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. And in case you wondered, yes, dormant oil is accepted as an organic treatment.

Dormant oil is designed for application on deciduous woody plants: trees, shrubs, conifers, vines … any plant, in fact, that looses its leaves in fall and overwinters above the ground. This product is especially popular with fruit growers (such deciduous fruit trees and shrubs as apples, pears, plums, cherries, nectarines, currants and gooseberries), but that’s simply because fruit trees have more than their share of pests that need to be controlled. It’s also probably the best treatment for magnolia scale.

Dormant oil acts on insects and mites that overwinter on branches and bark, either as adults, pupae or eggs, a category 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.

Dormant oil has little to no impact on beneficial insects, such as bees, most of which are simply not out and about at treatment time. It can negatively impact, unfortunately, certain predatory insects that help control scale insects and mealybugs, so you’ll have to judge for yourself whether or not to treat trees harboring such pests. 

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, an organic fungicide. Both products are compatible and, indeed, 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. 

There are also copper-based fungicides, also considered organic, that can be mixed with dormant oil (but not with lime sulfur). Read the fungicide label to be sure such a use is recommended. 

How It Works

Dormant oil is nontoxic; 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. That includes such pests as apple maggots and white grubs. (To help control apple maggots, pick up and destroy fallen fruits.) Note too that the residual effect of dormant oil is minimal: it degrades and disappears after just a few days, so it won’t prevent insects, only eliminate those that are already present.

Timing Is Everything

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’s 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 can be thick enough to smother tender young leaves. You can apply it when the buds are swollen and ready to open, but not when they are showing any green.

Application

Dormant oil and dormant oil/lime sulfur blends are designed to be sprayed. 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 if you’re spraying dormant oil alone. That means you can use it as is, eliminating the need for mixing it with water beforehand.

And of course, you can also hire someone to spray your trees (yep, the ideal approach for the richer laidback gardener!). Most tree specialists offer this service. Just make sure they’re not getting overly ambitious: you only want them to spray trees and shrubs that have problems worth treating, therefore, more likely your fruit trees than your ornamental ones.

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. 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 (if you spray early enough, they’ll still be dormant and that won’t be necessary).

Also, dormant oil will unfortunately 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.

Restrictions

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 mensiesii), and arborvitaes (Thuja spp.).

Although widely used on fruit trees and small fruits, there are a few apple varieties, including ‘Empire’, ‘Mutsu’ and ‘Red Delicious’, that are sensitive to dormant oil.

Lime sulfur 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!

Article originally published on March 31, 2015.