Dormant oil is a horticultural oil spray used to control insects and mites that survive the winter on trees and shrubs. It is typically made from highly refined mineral oil or plant-based oils mixed with water and sprayed onto branches, bark, and buds while the plant is still dormant.
Rather than acting as a poison, dormant oil works physically. The oil coats pests such as scale insects, aphid eggs, spider mites, and other overwintering insects, blocking their breathing pores and effectively smothering them. Because of this mechanical action, pests are much less likely to develop resistance compared with chemical pesticides.
Dormant oil is best applied while plants are still in their dormant stage, usually in early spring, before new growth begins. Applying it at this time helps smother overwintering pests and their eggs and can also reduce certain plant diseases.
Avoid applying dormant oil after buds begin to swell or open, as spraying too late can damage tender new growth. For best results, apply it just before bud break, when temperatures are suitable and plants are still dormant.
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