An old song goes, “April showers bring May flowers.” However, spring brings much more, including some pests you don’t want.

Many insects, especially aphids, mites and scale, overwinter on trees. When spring arrives, buds break and green leaves unfurl. That’s also a signal for overwintering insects to wake up and begin chomping on the new leaves. They don’t have to wake up. They can be smothered in their sleep with a product called dormant oil.

I like dormant oil. It’s easy to apply, it’s effective and it has very little environmental impact. As soon as the temperature rises above freezing for awhile, I apply dormant oil to the trees and shrubs in my yard. Because of its low environmental impact, I don’t have to closely examine whether my trees have aphids or other overwintering insects. If a tree or shrub had any of these pests last year, I can simply spray them this year, confident that I am not harming the environment. These would be very definite concerns with more aggressive pesticides.

Dormant oil is one in a family of products called horticultural oils. These products are highly refined petroleum jelly like you use on burns. They are applied to coat the insects and smother them. Dormant oil is the thickest, and the most effective. Due to its thickness, it has to be applied while the both the pest and the tree are still in winter dormancy. It cannot be applied when the tree has leafed out and it cannot be applied when the air is too cold. If you spray it when it is too cold, it will coagulate and get all sticky. If you spray it after the tree leafs out, the coating of oil can interfere with the tree’s respiration and kill it, right along with the bugs. There is definitely a small window of opportunity, but one that is worth taking advantage of.

Due to the warm weather we’ve seen this winter, our plant health care professionals have already begun applying dormant oil but it’s not too late to make applications until the leaf buds break.

Other horticultural oils can be applied after the leaves are out. These oils may be called summer oil, superior or just horticultural oil. It is more highly refined than dormant oil, which means it is thinner and won’t damage leaves. Like dormant oil, however, these oils cover insects and smother them.