Just as the name indicates, dormant oil is an oily product that is sprayed on plant foliage during the plant’s dormant season. It is a relatively safe method for controlling certain plant attacking pests, in that it does little harm to beneficial insects, birds and animals. In the Chicago area, this application is typically made in mid-March to early April. Applications should be made when temperatures will remain above freezing for a 24 hour period. It’s also important the spray application is completed before plant bud break and before leaves have started to emerge in the spring. Spraying oil on new leaves can cause leaf edges to turn black and may kill the new plant growth.

Dormant oil gives homeowners the opportunity to get a jumpstart on specific plant harming insects, before the destructive pests become active in the spring. It is a horticultural oil spray that coats the plant and suffocates or penetrates the outside cuticle of the targeted pest. In particular, Aphids, Honey Locust Plant Bug, Ash Flower Gall, Sawfly and certain Scale and Mite insects are the targeted pests for this application. Dormant oil controls Scale varieties that overwinter in the nymph or adult stage, which includes Cottony Maple, Euonymous, Lecanium, Magnolia and Obscure Scale. Mite varieties that are controlled by dormant oil include the Honeylocust Mite, European Red Mite and Spruce Spider Mite. These Mite varieties overwinter as exposed eggs on the plant.  Many of these insects can be difficult to control, due to their rapid growth and body types. One of the advantages of dormant oil is that it can be used on a wide variety of both deciduous and evergreen trees and shrubs to manage these bothersome pests all season long. Keep in mind new pest infestations may require additional treatments.

It’s not too late to reap the benefits from this treatment in your own landscape this year. Contact the Plant Health Care department with any questions and to have a Certified Arborist evaluate your property.  Our Certified Arborists are trained to properly diagnose and provide treatment options best suited for your landscape needs.

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