March 01--Did you have insect problems on small trees or shrubs last year? You may have heard advice to spray them with dormant oil in March.
Dormant oil is a heavyweight horticultural oil that is commonly applied to fruit trees to smother insects, according to Sharon Yiesla, plant knowledge specialist at the Plant Clinic of The Morton Arboretum in Lisle.
It needs to be applied when temperatures are above freezing but trees are still dormant, before they open their leaves. That often means early March. Horticultural oils are widely recommended as relatively safe insecticides, since they work by smothering pests rather than poisoning them.
The problem, Yiesla says, is that some people use them indiscriminately. "Dormant oil can be effective, but it's not a cure-all," she says. As with any insecticide, "you want to be sure you know what your problem is, and you want to be sure that dormant oil is the right treatment."
The oil works against overwintering insects or insect nymphs by sealing them against air. "The effect is not going to be highly visible," Yiesla says. "You won't see dead bugs falling from the tree."
Dormant oil won't kill every insect, Yiesla says. It works better in some years than others, depending on the weather, and it's not suitable for every kind of tree. For example, smoketree, black walnut and all kinds of maples are known to be sensitive to damage from the oil. Horticultural oil also can remove the blue color from blue spruces if it's overused, Yiesla says.
For advice on whether it's right for your situation, contact the Plant Clinic at 630-719-2424.
The oil can damage the leaves of any tree if it's applied after they start to unfold -- which can happen quickly if we get another warm spell in March and the trees come out of dormancy early.
Insecticides kill beneficial insects -- the vast majority of insects in your garden -- as well as those that cause problems. Although dormant oil is somewhat less harmful to beneficial insects than other insecticides, it still should be used with caution, Yiesla says.
To avoid doing more harm than good, apply it only if you have an identified problem that is severe enough to warrant treatment. "You don't want to apply it as a general preventative," Yiesla says.
Commercial horticultural oils are usually made from refined petroleum, though a few vegetable oil products are available. The products also contain an emulsifier to help keep the oil dissolved in water. Even vegetable oil products can be irritating to the skin, so you should always wear long sleeves, gloves and eye protection to spray them.
"It's better to use a commercial product than to try mixing your own," Yiesla says, because commercial products have been tested on plants.
To be effective, dormant oil needs to be applied when the temperature is above 40 degrees and you're sure the weather will stay above freezing for at least 24 hours after the application. Never spray any pesticide on a windy day, when the spray might drift onto plants that could be damaged by it.
"Dormant oil can be a useful product," Yiesla says, "but only for the right problem."
Beth Botts is a staff writer at The Morton Arboretum in Lisle (mortonarb.org). For tree and plant advice, contact the Arboretum's Plant Clinic (630-719-2424).
