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Ask a Master Gardener: Kill the pest, not the plant

Insecticidal soap can damage a few plants, too. It's worth checking the label to make sure whatever plant you’re treating isn’t sensitive to soap.

The Woolly Aphid
Insecticidal soap can be used to combat aphids.
Contributed photo
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Q: You recently wrote a column recommending insecticidal soap for use against aphids. What is insecticidal soap? Is it different from dish soap? Can I make my own at home?

A: Insecticidal soaps are specially formulated for use on plants. You can buy them at garden stores, hardware stores and big box stores. Other soaps you may have around the house, such as dish soap or laundry detergent, may contain additives that can damage plants. Insecticidal soaps can damage a few plants, too; it’s worth checking the label to make sure whatever plant you’re treating isn’t sensitive to soap.

Years ago, when I was just starting out as a gardener, one of my plants looked peaked and a friend suggested insecticidal soap. I doused the plant and it only looked worse. I have since learned that this is not the way to use insecticidal soap — or any insecticide, for that matter.

The first thing to do is to figure out what insect, if any, is causing the damage. Then you figure out what to do about it. Some insects can be picked off by hand or sprayed off with a hose. Some won’t do much damage and can be tolerated. Some can be killed by insecticidal soap, but you don’t drench the plant. You spray the soap directly on the pest, or it won’t work.

Pests that are vulnerable to insecticidal soap include aphids, mealybugs, thrips and spider mites.

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Plants you should not spray with soap include sweet pea, ferns, mountain ash, bleeding heart, begonia, geranium and impatiens. Don’t apply to trees and shrubs in early spring when their foliage is young. Don’t apply in direct sun or on days when the temperature is above 90.

One big advantage to insecticidal soap over many other insecticides is that it generally will not harm beneficial insects, such as pollinators, because you don’t spray it on them — you only spray your target insect — and it doesn’t continue to be toxic to insects after it dries. Another advantage is that you can eat vegetables sprayed with insecticidal soap. Just rinse them off first.

There’s more information on insecticidal soap at hgic.clemson.edu/factsheet/insecticidal-soaps-for-garden-pest-control .

Written by U of M Extension Master Gardeners in St. Louis County. Send questions to  features@duluthnews.com .

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