Dormant spray
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Martha Proctor
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Dormant spraying a fruit tree in a West Marin garden. Photo: Martha Proctor
Dormant sprays, when mixed with water, are highly refined petroleum products or vegetable oils used for applying pesticides to plants and trees in their dormant stage after all or most of their leaves have fallen. Thorough application and coverage are essential as many insects prefer the underside of leaves or reside under sepals or buds that are hard to reach. Don't spray unless you have confirmed that the insects are present in damaging numbers and cannot be controlled by any other means. Wear protective gear, long sleeves, and long pants when spraying. Don't swallow, inhale, or expose your skin and eyes to the oil.
Proper timing is critical. Dormant oils should be applied at the beginning of dormancy in late November and again during leaf bud swell, just before buds begin to open in February or early March. The timing coincides with the increasing activity of the larvae of overwintering insects. These oils are temperature sensitive, so apply on a clear, non-windy day in the 50oF to 70oF temperature range when the temperature is not expected to drop to freezing for 24 hours after application.
A good time to apply dormant oil is right after a period of rain or foggy weather. Do not apply to drought-stressed trees or to any tree during fog, rain, or during or prior to hot or freezing weather. If it rains within hours of application, reapply the treatment. Do not treat within 30 days before or after applications of sulfur or certain other fungicides to avoid damage to trees. Dormant oil applications are not appropriate on citrus or avocado trees as these species do not enter a winter dormancy.
Raking up dead leaves to avoid overwintering pests. Photo: Martha ProctorDormant sprays should only be used in conjunction with good garden sanitation. Incorporating good cultural practices in your garden maintenance program will set the stage for more successful pest management. Rake and dispose of fallen leaves around your fruit tree in the green can. Space tree plantings and prune to provide good air circulation and light penetration. Avoid overhead irrigation; plant fruit trees in full sun.
By applying dormant oils annually as directed to well-maintained fruit trees, you will help protect your trees from several overwintering insects and prevent damage to flower and foliage tissue. Sanitation and dormant sprays are the "ounce of prevention" that will save you a pound of cure next spring. If appropriately used, horticultural oils can be a less toxic approach to controlling many overwintering pests than chemically based insecticides or fungicides.