Dormant oil, also called dormant oil spray and dormant spray, is a horticultural oil normally applied in the spring at snow melt, somewhere between late January and mid-May, depending on where you live. You can also use it in the fall, after leaf drop. It’s more viscous than the lighter summer horticultural oils, yet commercial dormant oils still contain an emulsifier, so will nevertheless mix quite readily with water, making application easy. Dormant oil is designed for application to woody plants: trees, shrubs, conifers, vines… every plant, in fact, that overwinters above the ground. This product is especially popular with fruit growers, but that’s simply because fruit trees have more than their share of pests that need to be controlled.
Dormant oil acts on insects and mites that overwinter on branches and bark, either as adults, pupae or eggs, a group that includes many aphids, mites, mealybugs, and scale insects. It works by clogging the pest’s breathing pores, leading to their rapid demise. Also, dormant oil melts away the outer waxy covering that protects some insects (especially scale insects and mealybugs), killing them by exposing their fragile bodies to dry air.
Although dormant oil is essentially an insecticide, it does have some effect on fungal diseases (powdery mildew, black spot, sooty blotch, etc.), as it can also destroy spores that overwinter on treated trees and shrubs. But for a more efficient fungicide effect that will control a wider range of diseases, dormant oil is usually combined with lime sulfur, a biological fungicide. Both products are compatible and often sold together. They can be mixed and applied as a single spray, but do be aware that they won’t remain in solution for a very long time, especially not lime sulfur. You’ll need to shake the applicator tank every now and then when you’re spraying.
Dormant oil is non-toxic; its action is strictly physical. It is therefore harmless to humans, mammals, birds and fish when used correctly. To be effective against insects, however, it must actually coat them: it will not be effective against insects that overwinter on the roots or elsewhere underground, nor in leaf litter. This would include pests such as apple maggot and white grubs. Note too that the residual effect of dormant is minimal: it degrades and disappears after just a few days, so it won’t prevent insects, only eliminate those that are already present.
Before applying dormant oil, wait for a dry, fairly breezeless day when it’s above freezing (ideally between 40 and 80˚F/4 and 27˚C), when tree and shrub branches and trunks are dry (therefore, no rain the day before) and when there is no risk of frost or rain for at least 24 hours and, preferably, 48 hours. It is also better to apply it in the morning, after any dew has evaporated, so that the oil has time to dry before the dew appears the next morning!
If this product is called dormant oil, it’s because it is designed to be applied to dormant plants, before their buds have started to open, because dormant oil is thick enough to damage tender young leaves. You can apply it when the buds are swollen and ready to open, but not when they are showing any green.
Dormant oil and dormant oil/lime sulfur blends are designed to be sprayed on to plants by spraying. Just mix the product with water according to the proportions indicated on the container and shake thoroughly. You’ll find a wide range of appropriate pump sprayers in any garden center or hardware store. Mix only what you will need: you can’t store the diluted product for further use. You can also use a hose-end sprayer which means you can use it as is, eliminating the need for mixing dormant oil with water beforehand.
Even though dormant oil is non-toxic, you should use this product only when needed and even then directly on the affected plant only, not on nearby vegetation. That’s because it can also kill certain beneficial insects. Any plants that are already growing or blooming at the base of the trees or shrubs you are spraying could be damaged, so cover them with a plastic sheet or tarp (if you spray early enough, they’ll still be dormant and that won’t be necessary). Also, dormant oil will kill mosses and lichens, harmless residents on the trunks of trees and shrubs that any environmentally aware gardener will want to preserve, another reason not to use dormant spray indiscriminately. I spray only my fruit trees and shrubs: I grow no ornamental woody plants that have serious enough pest problems to merit this kind of treatment.
Dormant oil can be safely used on most plants as long as you follow the conditions explained above, but some plants are sensitive to oils in general and should not be treated. This group includes smoke bushes (Cotinus spp), hickories (Carya spp.), red oaks (Quercus rubra), sugar maples (Acer saccharum), Japanese maples (A. palmatum), red maples (A. rubrum), Amur maples (A. tataricum ginnala), beeches (Fagus spp.), evergreen hollies (Ilex spp.), yews (Taxus spp.), walnuts (Juglans spp.), white pines (Pinus strobus), Douglas firs (Pseudotsuga meniesii), and arborvitaes (Thuja spp.). It is widely used on fruit trees and bushes, but there are a few apple trees, including ‘Empire’, ‘Mitsou’ and ‘Red Delicious’, that are sensitive to it. Lime sulphur can be harmful certain fruit trees as well: read the label before applying it.
Note too that dormant oil (and indeed any oil or soap) will melt away the white waxy coating that gives certain conifers their bluish tinge, so if you want your blue spruces (Picea spp.) and blue junipers (Juniperus spp.) to remain a true blue, spray them with something other than oil or soap.
Enjoy this spring’s spraying session!