Dormant Oil in the Organic Garden
Use Horticultural Spray to Kill Pests on Plants and Trees
Dormant oil kills garden pests on woody plants like trees and shrubs just when damaging insects are waking up from their winter's nap.
What is Dormant Oil
Dormant oil kills many chewing and sucking pests in the organic garden, including aphids, scale, whiteflies, fruit moths, and all kinds of mites. You can combine dormant oil with good garden hygiene practices, such as removing fallen fruits and foliage, to control many damaging insects.
As a means of pest control, dormant oil is similar to other physical methods of control like floating row covers because its efficacy comes from its ability to form a suffocating film over insects and their eggs. This mode of extermination means that harmful chemicals, which can affect humans and wildlife, aren’t necessary.
What to Spray with Dormant Oil
Dormant oil is sprayed during the winter, which means you can’t spray herbaceous plants or perennials that don’t exhibit any top growth. Gardeners usually use dormant oil on trees and shrubs, but fruit trees especially benefit from an annual dormant oil application because of their susceptibility to pests.
The new generation of dormant oils is less viscous, which make them evaporate more quickly. This decreases the chances that the dormant oils will damage your trees or shrubs. Nevertheless, some plants are especially sensitive to dormant oil and require alternative pest control methods. Maples, redbuds, hickories, and walnut trees are susceptible to dormant oil damage, as reflected by blackened branches or yellowing foliage.
You must use dormant oils with caution on evergreen trees and other non-deciduous trees like citrus. If you must treat these plants, use the lightest oil formulation, apply on a cloudy day, and water the plant well to reduce foliage stress.
How to Use Dormant Oil
Look for soybean-based dormant oil. You can also buy petroleum-based dormant oil, but petroleum-based products are falling out of fashion with organic gardeners. Organic gardeners must not purchase products that combine dormant oil with toxic pesticides. Read the product label carefully.
Although you must apply dormant oil during winter’s garden intermission, you don’t want to apply the oil in the most frigid part of the year when all insect activity is suspended. Timing is tricky in some regions, especially as spring seems to arrive a bit earlier every year in some areas. Better to err on the side of caution than to apply oil to plants with tender green growth. Act on observation rather than by the calendar: apply dormant oil when buds start to swell, but before buds open. This should maximize your insect kill and minimize the possibility of plant damage.
Source:
Rodale, J.I. (1999). Encyclopedia of Organic Gardening. Rodale Books, Inc: Emmaus, PA.
