What Is Horticultural Oil
and Dormant Oil?
During the winter months, when plants and trees are dormant, we see the recommendation to spray dormant oil or horticultural oil to control insects on everything from fruit trees to lilacs. But what kind of oil is it that we are being told to spray?
What Is The Difference Between Horticultural Oil and Dormant Oil?
The main difference between many of the newer commercial horticultural oils is that they are more highly refined than past dormant oil products and they can be used year-round.
Most commercially available horticultural oils (i.e. Sunspray, Scalecide, Volck) are refined from petroleum oil with the impurities filtered out.
The final oil is then combined with an emulsifying agent that allows the oil to mix with water so they can be sprayed.
A true dormant oil originally referred to heavier weight, less well-refined oils that were unsafe to use on plants after they broke dormancy because they would burn and kill plant foliage.
These older dormant oils, however, have been replaced with more refined, light-weight oils that can be applied to plant foliage year-round without damage.
Dormant oil now refers to the time of application rather than to any characteristic type of oil.
That said, there are other oils that are used.
Other Sources of Spray Oils
The new formulations are labeled to be used, usually at a reduced rate, during the growing season. These are generally listed as horticultural, ultrafine or summer oils.
Mineral oil: A petroleum-derived oil (as opposed to vegetable oils). A narrow-range oil that is designed to be mixed with water and applied to plants as a spray for pest control.
Summer oil: An oil used on plants when foliage is present (also called foliar oils). As with dormant oil, the term now refers to the time an application is made rather than to the properties of the oil.
Supreme oil: A term used to categorize highly refined oils that distill at slightly higher temperatures and over a wider range than the narrow-range oils. Most supreme oils meet the characteristics of a superior oil.
Superior oil: A term originated to categorize summer-use oils that meet certain specifications that allow year-round use without phytotoxicity, or killing of plant tissue.
Vegetable oil: An oil derived from the seeds of some oil seed crop (e.g., soybeans, canola, cottonseed). Vegetable oils can be used as insecticides, although the type of oil can greatly affect its activity.
Cottonseed oil: is generally considered the most insecticidal of the vegetable oils.
Soybean oil: the most commonly available vegetable oil used in cooking, has often provided fair to good control of some insects and mites.
Neem oil: the most wide-ranging and beneficial oil. Its oil is extracted from seeds of the neem tree (Azadirachta indica). Several neem-derived insecticides have been made as well as fungicides and bactericides.
So What Does Horticultural Oil Do?
Horticultural oils are safe, and effective plus they have the benefit of not harming beneficial insects.
They kill pest insects by smothering them, because the oil blocks the air holes (spiracles) through which insects breathe, causing them to die from suffocation.
In some cases, oils also act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism.
Oils can also disrupt how an insect feeds, a feature that is particularly important in the transmission of some plant viruses by aphids.
Historically, the primary reason oils were developed was because of their effectiveness on otherwise hard-to-control pest problems on fruit trees.
They were used as a dormant-season application (before bud swelling and bud break) to kill mites and insects, such as scales and aphids, that spent the winter on the plant. Dormant oil applications also control certain overwintered shade tree pests.
Recently, improvements in refining have produced oils with increased safety to plants which have expanded their uses. Summer or foliar treatments are now possible for a variety of pests during the growing season.
Oils are sometimes applied to prevent transmission of viruses. Many viruses spread by aphids (nonpersistent viruses), as well as some that are mechanically transmitted by people, can be inhibited by oil applications.
Oils also are useful against powdery mildew. Diluted horticultural oils, often mixed with a small amount of baking soda, can be an effective control for this common plant disease.
The neem oil products have been effective against several types of powdery mildew and rust.
Horticultural Oil Limitations
The main limitation of spray oils is their potential to cause plant injury (phytotoxicity) in some situations. Do not use oils on certain sensitive plants (read all labels), or plants under drought stress may have increased risk of injury.
Oils also can stain some surfaces, particularly dark-colored house paints. Some of the newer spray oils, however, have largely eliminated these problems if they are properly applied.
What Insects Do They Kill?
Various oils have been used for centuries to control insect and mite pests. Oils remain an important tool to manage certain pest problems on fruit trees, shade trees and woody ornamental plants. Several recently developed oils extend this usefulness to flowers, vegetables and other herbaceous plants. They kill:
Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)
Mites that winter on the plant (e.g., conifer-infesting species)
Scale Insects (e.g., pine needle scale, striped pine scale, Kermes scale, cottony maple scale)
Some aphid-transmitted viruses
Overwintering eggs of red spiders
Always read the label carefully and follow precautions that are recommended whenever using an oil on a woody plant.
Avoid using oils on plants that tend to be oil-sensitive (see list below).
Avoid drift onto sensitive plants.
Do not apply when temperatures are excessively high, above 100° F (38° C), or low, below freezing.
High temperatures need to be avoided because the plant is stressed due to water loss and plants under stress may be damaged.
Do not apply oils during freezing weather because this can cause the emulsion to break down and produce uneven coverage.
Do not apply oils if plant tissues are wet, rain is likely, or there is high humidity (above 90 percent). These conditions inhibit oil evaporation.
Lastly, do not spray when shoots are growing.
Plants that tend to be sensitive to oils.
Junipers and cedars
Maples (particularly Japanese and red maple)
Spruce (particularly dwarf Alberta spruce)
The neem oil insecticides (Trilogy) have been most widely used on greenhouse-grown ornamentals. They have shown good plant safety, but there are some precautions for use on impatiens, fuchsia, hibiscus, some roses, ornamental olive and some carnation varieties.
When To Use
It is best to spray before buds begin to swell. If buds of trees and shrubs have begun to swell slightly, go ahead and spray. Although some of the buds may be damaged, the benefits will outweigh the damage.
Do not spray trees which are in full bloom.
Spraying of dormant oil should occur on a clear day when the temperatures are expected to remain over 50° F (10° C). for at least twenty-four hours. The ideal temperatures for application are between 40° and 70° F (4° to 21° C). in order to get the oil to spread out over the tree and cover all crooks and crevices.
Try to avoid applying dormant oil when severe freezing trends are expected in the 3-4 days following application.