Dormant oil is a horticultural oil applied to trees and shrubs in late winter or early spring, before the buds open. It coats the bark and twigs and smothers the insect eggs laid there the previous year. As temperatures rise in spring, the eggs begin taking in oxygen to develop; the oil prevents that, and the insects do not hatch.
What it controls
Dormant oil is most useful against pests that spend the winter on the plant as eggs or immature insects, including scale insects, mites, and aphid eggs. It is a common part of a fruit tree program and is also used on ornamental trees and shrubs.
Timing and weather
The application has to be made after the plant is fully dormant and before bud break, on a day warm enough for the oil to spread evenly, with no hard freeze expected immediately afterward. On the Western Slope that window usually falls in late winter or early spring, and the exact day depends on the weather.
Where it is not used
Oil strips the waxy coating that gives Colorado blue spruce its blue color. It is not applied to blue spruce, or to other plants known to be sensitive to it.
Dormant oil pairs well with winter pruning. All applications are made under Brookstone Outdoors' Colorado Department of Agriculture commercial pesticide applicator license.