Using Insecticidal Soap In Your Garden
For many of us the most difficult aspect of gardening is dealing with pests, especially insects. Insecticidal soaps are among the safest choices available for controlling pest problems in your vegetable garden or on your ornamental plants. They are effective primarily on soft-bodied insects such as caterpillars, aphids, thrips, mealybugs and whiteflys. If you are interested in using insecticidal soap, here are some guidelines to follow for good results.
Insecticidal soap works on contact which means that the spray solution must coat the pest you are trying to control. Once the soap has dried, insects will not be harmed by walking over the residue. This means that good coverage is extremely important. For example, spraying only the upper leaf surfaces will leave whiteflies alive and well since they usually feed under the leaves. For best results direct spray both above and below the leaves.
Because insecticidal soap works only on contact, it can’t be used as a preventive measure. Spray only when an infestation appears. Symptoms such as leaf or shoot distortion, black sooty mold and holes in the leaves will require some further looking to pinpoint the cause of the infestation. Aphids, for example, will sometimes be found on a single shoot and sometimes throughout a planting. Identify the pest and where it is before spraying.
Watch for phytotoxicity, a form of plant injury, from soap treatments. Symptoms on foliage include yellow or brown spotting, “burned” tips and/or yellow or brown scorching along the leaf edges. Plants which are likely to be damaged by soap sprays are those with dull leaves (little protective wax) or hairy leaves. Soap sprays can also cause markings on some fruit varieties.
Phytoxicity can be a problem when using insecticidal soap, but observing a few pointers will reduce the chances for injury. Don’t treat plants when they are suffering from drought or any other kind of stress. Make sure that the plants to be treated are well-watered and look healthy. Plants under stress can be particularly intolerant of insecticidal soap solution. Conifers, for example, are very sensitive when under drought stress.
Apply soap solutions when the temperature is below 90 degrees F and the plants are not exposed to full sun. High temperatures and high relative humidity may increase plant stress, and therefore, sensitivity. The best time to apply insecticidal soap is the early morning. Since insecticidal soap works only when it is wet, the slower drying conditions of early morning favor better control.
In spring and early summer wait for new growth to harden off before treating. The tender young foliage of evergreen trees or shrubs can be particularly sensitive. Fruit tees in bloom should not be sprayed. If in doubt, test a small part of the plant first. If the plant is sensitive, phytotoxic symptoms should appear within 48 hours.
Lastly, avoid treating sensitive plants. The following list of plants is known to show phytotoxic reactions after treatment: horse chestnut, mountain ash, japanese maple, bleeding heart, sweet pea, maidenhair fern, crown of thorns, lantana, nasturtiums and Easter lilies. Certain varieties of azaleas, begonias, fuchsias, geraniums and impatiens may also be sensitive. Rinse these plants with a clean water spray if they show signs of wilting within a few hours after treatment.
When using insecticidal soap be aware that soap combines with and is precipitated by certain minerals in hard water (especially calcium, iron and magnesium). Check for compatibility with your water source by allowing a quart of the mixed spray soultion to stand for 15 minutes. A scum or “curd” of soap scale indicates that you need a conditioner. The solution should be a light, milky color.
For more information visit our web site or
call the UNH Cooperative Extention’s Family, Home & Garden
Center ’s Info-line (toll free) at 1-877-398-4769. Trained volunteers
are available to answer your questions Monday through Friday 9:00am to
2:00 p.m.
