Canadian County OSU Extension Service

Dormant Oil

You may have forgotten about those pesky insects that can be a constant nuisance in the summer time, but if you treat with dormant oil now you can easily eliminate some of the hassle this summer. 

Dormant oil is used on woody plants during the dormant season. Many insects such as aphids, scale, mealybugs, and mites often go unnoticed during the winter months as they overwinter on plants. However as soon as the temperatures rise and the new, soft leaves begin to emerge, these insects will strike and explode in population. Also, if you have notice galls on your plants, this treatment will kill some overwintering gall-forming insects as well. By treating with dormant oil now you can control these insects and give your plants a fresh start this spring.

The term ‘dormant oil’ originally referred to heavier weight, less well-refined oils that were unsafe to use on plants after they broke dormancy. However, these older oils have been replaced with more refined, light-weight oils that have potential application to plant foliage. Dormant oil now refers to the time of application rather than to any characteristic type of oil. Dormant oils is just one of many types of horticulture oils.

Oils have different effects on pest insects. The most important is that they block the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation. Oils pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests. This allows oils to integrate well with biological controls. Toxicity is minimal, at least compared to alternative pesticides, and oils quickly dissipate through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment.

You should only apply dormant oil on dormant deciduous trees or shrubs when the temperature is between 40-70°F in late fall and winter (usually November-February). Do not apply oils during freezing weather as this can cause the emulsion to break down and produce uneven coverage. Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation. Dormant oil should not be used on evergreens. Read and follow all label direction for proper timing and rates.

Precaution should be taken as the use of a dormant oil mixture will kill annual flowers such as pansies growing under or near plants to be treated. These plants should be completely covered before spraying nearby trees and vines with dormant oil.

During the slow winter months, applying dormant oils is a great way to eliminate those annoying pest this summer and to help reduce your summer spraying. 

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