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What Are Dormant Oils?

What Are Dormant Oils?

Distilling petroleum to 92 to 99 percent purity creates oils and removes any phytotoxic compounds such as aromatic compounds and compounds containing sulfur, nitrogen, or oxygen. Since oil and water do not mix, adding an emulsifier allows mixing horticultural oils with water for application. Typical dormant oil solutions are at a two percent oil concentration. Below are various terms for oils used to control insects currently on the market.

  • Dormant Oils – These less refined oils are sprayed on trees in the winter to control insects but not safe to use on plants after leaf-out.
  • Spray Oils – A oil with emulsifiers mixed with water to be applied to trees for insect control.
  • Summer Oils – Horticultural oil sprayed on trees during the spring or summer. These oils are formulated not to burn or harm foliage.
  • Supreme or Superior Oils – These highly distilled oils contain paraffinic hydrocarbons, allowing use on trees and plants with foliage. Sometimes these products are called narrow range or summer oils.
  • Vegetable Oils – Some horticultural oils come from Soybean, Cottonseed, and Sesame seeds. Cottonseed oil is the most insecticidal of the vegetable oils. However, plant-based oils are less refined and may have some phytotoxicity.

The oil suffocates tree insects’ eggs, larvae, pupae, and adults by clogging their breathing tubes. Dormant oils dissipate quickly by evaporation, leaving little residue or toxicity. In the winter, dormant sprays kill overwintering insects and their exposed eggs, larvae, and pupae.

The Advantages and Disadvantages of Using Dormant Oils

Dormant oils show low toxicity to humans and wildlife.

  • Since the oils evaporate quickly, they are less disruptive to beneficial insect populations than chemical insecticides.
  • Since these oils suffocate insects, they cannot develop resistance.
  • Do not apply when below freezing because the emulsion breaks down, causing uneven coverage.
  • Do not apply oils in wet conditions because the additional moisture inhibits oil evaporation.
  • Do not apply dormant oils until winter hardening has occurred. Fall treatments sometimes increase susceptibility to winter injury.
  • Do not apply oils in combination with sulfur-containing pesticides because they form phytotoxic compounds.

The following trees are sensitive to dormant oil applications.

  • Black walnut
  • Dwarf Conifers
  • Douglas Firs
  • Junipers and Cedars
  • Some Maples
  • Redbud
  • Smoke tree
  • Spruce

For a Fall Dormant Oil Application:

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