Dormant oils have an interesting history in their use on fruit trees. One challenge many conventional farmers faced with their trees was how to deal with the many insects that like to feast on the developing fruit. Because of the edible fruit, spraying with insecticides was out of the question.
As a result, a heavy oil was first used during the dormant season of the plant to kill off over wintering insects like aphids, mites, and scale. Originally oils were heavy and could not be used directly on the leaves. However as technology and testing have improved, newer oils are light enough to be used during the growing season without damage to the tree and dormant oils now refer to the main time when these oils are applied rather than the only time they could be applied.
Dormant oils work by suffocating the insects. It must be applied directly onto the insects which then blocks their spiracles through which they breathe. The oils can also affect the insect eggs causing them to starve due to their inability to feed.
The best time to apply is before the leaves or flowers of the trees show signs of breaking open. If applied too early, the insects may not be active and breathing and so the oil may not be as effective. Dormant oils should not be used in very hot weather (over 38C) or below freezing temperatures.
Oils are considered safe to use around humans and pets and is something we recommend for many gardeners with fruit trees. We like to Green Earth’s Dormant Spray Kit, though there are others on the market as well.
Dormant horticultural oils should not be used on sugar maples, beech trees, blue spruce, hickory and walnut. Lime sulphur should not be used on peach trees or on plum trees that have already leafed out