When to Spray Dormant Oil on Fruit Trees

Horticultural oil is applied before buds break in the spring.

At one time, dormant oil was a heavy oil applied to dormant fruit trees in winter to smother overwintering insect pests. Over the years, dormant oil, now known as horticultural oil, has become highly refined, much lighter and is used on dormant trees as well as some foliage plants in spring and summer to control a wide variety of insects. Dormant oil now refers to the time horticultural oil is applied, rather than the grade.

What is Horticultural Oil?

Horticultural oil is a petroleum or plant-based oil that is highly refined to remove impurities that damage plants. It may contain petroleum-based oil, neem oil, soybean oil, vegetable oil or cottonseed oil. It is often sold as a concentrate, called spray oil, and contains an emulsifying agent allowing it to be mixed with water. Other names for horticultural oil are summer oil, dormant oil, supreme oil or superior oil. The different names reflect the level of impurities removed from the oils during the refinement process.

Pests Controlled

Horticultural oil controls scale, mites, aphids, leaf hoppers and some diseases such as powdery mildew. In most cases, it works by cutting off the air supply to the insects, eggs or disease pathogen. It only kills insects that are on the plant, so direct contact is necessary for control. Horticultural oils have little residual effect and may need to be applied several times during the season.

When to Apply

Horticultural oil is applied on fruit trees in early spring just before bud break. If it is applied after the young leaves appear, it may kill or damage the young leaves. In all applications, it must be applied when temperatures remain above freezing and below 100 degrees Fahrenheit for at least 24 hours after application. Horticultural oil can damage plants stressed by drought. Because there are different grades of horticultural oil, the types of trees damaged by a specific grade can vary, as well as the extent of damage, so follow directions and warnings carefully.

How to Apply

Horticultural oil is applied with or without dilution depending on the type of oil, the time of year and the target pest. It may be sprayed on the stems, trunk or leaves of the plant when it is dormant or during the growing season. Lighter grades of oil are usually sprayed on the leaves of plants during the growing season and heavier grades are sprayed while the plant is dormant and leafless. Do not spray any type of horticultural oil on the leaves of an evergreen or other plant unless the directions say it is appropriate for that specific plant.

References

• University of Illinois Extension; Just What is Dormant Oil?; Sandra Mason; March 2005
• Colorado State University Extension; Insect Control: Horticultural Oils; W.S. Cranshaw et al.; May 2010