In horticulture, dormant oil is an inaccurate term today. It originally referred to less refined oils that were unsafe to use on plants after they broke dormancy. However, these older oils have been replaced with more refined, light-weight oils that have potential application to plant foliage.
Horticultural oil is a more correct term. The concentration of oil in your spray, and time of application, determine whether it is being used as a dormant oil. There are still some differences in the quality of horticultural oils so follow the label of the product you are using.
Phytotoxicity (damage to leaves) can occur if the wrong product or wrong concentration is used on actively growing plants.
Some people may view horticultural oil as a less effective alternative to conventional insecticides. Horticultural oil is frequently used as a primary insecticide in organic orchards. However, its value as a pest control tool can stand on its own merits regardless of what other treatment measures you choose to employ.
Horticultural oils work by smothering insect and mite pests/eggs, and in breaking down their protective coatings, causing them to desiccate (dry out). Thorough coverage of trees is critical. Cracks and crevasses in the bark and around buds deserve special attention. Dormant and delayed-dormant applications are typically applied at a rate of 2%, which is five tablespoons in one gallon of water. If you are not growing organically, an insecticide can be added for residual control. When spraying apple trees, add in copper for fire blight, if this has been a problem in past years. Oil should be applied on a clear, non-windy day in the 50 to 70°F temperature range.
The timing at which you can effectively use “dormant” oil concentrations (2%) varies amongst fruit varieties. Generally, you want to target the window between bud swell and bud break. By the time you read this I suspect most apricots will be out of “dormant” oil timing. The last point at which you can safely apply oil for each crop is listed below. For images of what the development stages look like, see “Cold Effects On Fruit” at https://intermountainfruit.org.
Species — Growth Stage
Apple — Half-inch green
Apricot — Before first bloom
Cherry — White bud
Pear — Green Cluster
Peach — Before first bloom
Plum — Green Cluster
There are two big benefits to incorporating horticultural oils into your orchard pest management plans. First, they are less damaging to beneficial insects and can reduce reliance on other insecticides later in the season.
Second, no insect or mite pests have developed resistance to horticultural oil despite their being in use for many years.