Learn / Ask The Landscape Professional
A long time ago, my grandmother spoke of "dormant oil" as a way she used to control insects on her fruit trees. What is that and is it still effective?
Answer
Dormant oils have been around for centuries and
are extremely effective in killing insect pests on plants, shrubs
and trees. Back in your grandmother's day, "dormant oils" were
heavy oils, weren't as highly refined as they are today and were
generally limited to fruit tree applications before bud break. Today,
there are a number of highly refined oils that are safe at different
times of the year depending on the insect pest. Dormant oils
are much safer to humans and the environment than synthetic
pesticides that are on the market today. Scientists also believe
that insects have not developed a resistance to oils like they have
to synthetic pesticides. While oils don't work for all pests or on all
plants, they are effective in a wide range of applications. These
highly refined oils are generally referred to as "horticultural" oils and originate from various petroleum, vegetable
and animal based sources. All of the oils are effective in the same way; they either suffocate the insects or
penetrate the cells of the insects. In any case, you must be able to see the insect or the eggs and directly apply
the oil to be effective. You may also need to apply it multiple times. The fact that oils are directly applied to
insects to suffocate them also means that they are not harmful to humans like chemical pesticides which kill pests
through disruption of metabolic processes. Horticultural oils kill: Powdery mildew, Aphids, Whiteflies, Mites, Scale
Insects, Leafhoppers and a host of other pests.
Horticultural Oils need to be applied when the temperature is above 40 degrees and no freezing is expected
before bud swelling and bud break (when the plant is dormant). There are some oils that can also be used as
foliar treatments in the summer, so read the labels carefully for your specific application. Horticultural oils are
not a "silver bullet." Some plants are susceptible to injury from horticultural oils so be cautious and read labels
carefully. Susceptible plants include: Hickories, Maples, (especially Japanese and Red Maple), Black Walnut,
Smoke Tree, Redbud, Junipers, Cedars, Spruce (especially Dwarf Alberta Spruce), Plume Cedar and Douglas
firs. Another caution: horticultural oils should not be used on plants that are blue in color like blue hosta or Blue
Spruce since they get their color from a waxy coating that could be removed by the oil application which would
turn them to green.
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About Linda Lillie
Linda K. Lillie is the President of Sprigs & Twigs, Inc, the premier
landscape design and maintenance, tree care, lawn care, stonework, and carpentry
service provider in southeastern Connecticut since 1997. She is a graduate of
Connecticut College in Botany, a Connecticut Master Gardener and a national
award winning landscape designer for her landscape design and landscape installation work.
