Dormant oil is nothing more than a highly refined petroleum product that is sometimes referred to as horticultural oil. There are types of oils that can be used when a tree is in leaf that won’t cause burning of leaf tissue, but this article is about actual dormant oil, used while the tree is dormant.
Dormant oil is used as an insecticide to suffocate insects and insect eggs that have overwintered on trees and shrubs. If you had problems last year with aphids, scale or spider mites, dormant oil is a great way to help you get a handle on the insect problem before it starts this year. Dormant oil, used alone, does not control plant disease!
Dormant oil should not be used on blue trees or shrubs. Blue spruce, blue atlas cedar, blue junipers and other blue trees and shrubs get their color from a waxy coating on the needles. An oil product will dissolve that coating, turning your blue plant green. This won’t kill the plants, but they will look multi-colored for a while. The blue color will eventually come back to the parts that were sprayed but it could take several years for the plant to regain its natural blue tone. New growth will be the natural blue color.
Dormant oil, as the name implies, is used when the plant is dormant and without leaves because the oil can damage green leaf tissue or buds. Without leaves, it’s also easier to get the product into cracks and crevices in the bark where insects overwinter and have laid their eggs. This is effective for adult insects on the tree, but not as effective on over-wintering eggs. The closer the insect egg gets to hatching time, the greater oxygen exchange it needs. Interruption of oxygen in early spring will be more detrimental to that egg than one covered with oil in the fall. Although, oil products can be used in the fall after leaf drop as well.
Dormant oil should be used when the temperatures are going to be above freezing with no sign of rain for 24 hours. Most labels will suggest that you mix the oil half rate for use on evergreens. Because rose canes are green tissue, follow the evergreen directions when spraying roses unless otherwise directed. As with any garden product, read your label carefully.
Fungal diseases are treated when the tree is dormant and the leaves are off the tree. Some fungicides are registered for use with oil but some aren’t There are also products that are specifically listed as a dormant oil/fungicide already mixed for you. But, I can’t repeat it enough…read the label.
A tip when spraying dormant oil; be careful of plants you may have growing under the trees you spray. Things like pansies, snapdragons or other green plants may be damaged, if not outright killed, by the oil if it’s allowed to get on them. To keep everybody happy, cover them with a tarp before beginning to spray.