Dormant Oils: An Eco-friendly Strike Against Pests

Oils are often recommended as a “clean slate” treatment for professional arborists and eco-conscious homeowners. Oils are highly refined petroleum or plant-based products (such as mineral, canola, or soybean oil) used as non-toxic insecticides and miticides.

Applied to trees and shrubs during late winter to early spring, these oils are exceptionally effective against pests that “hide” in the bark over winter, such as aphids, mites, caterpillars, and scale insects. 

Timing is critical with dormant oils. So is the weather. If you apply them too early, the pests aren’t active enough to be smothered; too late, and you can “burn” new leaves.

• Temperature: Apply when the air temperature is between40°F and 70°F.
• The 24-Hour Rule: Ensure temperatures will stay above freezing for at least 24 hours after application.
• Dryness: Apply on a clear, non-windy day when rain is not expected for 24–48 hours to ensure the oil dries and sticks properly.

Not all trees tolerate oil well, however. Before spraying, check the label for these specific sensitivities:

• Aesthetic Damage: BlueSpruce and Blue Junipers will lose their signature blue color and turn green. The oil dissolves the waxy “bloom” on the needles that creates the blue hue (it takes 1–2 years for new growth to restore the color).
• Phytotoxicity (Tissue Burn): Maples (especially Japanese and Red), Black Walnut, Hickory, Beech, and Redbud are notoriously sensitive to oil and can suffer branch dieback if the concentration is too high or timing is off.
• Chemical Conflicts: Never apply dormant oil within 30 days of a sulfur-based treatment, as the combination is highly toxic to plants.

Dormant oils protect trees from destructive pests by suffocating insects and their eggs before they hatch. Oils clog the breathing pores of pests like scale insects, mites, and aphids, stopping infestations in their tracks.

Applying them during the dormant season reduces the need for synthetic chemicals later on, safeguarding pollinators and beneficial insects inactive during winter. But they need to apply at the right time and the right amount.

See our blog post for more information on dominant oils.