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Knowledge to Grow by Karen Weiland - Horticultural icon

When it comes to gardening there is one thing we all have in common and that is battling pests and disease. Aphids, scales, whiteflies, spider mites, powdery mildew, rust and leaf spot are but a few of the combatants that we all must deal with now and then.

Horticultural oils are pesticides that can be used to control the damage from pests and some diseases. They are specifically designed to control plant pests. After distillation and filtration, these highly refined petroleum products – being 92 to 99 percent pure – are mixed with an emulsifier to allow the oil to mix with water and then be applied as a spray.

There are some plant-based horticultural oils available. They are less refined and may burn plants (phytotoxicity). Some plant based examples are cottonseed, sesame, soybean and neem.

I’m sure many of you have heard of or maybe used neem. It is a botanical pesticide found in the seed of the neem tree. Azadirachtin is the most active insecticidal ingredient in the neem seed. It reduces insect feeding, growth and egg laying. It also acts as a repellant and is effective against immature stages of insects. The neem seed oil without the azadirachtin protects plants against insects, mites and fungi. It reduces fungal infection by preventing spore germination. With or without azadirachtin, neem oil is practically non-toxic to birds, plants, mammals and bees (if applied late evening or early morning when bees are not active). It is toxic to fish and other aquatic animals.

The insect must take a direct hit for the oil to do its’ job. When oil is sprayed onto a plant, it covers any exposed insect or egg and suffocates them by clogging their airway. The oil also may disrupt an insect’s metabolism and how it feeds.

You may be familiar with the term “dormant” oil. It is a heavier weight oil and is applied only when plants are in their dormant stage of growth, prior to bud break in the spring and after leaf drop in the fall. It is particularly applied to shade and fruit trees. Over time superior oils have been developed which are lighter weight and contain no sulfur, which makes them less likely to burn plants and have thus been called “summer” oils. They can be applied to plants that are in active growth and in full leaf.

There are some plants that are sensitive to oil application. Among them are beech, black walnut, maples, hickory, azalea, redbud, Japanese holly, smoketree, spruces and Douglas fir. Read and follow all label directions for proper timing, uses and rates.

As always, Happy Gardening!

The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.