Landscape: Use of Dormant Oils

The dormant season will be upon us soon. Use of horticultural oils during the dormant period can be very effective on a number of pests including mites, soft scales, aphids, and adelgids. The following is an article on using oils during the dormant period.

In Delaware, dormant oils (3-4%) are typically applied during March and April. Many landscapers, however, also apply dormant oils during the late fall months. Dormant oils have proven to be an effective material against a wide range of immature insects/mites and their eggs. Common pests controlled include spider mites, soft scales (armored scales to a lesser degree), aphids, eriophiid mites and adelgids. Are there any potential problems associated with applying dormant oils during the months of November and December in Delaware?

Some advisors within our industry have stated that fall oil applications are not recommended. It has been suggested that spraying oils on deciduous trees just after leaf drop, when plants have not yet “hardened off” may interfere with interior growth of twigs, casing damage and even twig death. Also, it has been stated that spraying oils on conifers in November and December removes the protective waxy bloom necessary for winter protection, and should therefore be avoided.

Although the potential negative side effects need to be considered, they do not appear to be widespread problems. The use of a more conservative 2% oil mixture during the fall season on conifers may be a safeguard against undesirable side effects and yet still maintain spray effectiveness. Dormant oils are best applied when temperatures will remain above 40°F. for at least 24 hours. However, several university studies determined that no phytotoxicity occurred at lower temperature applications. When sprays are applied just prior to temperatures dropping below freezing, the emulsion breaks down, causing the oil to adhere to the bark/leaves instead of insects and thus produces poor results. Before applying dormant oils, monitor the plant to be sure of the susceptible life stage and location of the pest. Properly direct the spray to where the pest is located on the plant (underside of foliage, bark, new growth, etc.). When mixing, add water to the tank first, and then add the oil. The mixture should look like skim milk. Constant agitation is necessary during application (do not apply if the solution has been sitting 10 minutes or more without agitation).

Adapted and modified from "A Few Things to Remember When Applying Fall Dormant Oils" by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers Cooperative Extension (NJ), in the November 2, 2006 edition of the Landscape, Nursery, and Turf Edition of the Plant and Pest Advisory.