Horticultural oils are oil-based products used to control pests and diseases. Most horticultural oils available to home gardeners are petroleum-derived, but you can find plant-based oils, too. Manufacturers refine the oils to remove impurities damaging to plants, and then combine the oils with emulsifiers so the end product can be mixed with water and sprayed on plants. Horticultural oils are available at home-improvement and garden centers and in catalogs. Look for types approved by the Organic Materials Review Institute (OMRI).
Gardeners use horticultural oils on woody ornamentals and trees to kill scale, aphids, mites, and other soft-bodied insects. Horticultural oils are nonselective, meaning they kill both pests and susceptible beneficial insects (predatory mites, for example). The oils work by smothering the insects or their eggs, poisoning them, or disrupting the way some insects (generally aphids) feed. The pest must be thoroughly coated with the oil in order for it to work. It has no effect after it dries on the plant. Horticultural oils may also control powdery mildew and prevent the spread of plant viruses transmitted by aphids.
In the past, horticultural oils were heavier and used only when plants were not in their growing season, so they were called dormant oils. The old oils effectively managed pests that overwintered on woody plants, but they were too heavy to be sprayed on plants with foliage. The term dormant oils is still used, though now it generally refers to the application time of the oils. New, lighter formulations allow you to use oils during the growing season. These products, sometimes called summer, superior, or supreme oils, can control spider mites and whiteflies.
Horticultural oils are safe to use around mammals, birds, and reptiles but are toxic to fish. Follow package directions carefully. Applying oil in freezing conditions or extreme heat, or during drought or shoot growth, can injure your plants. Some plants, including maples, junipers, cedars, and spruce, are sensitive to these oils. Be cautious when using horticultural oils, and apply them only as a last resort. You could potentially damage your plants and beneficial insect populations.