Dormant oil, which is applied to trees and shrubs to protect trees from over-wintering pests and larvae, should be sprayed either before or just after the buds begin to swell. Conversely, it should not be sprayed when trees are in full bloom. Dormant oil is also called scale emulsion.
When spraying dormant oil, it is important to maximize the substance's coverage on the tree or shrub. Consequently, it should not be sprayed when temperatures are below 40 degrees Fahrenheit. Applying the dormant oil in colder temperatures prevents the oil from spreading and entering all the tree's crevices. If possible, try to select a time when the temperature is expected to stay above the 40 F threshold for several days after application.