Extension

As gardeners prepare their landscapes for winter, they may think pest control is done for the moment. However, there’s one pest-management tool worth keeping in mind – dormant oil.

Dormant oil is a refined petroleum product formulated for use on trees and shrubs. As the name suggests, it is applied when plants are dormant, typically in late fall and late winter or early spring. These applications can help control insects and insect eggs that spend the winter on plants, giving trees and shrubs a better start when the growing season arrives next year.

A single application usually won’t provide complete control, so applying dormant oil in both late fall and early spring can be more effective. However, don’t automatically reach for the sprayer. Remember, dormant oil should be used when a pest problem is present or has been identified, not as a blanket treatment.

Dormant oil can help control a variety of common landscape pests, including aphid eggs, certain mites and mite eggs, scale, lace bugs, cankerworm eggs, leaf tiers, psyllids and plant bugs. For the best results, scout plants first and concentrate applications where pests are present.

For a fall application, late October through early December is generally a good window, after trees and shrubs have gone dormant. Late February through March is good for late-winter or early-spring applications. Spring applications should be made before buds swell and break and before new growth emerges.

Oklahoma weather, of course, doesn’t always make timing easy. It’s not uncommon to experience all four seasons in the span of a week. Ideally, temperatures should be between 40 and 70 degrees Fahrenheit when applying dormant oil. Avoid applications when freezing temperatures are expected or rain is forecast for the next few days.

Applying oil too early, before plants have hardened off for winter, can increase the risk of winter injury. If temperatures are too cold, the oil also may not mix properly with water, resulting in poor coverage and reduced pest control.

Remember, coverage is key. Dormant oil needs to be mixed with enough water to thoroughly coat the plant, so always follow the product label for mixing and application instructions. The spray should reach the cracks and crevices of bark where many overwintering insects and eggs hide.

The oil works by coating insects and blocking their spiracles, the tiny openings insects use to breathe. The oil suffocates them. By reducing overwintering insects and eggs, dormant oil can help slow the buildup of pest populations in the spring.

Fruit trees are among the plants that can benefit from dormant oil applications. Apple, pear, plum, pecan and crabapple trees are good candidates for late-winter applications. Peach, nectarine and apricot trees often don’t require dormant oil, but treatment may be beneficial if certain insect pests have been a problem in previous years.

Dormant oil can also be useful on shade trees and woody ornamentals. Check the product label before treating a plant because some plants are sensitive to oil applications.

There are a few other precautions gardeners should remember. Dormant oil can damage annual flowers, so avoid spraying trees near annuals when the flowers are present. Also, don’t combine dormant oil with sulfur-containing pesticides such as captan, as the combination can cause plant injury. Follow the label for any additional restrictions or precautions.

When used properly, dormant oil offers several benefits. It is relatively inexpensive and generally has lower toxicity to birds and mammals than many other insect-control products. Plus, it can reduce pest populations before they have a chance to get established in spring.

For gardeners looking to get ahead of insect problems, a well-timed dormant oil application is a simple step toward healthier trees and shrubs.