Although fruit trees are not actively growing in the wintertime, this is an excellent time to do some maintenance to improve fruit production in the coming growing season. One of these maintenance practices is to apply a dormant oil spray. A dormant oil spray is just as it sounds – spraying oil on a tree while it is dormant.
First, why do this? Because it is very useful for the control of overwintering scale and mite eggs. The oils used for these applications are highly refined oils (not motor oils) which spread uniformly on the bark of trees to which it is applied and coat non-mobile, dormant insects on the tree smothering them to death.
It is best to spray before buds begin to swell. If buds of trees and shrubs have begun to swell slightly, go ahead and spray. Although some of the buds may be damaged, the benefits of spraying dormant oil far outweigh the possible repercussions.
Do not spray trees which are in full bloom. Application can be made on apples as late as the half-inch green stage, which is when new growth from buds is one half of an inch long and on pears as late as the green cluster bud stage, which is when buds are in cluster before they open. Oil should be applied to stone fruit trees, such as peach and cherry, while they are still dormant.
Do not apply oil when the temperature is below 35 degrees Fahrenheit, or predicted to drop below 35 Degrees Fahrenheit within 24 hours.
The ideal temperatures for application is between 40 and 70 degrees Fahrenheit in order to get the oil to spread out over the tree and cover all crooks and crevices.
Oil sprays can also be very useful for pest control on many ornamentals, particularly scale insects like Magnolia scale. Scale insects look like tiny oyster shells attached to the tree branches. These sap sucking insects can weaken a tree significantly if left unchecked.
If you determine you have scale insects on a deciduous tree, it would be helpful to identify which scale insect you have before applying the dormant oil. With certain scale insects, the susceptible stages of the species are not present during the winter. For those species, the application is more effective after the buds swell in the spring, but before they open.
Remember that oil sprays are for the dormant season only, as they will damage green foliage. The exception would be a superior oil that is designed specifically for application when leaves are on.