It should be noted that hard water can reduce the effectiveness of the soap, so if your water is high in calcium, magnesium, or iron, use distilled or bottled water for the solution.
How to Make It
Homemade Insecticidal Soap
Makes 1 gallon of a 1% soap solution
1 gallon water
2 1/2 tablespoons pure-castle liquid soap
1 tablespoon vegetable oil (optional)
Fill a gallon-size spray container (I use this one with great results) with water, then add the soap and oil. Mix or shake the container thoroughly before using.
The oil helps the solution stick around longer after being sprayed. Since the oil can go rancid, I mix up a fresh batch of this insecticidal soap every time I need it. If you want to keep some on hand at all times, omit the oil.
To scale the recipe for smaller applications, use 1 teaspoon pure-castile liquid soap for every 1 quart water. (This spray container works well when you have fewer or smaller plants.)
Where and How to Use It
High temperatures (above 90°F) and dry conditions can increase plant stress and increase their sensitivity to the soap, so avoid spraying on a hot, sunny day and make sure your plants are well watered first. If you're trying to treat houseplants, be sure to protect the surroundings from overspray or move the plants to an area where you can spray freely, like a patio or garage.
Insecticidal soap is best applied in the early morning or early evening, as the cooler temperatures slow evaporation of the soap and favor better pest control. Pollinator activity tends to be low during these hours, so you have less of a chance of impairing bees, hoverflies, and other beneficial bugs.
Insecticidal soaps are not systemic insecticides — that is, they don't absorb into plant tissue. They only work on direct contact with insects, so make sure you cover all plant surfaces where you see pests with a fine spray, including the undersides of the leaves where many pests like to hide. Note the emphasis on where you see pests. Simply spraying the whole plant won't work — the soap needs to coat the insects thoroughly, not the leaves, in order to kill them.
Spray once a week (or for more serious infestations, every 4 days) for 4 weeks until you see improvement. Any more or longer than that, and you risk leaf injury, as the soap will remove all the natural oils and waxes that protect the leaf, and thus remove the plant's natural defenses against pests and diseases.
Speaking of leaf injury, some plants are more susceptible to soap than others, so I suggest a test spray on a small area first if you aren't sure how sensitive your plant is. Wait 24 to 48 hours and check for leaf damage (such as burned tips or yellow or brown spotting) before proceeding with a full application. (If you do spot damage, rinse the leaves with clean water to remove any residual soap.)
According to Clemson University's Cooperative Extension, susceptible plants include hawthorn, sweet peas, cherries, plums, horse chestnut, mountain ash, Japanese maple, bleeding heart, maidenhair fern, crown of thorns, lantana, nasturtiums, gardenias, and Easter lilies, and to some extent azaleas, begonias, fuchsias, geraniums, and impatiens.
Seedlings, new transplants, newly rooted cuttings, and drought-stressed plants are also sensitive to insecticidal soap, so try to incorporate other means of pest control (like row covers or other physical barriers — I'm a fan of this mesh pop-up tent) before resorting to soap.
Remember: Less is more when it comes to spraying anything on your plants, even when you're using natural pest control sprays.
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