Answer: Dormant oil is an extremely refined oil, intended for use on dormant outdoor plants to smother overwintering pests, insect eggs, and fungal spores. An even lighter weight oil, called superior oil or summer oil, is sold for use during the growing season to control mites and scale on citrus, and powdery mildew on roses, among other uses. It's an important distinction because a heavy application of oil when temperatures are high can cause burning of plant tissues. These refined oils can be from petroleum or from plant oils, depending upon the manufacturer.
Be sure to read the labels on your dormant oils for warnings of phytotoxicity on selected plants, and follow application instructions carefully. Ferns are particularly sensitive to oil applications and conifers can be burned by an improper application. Spraying on hot or humid days can also cause phytotoxic reactions especially if the plants are under water stress.
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