Dormant Sprays: Why, How, and What

Few pesticidal applications are as misunderstood as the dormant spray. For instance, even after more than a decade in the industry, I still have customers that believe that dormant sprays will keep their fruit from getting "wormy." Nowadays we explain to new customers what the spray will do when they order it so that there won't be any misunderstandings.

So what does a dormant spray do? Dormant applications target insects and disease during their dormant period, most often late in the winter when insect and mite eggs are at their most vulnerable. Insects, mites, and their eggs are smothered and suffocated by the physical action of the oil. Fungal spores and bacteria are inhibited from developing into full-scale infections by the copper-based part of the solution. Only certain pests - such as aphid - overwinter on the tree itself, thus limiting the spectrum of affected organisms. For this reason, a dormant spray will not be effective against all insects, mites, or diseases.

What comprises the usual dormant spray mix? Generally, a dormant spray will include a mineral oil and a copper- or lime sulfur-based fungicide. Because oil and sulfur mixes are more likely to lead to injury to the target plant, I recommend sticking with a mineral oil and a copper-based fungicide.

When is the best time to treat? In the past, dormant sprays were applied before bud break to avoid burning the foliage. In recent years, a new generation of horticultural oils has emerged with less potential for injury. Some oils may be sprayed at temperatures as high as eighty degrees without ill effect to the tree. So, as long as you aren't using lime sulfur - I told you not to, darnit - it is pretty much impossible to be too late. Of course, if you wait until the eggs have hatched and the target insects and mites have had a good feeding, they will be more robust. So aim for the narrow window between bud swell and bud break - that's when they'll be at their weakest, because they have exhausted their food stores for the winter. However, horticultural oils will smother adults when applied properly, so if you go on vacation and forget to treat your trees, don't fret - just spray when you get back.

Some caveats: don't expect any activity against diseases that winter over as spores away from the plant or don't attack the plant until late in the season. Also, be sure that any diseases particular to your situation are on the label of whichever fungicide you buy; some will not be adequately controlled by copper-based fungicides. Scale insects are mostly invulnerable to dormant applications, because the eggs are sheltered from the oil by the deceased mother's body; you'll need to treat again when the young emerge, which can be months later for some species of scale.

Lastly, know that you need not limit yourself to treating your home orchard. You can treat your roses, as many gardeners do, and indeed, most of the plants in the average landscape. Follow the label, and for those with generic guidance, use this rule of thumb: if it's shiny, waxy, or exotic, don't spray it. Plants with natural protective coatings - such as blue spruce or oregon grape - don't need the help anyway.