Source: Roger Williams For Release: January 31, 2016
Extension Ed., Ag/4-H
Cherokee County
456-6163
Using Dormant & Horticultural Oil
Various types of oils have been used for many years to control insects like aphids, scale and spider mites on fruit trees, providing good control of these insects with little risk to the plant, beneficial insects or the applicator. Oils kill insects in several ways, most importantly by blocking the air holes through which the insects breathe, causing them to suffocate. Oils may also act as poisons by interfering with insect metabolism or to disrupt insect feeding. They are also effective against powdery mildew and some aphid-transmitted viruses. In fact, oils can easily be worked into an organic spray program for fruit trees as well as small fruit plants, vegetables, flowers, tree and shrubs.
Before the development of current oil refining techniques, oils were used primarily on fruit trees during the dormant season, before the spring bud break. The application was timed to kill insects over-wintering on the trees but also to avoid burning or damaging the leaves, which frequently happened with heavier, less highly refined oils. These oil products could not be used on plants during the growing season due to their phytotoxic effects. For this reason, the term ‘dormant oil’ was coined. Many oil products currently available are more highly refined and the leaf damaging components including sulfur have been largely removed.
The EPA’s decision to ban the use of dursban, diazinon, and lindane pesticides has prompted many homeowners to resume the use of dormant oil. This is a good practice but there are precautions that should be considered. Dormant oil needs at least 10-12 hours to dry. If it freezes before the oil is dry it can severely damage the tree. The standard recommendation for dormant oil is that the temperature not be forecast to drop below 40o that night. I received a call from a lady last spring whose peach trees had died from a spraying of dormant oil. The day that she sprayed dormant oil the temperature dropped to 30o that night.
Oil sprays also speed up spring bud development and reduce the ability of buds to withstand cold temperatures. For species that tend to freeze out in this area like peach and apricot it is best to delay dormant oil sprays until the air temperature and bud development are at the proper stage.
Spray coverage is very important in oil sprays. There is no residual chemical activity so the trees must be sprayed with an adequate amount of prepared spray mixture to cover all the tree parts.
Usually a 15x15 size tree will require 3 gallons of spray but a poorly pruned tree of the same size may take twice as much to get adequate coverage.
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