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Spray horticultural oils now to reduce insects, diseases

Over the past two weeks we have been talking about pruning ornamental and fruit bearing shrubs and trees. The last step in getting them ready for spring is applying horticultural oil to reduce the later occurrence of some insects and diseases.

Horticultural oils used to be called “dormant oils” in the lexicon of our grandparents. These old oils were applied only during the dormant season because they could easily damage plants when they were in leaf. Modern horticultural oils are much more highly refined products from both petroleum and vegetable sources. These newer oils can be applied throughout the season and have far less potential to damage plants as long as they are applied following the directions.

Vegetable-based oils are usually from cottonseed, soybean or neem seed and are nearly as effective as petroleum based oils. They are more likely to be available at independent garden centers. Don’t be tempted to make your own sprays.

Horticultural oils are mixed with water and sprayed on the target plant. The spray coats overwintering insects and eggs with a film of oil that then plugs up the breathing tubes on their outer coverings and basically suffocates them. The oil then evaporates leaving no residue. Because this is a mechanical process and not a chemical one, there is very little potential for environmental impact or the development of resistance to the oil. Beneficial insects and bees aren’t affected by horticultural oils because they have dissipated long before the good guys are out.

Some of the most common insects that are killed by a winter application of horticultural oil include most but not all scales, mites and aphids. Horticultural oils also can reduce powdery mildew and some of the viruses that are carried by the insects. It does not affect other bugs like codling moth, cherry fruit fly or apple maggot fly.

Often horticultural oils are mixed with copper which acts as a fungicide and broadens the usefulness of the application. This is particularly effective on peach leaf curl and is the only time the fungus can be controlled. Be aware that lime sulfur sprays of any kind are no longer registered for homeowner use.

Read the label carefully before you spray. Horticultural oils are safe but they can easily burn emerging foliage if applied incorrectly. Walnuts, redbuds and maples are sensitive to the oil and should not be sprayed at all. Conifers with bluish needles like Colorado blue spruce and Douglas fir will irreversibly lose their bluish color if sprayed because the oil breaks down the wax coating that creates the color.

Apply horticultural oils when the plant buds are still very tight and haven’t swelled to avoid damaging newly emerging growth. In the Inland Northwest that usually means spraying needs to be done in early to mid-March. Apply the oil on a calm day when the temperatures are expected to stay above 40 degrees and the nights are going to be above freezing. Be sure to spray bark cracks and cervices well to reach insect hiding places.

Pat Munts is a Master Gardener who has gardened the same acre in Spokane Valley for 30 years. She can be reached by e-mail at