Dormant Oil is a known application that is sprayed on landscape trees and shrubs in fall to help protect them from overwinter damage from insects. Prepared from a mixture of highly refined petroleum oils combined with an emulsifying agent, dormant oil can be mixed with water and sprayed on trees killing exposed insects and mites by either suffocating them or destroying internal cells. These oils are effective in controlling most species of scales and mites that overwinter as nymphs or adults such as cottony maple scale, obscure scale, euonymus scale and lecanium scale which can do early damage. Dormant Oil is also effective on insect eggs that are laid in September and overwinter such as aphids, leafrollers, Spruce Spider Mite, Honeylocust Mite and European Red Mite. Dormant Oils are effective for both immediate and preventive care and have been developed to be less harmful to beneficial insects. They are also safe for birds, humans and other mammals.
When to Apply: Dormant oil applications should be applied in fall when temperatures remain in the 50’s at night (Early October) and must be done when temperatures stay above freezing for 24 hours. Dormant Oil is also best applied on a clear day with no wind and no threat of rain within a least six hours so that the oil can dry. It is preferable to choose a time when no rain is in the forecast for a few days to ensure effectiveness.
Warnings: Be sure to follow all label directions because oil sprays may damage certain plants, including Japanese maple, Eastern Redbud, sugar maple and Amur maple. It can also cause the foliage (needles) of Colorado blue spruce to become discolored (change from blue to green) since the pigment is formed from the oils on the surface of the plant. When in doubt consult with your landscape professional.
Dormant Oil when applied correctly will help to protect your trees and shrubs from winter and early spring harm from a variety of damaging insects and will ensure the health and vitality of your landscaping.
For more information on dormant oil application visit: Colorado State University horticulture Insect Control
As Always..Happy Gardening!
I use it no more now that plants got too big, but it is helpful when plants are newly planted and very tasty to insects.