Time to Apply Dormant Oils for Pest Control

Although you may not see them, or be thinking of them right now, there are insects overwintering in your garden. A good way to nip a potential insect problem in the bud, so to speak, is with the application of a good dormant oil now.

Horticultural oils have been around for many years and are a safe way to manage pests and diseases. They are a refined petroleum product and may also include a few vegetable based oils as well which are also effective pesticides such as cottonseed and soybean oil. The newer oils are more highly refined than those used in the past and many of them can now be used year round, but the first application in the spring will help to greatly control any problems down the road. They were first widely used in orchards as a way to control pest issues on fruit trees, mainly because they are so safe to use, particularly on edibles.

Dormant oils are recommended for insect control in home orchards and for all deciduous trees, shrubs, and rose bushes. They are best applied when temperatures are over 50 degrees for at least 24 hours and preferably at a time when it will not be freezing for three to four days following application. As the name implies, dormant spraying needs to be done while the plant is dormant, so you want to apply before buds begin to swell in the spring.

Dormant oils kill pests by smothering them because the oil blocks their breathing, which is why you’ll need to choose a warm day. The insects don’t have to necessarily be active yet- just breathing, in order for this to work well. The oil will also smother any insect eggs that are overwintering on your plants and works best on young, soft or slow moving insects which is why you want to do this early in the season. Oil can also work by disrupting the way an insect feeds which is helpful if you have an outbreak of aphids, for instance, since they are notorious for transmitting diseases from one plant to another.

Oil is combined with an emulsifier so that it can be mixed with water and sprayed onto your plants. Once applied liberally, it virtually suffocates all the common sucking insects such as aphids, scale, red spider mites, thrips and others that have overwintered on trees, shrub buds, twigs and bark. Since it also smothers fungal spores, diseases such as powdery mildew, apple scab, apple blotch, rust and peach leaf curl, to name a few, can all be eliminated with early spraying.

Although it’s very safe and fairly easy to use, begin with a good quality oil and follow all label directions because horticultural oil can cause damage if you don’t follow the directions. For instance, oil can smother beneficial insects too, so, as with any pesticide, you want to be sure to use it only when and where necessary. It can also cause foliar burn on evergreens which is why you need to be careful using it. Apply to all branches, trunks, and the ground around the base of the plant to the point of run off. The excess evaporates and dissipates quickly so there is no residue. Most horticultural oils are considered safe for use around pets and on edibles when the directions are followed.

Do not apply horticultural oils when temperatures are cold or freezing. The emulsifier usually will not hold the solution together and coverage will be spotty at best. It’s a waste of your time and money to do it then. Also avoid applying oil when your plants are wet. Maples, junipers, redbuds, spruce, firs and some others all tend to be particularly sensitive to these oils so do not apply oil on a windy day if any of these are nearby. The drifted oil carried by the wind can cause quite a bit of harm.

Since we had such a mild winter this year, overall, I think that the bug populations are going to be quite bad this summer. Aphids, especially, seem to be prolific as their numbers were not killed by prolonged cold weather. Although you can use many of the oils year round, a good application of dormant oil now could be extremely beneficial in knocking back some of the infestations for your home orchard and other plants come the summer season.