Horticultural oils are great tools for controlling and reducing pest insect populations and for treating fungal problems.  The oils kill on contact by coating and suffocating the larvae, eggs, adults and nymphs of soft-bodied insects, including aphids, whiteflies, scale, leaf-miners and red spider mites. They also disrupt the insects' summer feeding patterns.

In winter, when sprayed on fully dormant trees and bushes, the oils further reduce pest insects' populations before they become active again in spring. When added to the regular practice of good garden sanitation (cleaning up litter, weeds and pest-infested leaves and debris as well as picking up fallen fruits and removing dried fruit mummies from bushes and trees) home gardeners should see very few, easily manageable infestations of soft-bodied pest insects.

Horticultural oils can also act as preventative fungicides. The oils coat leaf and stem surfaces, smothering the spores or pathogens and preventing them from attaching to the tissue and germinating. Success in controlling fungus problems depends greatly on the type of fungus being treated, weather patterns and timing of applications. Powdery mildew and black spot seem to be more easily controlled with applications of horticultural oils than rust or botrytis.    

There are several weights and types of horticultural oils available to home gardeners and label descriptions often use terms that can be confusing. Horticultural oils can be formulated from petroleum-derived mineral oils or can be plant-based, such as neem oil, jojoba oil and canola oil.  

Superior and supreme oils are interchangeable terms. Both are light weight, highly refined mineral oils and do not contain sulfur, which can harm sensitive plants. Supreme and superior oils can be applied when plants have leaves.  Some newer formulations of dormant oils are now labeled as supreme or superior oils. Dormant oils not labeled as supreme or superior should only be applied during the winter dormancy season.

Dormant oils are slightly more viscous or heavier than summer-weight oils and are sprayed on deciduous plants and trees in winter when the plants are fully dormant and leafless, before bud break in spring. Applying heavier-weight dormant oils in summer when temperatures are high risks burning plant tissues.

Summer oils are a type of superior oil. They can be applied in every season but might not be as effective as dormant oils in winter or during spring rainy seasons.

Year-round oils can be safely used in every season. Check labels for temperature ranges. Neem oil is a year-round oil, as are superior oils.

Horticultural oils should be sprayed to drench every surface – the undersides of leaves, branch crotches and bark cracks.  They are ineffective during freezing weather and can cause damage to plant tissues when temperatures are above 90 degrees. Labels provide information on application rates during every season and also list plants that are sensitive to the specific horticultural oil.

Horticultural oils should not be sprayed on drought-stressed plants. Monitor soil moisture levels and deep irrigate trees and bushes that may be drought-stressed before applying horticultural oils. During our hot summers do not spray plants when they are in full sun even if temperatures are below 90 degrees.

Sources:

Monterey Horticultural oil, GrowOrganic.com

Oils: Important Garden Pesticides, Pests in the Urban Landscape Blog. Author: Mary Louise Flint, March 11, 2014

Horticultural oil spray: How dormant oil works, EpicGardening.com, June 20, 2023