Horticultural Oil and Dormant Oil for Pest Control

Horticultural oils are oil-based products that are used to control pests and diseases on plants, especially trees and shrubs. These oils are typically mineral oil-based, although organic alternatives are widely available. The oils are refined to remove any impurities or toxins that could damage plants, then mixed with an emulsifier so they can be easily diluted and sprayed.

Horticultural oils became popular initially as a way to control destructive insects on fruit trees. Horticultural oils work very differently from systemic insecticides: the oil is sprayed so that it coats the plant and any pests on it, smothering the insects. The oils can be easily washed off, keeping fruits and flowers toxin-free.

Years ago, there were two different types of oils, a heavier oil, known as “dormant oil” that was sprayed in the dormant season to kill pests that may be wintering on plants, like aphids and scale. This dormant oil was heavier than other horticultural oils, and couldn’t be sprayed during the growing season. A “summer oil” was lighter, and could be sprayed without fear of damaging foliage. Today, all horticultural oils are lighter, and the terms “summer oil” and “dormant oil” refer to the time of spraying, rather than the formulation.

While there are many different types of horticultural oils available, they all work essentially the same way. They suffocate both insects and their eggs, or simply block the insect’s ability to feed. Oils are particularly effective in controlling:

  • Woolly adelgid on hemlocks
  • Mites – especially conifer-infesting ones – that winter on the plant
  • Scale insects like pine needle scale, striped pine scale, and cottony maple scale
  • Aphids
  • Whiteflies

…and many others. It’s also effective at controlling powdery mildew and diseases spread by aphids and other insects.

One of the reasons people really love horticultural oil is that – when used properly – it is safe for most plants and for use around animals, although it is harmful to fish. Overall, horticultural oils are a great and environmentally-friendly way to maintain your garden.

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