MASTER GARDENER: Garden-friendly pest control: Horticultural oils

Our yards and gardens are full of insects. Some of these insects cause damage to our plants, and we look for ways to control these annoying pests. But there are also beneficial insects in our yards that pollinate plants, devour undesirable insects or clean up debris from the garden. Unfortunately, using a broad-range insecticidal spray kills beneficial insects along with the targeted pests. Horticultural oils, or dormant oils (used when a plant is dormant in late winter, before “bud break”), provide us with a garden-friendly alternative to control undesirable insects while doing less damage to beneficial insects, people and the environment. These oils are available at local garden centers, and should be a part of any pest-management strategy.
What are horticultural oils?
A horticultural or dormant oil is an oil used to control a pest on plants. Most horticultural oils are refined petroleum products (mineral oils). The oils are put through a process of filtration, distillation and de-waxing to remove impurities that would cause plant injury. They are usually mixed with an emulsifying agent that allows the oil to mix with water, and are used at about a 2 percent dilution. Seeds of the neem tree (Azadirachta indica) are commonly used as a source of dormant or horticultural oils. Azadirachtin, a compound found in neem tree seeds, has proved useful as an insecticide.
How do horticultural oils work?
Horticultural oils control flea beetles, aphids, scale, whiteflies and mites by suffocating them. The oils also kill insect eggs by penetrating the shells and interfering with natural processes. The oils have few residual (lasting) effects, and so their impact on beneficial or benign insects is minimal. Since oil sprays only work by contact (coating the target pest), a thorough spray application of a plant’s leaves, stems and branches is required. Be sure to spray both leaf tops and undersides, small cracks in the bark and bud unions. Like all pesticides, read and follow label directions for each individual product, paying special attention to recommended temperatures at time of application.
Timing is important. Actively growing insects or mites are more susceptible than dormant ones, so the best time to apply dormant oils is after insect dormancy ends in late winter or early spring when insects resume growth. Dormant oils will damage plants if used during the growing season. The term “dormant oil” refers to the time of application rather than to any characteristics of the oil. Summer oils are lighter and more refined, and can be applied to both actively growing and dormant plants. Do not apply summer oils when the temperature is above 90 degrees. Read the label. It’s important to note that in order to protect the blooms, camellia growers don’t spray their prized camellias with horticultural oils until their bloom period ends.
A gardener’s experience
Master Gardener Kathryn Darwin was motivated to try dormant oils after reading Clemson Professor Bob Polomski’s Month by Month Gardening in the Carolinas, and has been using the product successfully for over ten years. She likes to use the oils because they are less toxic than many of the chemicals used to control the same insects during the growing season, because pollinators and most beneficial insects are in little danger of being harmed at the time that dormant oils are used, and because the oils are not very expensive. Kathryn says that she “has not had a problem with listed/controlled insects” since using the horticultural oil. She always follows label directions specific to the product being applied. She says that oil sprays may cause “burn spots” on some plant leaves, especially in full sun. Make sure the oil/water mixture in the sprayer stays agitated and do not use dormant oil on stressed plants, including those under drought stress.
Horticultural (either dormant or summer) oils should not be used on the following plants: maples (particularly Japanese and red maple), hickories and black walnut, cryptomeria, smoke tree, redbud, juniper, cedar, spruce and Douglas fir. Always follow label directions and safety guidelines when spraying dormant oils. Mineral oil may be listed as the primary ingredient in dormant or horticultural oils. Organic versions of these oils often list canola oil as the main ingredient. These are familiar as oils available in grocery stores and pharmacies; however, the oils you purchase from these locations are not recommended for use as horticultural sprays.
February yard and garden tasks
In addition to the application of horticultural or dormant oils, February is a good time to prepare your turf grass for spring. Soil temperatures are getting into the low 50s and that means it’s almost time to apply pre-emergence herbicides to prevent warm season weeds in turf grass. But before you apply the weed preventatives, you need to rake your turf grass. Pre-emergence herbicides work by creating a barrier that stops weed seeds from germinating, and you won’t want to disturb that barrier after an application of the herbicides. So now is the time to rake. This is also a good time for a soil test so that you’ll be ready to apply the necessary nutrients to your turf grass when it is actively growing in May or June.
The Aiken Master Gardeners will hold its lunch box lecture series at 12:30 p.m. on Feb. 17, at Trinity United Methodist Church, 2724 Whiskey Road. Jane Burkhalter, a local master gardener, will be the guest speaker. Her topic will be Roses to Love.




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