Dormant oil spray refers to the condition of the trees, rather than to the spray itself, the reason being that trees must be dormant before applying the oil. This is to avoid damaging the foliage of fully-leafed out trees (it may turn the edges of leaves black or even kill young leaves), but also to deal with insect eggs before they hatch.
To spray trees, make sure the temperature will stay above zero (about five degrees C is preferred) for at least a 24-hour period. Look for a windless, dry, sunny morning to apply the spray. From about now and until about 14 days before bud break would be an ideal time.
Dormant oil spray is generally made from highly refined petroleum based products but sometimes from vegetable oils. Both are equally effective and have little impact on the environment.
Dormant oil sprays have been used for hundreds of years as a defence against soft-bodied insects and the eggs of insects. They work by plugging breathing holes (spiracles) and suffocating the insects. They also penetrate egg and soft-bodied insects, dissolving their insides. Dormant oil can also help prevent powdery mildew.
The spray is effective against spider and other mites, scales, adelgids and any eggs laid by other insects, such as leafhoppers, mealybug, whitefly and the odd predatory thrip in the nooks and crannies of the tree branches. It is not useful in dealing with white grubs, cabbage worms or apple maggot, all of which overwinter in the soil.
It is safe for birds, pets and humans and does not affect beneficial insects such as bees, spiders and ladybugs.
On the downside, dormant oil spray has little residual effectiveness, as it eventually washes and wears away. Some gardeners will apply a diluted solution a little later into the season. There are some commercial sprays constituted for just this purpose sold under names such as horticultural oil, ultra-fine oil and summer oil.
Don’t use dormant oil spray on amur on Japanese or amur maple, black walnut or beech. It will turn Colorado blue spruce needles green. Check the label before using.
Homemade dormant oil spray
2 Tablespoons sunflower seed oil (or other light oil)
2 Tablespoons baking soda
5 Tablespoons hydrogen peroxide
1 teaspoon dish detergent
1 gallon water
Combine the soap and oil, then add the water slowly, stirring as you go. Add the peroxide and baking soda. Shake well. One gallon of the mixture should treat on fruit tree.
In this recipe, the baking soda and peroxide act against powdery mildew.
Dorothy Dobbie is the publisher of the Manitoba Gardener magazine. She broadcasts a weekly radio show on CJNU 93.7 FM. Call 204-940-2700 or go the localgardenr.net for a subscription