Horticultural oil is an ideal remedy to deter overwintering pests. The best time to apply it to woody plants and shrubs is before the buds break and leaves open but you also want the little pests to be active, not hiding, so wait until temperatures warm up to above zero for at least 24 hours.
Why apply Dormant oil (aka Horticultural oil)? reduce overwintering pests such as aphids, mites and scale on fruit trees, deciduous trees and ornamental woody shrubs.
What is Dormant oil – An oil used on woody plants during the dormant season. This term originally referred to heavier weight, less well-refined oils that were unsafe to use on buds or foliage. However, these oils have been replaced with more refined, light-weight horticultural oils and the term ‘Dormant oil’ now refers to the time of application rather than to any characteristic type of oil.
When to apply – Be sure the temperature will remain above freezing for at least 24 hours and there isn’t any precipitation in the forecast. Spray on a dry, sunny morning to facilitate fast drying.
Some pests are resistant – some overwinter as tolerant adults or eggs protected under the dead females: euonymus scale, oystershell scale, pine needle scale, golden oak scale, two spotted spider mites.
Precautions – oils can stain siding or painted surfaces. Do not apply to broadleaved evergreens when freezing temperature may be expected within 3 weeks after application. Avoid spraying during or immediately prior to hot weather (over 30C). Do not apply if frost is expected before spray dries completely. Avoid contact with plants that tend to be sensitive such as:
- Black walnut
- Junipers and cedars
- Maples (particularly Japanese and red maple)
- Smoke tree
- Spruce (particularly dwarf Alberta spruce)
- White Pine
Colorado State University – Insect Control via Hort. Oils http://www.ext.colostate.edu/pubs/insect/05569.html
University of Nebraska – Applications and use of Dormant Oil http://byf.unl.edu/Dormantoil
University of Missouri – Dormant Oil application to Fruit Trees