Should You Use Dormant Spray This Winter?

Are you contemplating using a dormant spray on your plants and trees this winter? If so, you’re certainly not alone. Dormant sprays seem to have gained a reputation, even if ill-founded, for being the great eliminator of all insects.

What are dormant sprays, what do they control and when are they applied?

A dormant spray is a pesticide applied to plants while they are sleeping - physiologically inactive - as they are now. The pesticide may be in the form of a fungicide, an oil or a mixture of an oil and an insecticide.

Fungicide sprays control certain fungus diseases, such as peach leaf curl and powdery mildew.

Oil sprays are used to smother the hard-to-control scale insects. It would be wise to familiarize yourself with these critters. The more common ones are pine needle scale and juniper scale, which take on the appearance of dandruff or droplets of white paint. San Jose scale and oyster shell scale resemble rough bark, while bud scale and lecanium scale look like swollen plant buds. Oil also smothers the overwintering eggs of aphids and spider mites. It is a misconception that dormant oil will control the nasty worms that bore into our apples and cherries - the codling moth and the cherry fruit fly. These insects must be controlled with summer sprays, not dormant sprays.

The third type of dormant spray is a mixture of oil and insecticide. These are applied in late winter. While the oil smothers, the insecticide zaps the awakening pests.

It is important to realize not all sleeping insects and their eggs are overwintering in trees and shrubs.

Many are in the soil where they cannot be controlled with dormant sprays.

Since many insects are not controlled with dormant sprays, and certain plants cannot tolerate oils, blanket spraying is not recommended.

Always read the label of any pesticide. The label will inform us which plants may or may not be sprayed. For example, oil should not be applied to blue spruce; it will discolor the tree. And oil should not be applied to Japanese maples or maidenhair ferns. When freezing or near-freezing temperatures are expected, oil should never be applied to conifers or broadleaf evergreens.

When do we apply them? This is where it gets a little ticklish for people in our area. The oil we are most familiar with is a highly refined horticultural petroleum oil. It is identified as superior oil, volck oil or ultra-fine oil. A true dormant spray is applied in the dead of winter. However, oil should not be applied when the day temperatures are below 35 degrees. Freezing temperatures combined with oil may burn conifers and broadleaf evergreens. If it is used on bright sunny days with temperatures above 35 degrees, the oil must dry on the plants before nightfall, when temperatures again dip below freezing.

For these reasons, delayed dormant sprays are most often recommended for our area. The oil is applied just as the buds begin to break open. This is usually sometime in March or April. At this time, certain insecticides can be mixed with the oil to control awakening insects. Again, it is important to read the label. Not all insecticides can be combined with oil, such as Sevin or sulfur. In fact, oil cannot be used 30 days before or after the use of sulfur. Nor should it be used 90 days before or 60 days after the use of the fungicide Captan., Read the label, read the label, read the label.

When applying the spray, make sure to apply enough chemical to cover the tree or shrub completely. Spray the top half first. Thoroughly spray the trunk, branches, fruit and both sides of the leaves, just to the point of runoff. If you don’t feel comfortable handling pesticides, reputable spray companies can easily do the job for you.

In our area dormant sprays are often over-emphasized and overused.

Good pesticide management calls for not introducing pesticides into our environment unless they’re needed. If your plants have been subjected to certain diseases or infestations of aphids, mites, or scale, then dormant sprays can be a valuable tool. If they haven’t, quite likely dormant sprays are not necessary.