When are dormant oils most effectively used on shrubs and trees?

Dormant oils are most effectively used on shrubs and trees during the period when the plants have entered full dormancy. This timing is crucial for several reasons. Dormant oils are designed to suffocate pests and their eggs that are residing on the surface of the plant. When applied during dormancy, the oils can effectively coat these pests without interfering with plant growth or function.

Applying dormant oils when the plants are dormant helps to ensure that the oils can penetrate and cover the target pests thoroughly. During this time, the risks of phyto-toxicity to the plants are significantly reduced since the plants are not actively growing and are less stressed. Additionally, the absence of leaves means there are fewer barriers for the oil to reach the pests that may be hiding on the bark or in crevices.

Using dormant oils at other times, such as when growth begins or during active growth periods, may result in ineffective pest control. When growth resumes in early spring, there is increased sap flow and foliage which can hinder the effectiveness of the oil. Additionally, applying oils when pests are first observed may not address the problem effectively if part of the pest population has already developed.

Thus, the best practice for using dormant oils on shrubs and trees is to apply them during full