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Horticulture Oils: Use and Safety

Horticultural oils are widely used to mitigate insect pests in a variety of settings such as ornamental trees, landscape plants, greenhouses, fruit trees, and orchards. But what are they, what are their appropriate uses, and what precautions should you take when using them?

What are horticultural oils?

Horticultural oils are derived from either petroleum or plant material. Mineral oils are petroleum-based while vegetable-based oils are derived from oil seed crops such as soybeans, canola or cottonseed. The two most common horticultural oils contain refined mineral-based paraffin and olefin. There are different oils available for different seasons.

Dormant oils are applied when plants, and often their insect pests, are dormant in winter. Summer oils are used when foliage is present, temperatures are higher, and insect pests are active. More refined versions of oils known as narrow-range, or Superior oil, allow for year-round use and have are less likely to be used improperly and cause damage to plants. Many products available in the market are labeled for all-year use, often as “Horticultural and Dormant Oil”, but may still have application timing limitations for specific pests or plant varieties.

Always read the label before purchasing the product to know when you can use it on specific plants. Additionally, essential oils intended for cosmetic or home use are not the same as horticultural oils, though there are pesticides containing essential oils. If a product is not labeled for use in gardens, do not use it. Not only is it illegal to do so, but you may cause permanent damage to your plants.

How do they compare to other pesticides?

Using horticultural oils within an integrated pest management approach can provide many benefits. They have low toxicity to plants and animals, can be applied easily, are cost-effective, and provide multiple modes of action that can help delay the development of pesticide resistance in insect populations. Since they often degrade by evaporation quickly after application, they are non-residual and have minimal effect on beneficial insects. But just like other pesticides, they are chemicals that must be applied according to the label instructions and can pose harm when not used properly.

Oils are not selective so if beneficial insects are present during an application, they will be harmed just like the pest insects. You must use extra care not to drift or impact water bodies since oils can kill fish by coating their gills, which prevents them from breathing.

How do they work?

Horticultural oils are generally classified as having a physical mode of action, rather than a chemical one like synthetic insecticides. The most common way that horticultural oils work is to coat and clog the spiracles, which are pores that insects use to breathe. Additional effects such as disruption of membrane functions and interfering with the sucking or piercing actions that some insects use to eat.

Fungal diseases like powdery mildew and sooty mold can be controlled with horticultural oils in three ways:

1. Controlling the insects that carry them.
2. Trapping fungal spores, which prevents them from spreading, and
3. Coating plant surfaces, making it difficult for fungi to adhere to the host plant.

Which pests do they affect?

Horticultural oils are often used to manage aphids.
Photo: Jeff Hahn
Horticultural oils are non-selective pesticides that can affect small, soft-bodied insects. The most common pests they are used to manage are mites, scale insects, and aphids as well as fungal diseases such as powdery mildew and sooty mold. Applications are effective on insects at all growth stages from egg to adult, and they can even deter some insects from laying eggs. However, the insect must be present at the time of application to allow for direct contact.

Application Do’s and Don’ts

Horticultural oils are applied as sprays directly to plant parts; they cannot be applied to the soil. They also cannot be used as preventative treatments since they do not provide residual control if the pest is not present. Most of the time, you will find pre-mixed products at garden and hardware stores; follow the label instructions for application rates and timing.

In some cases, you may find concentrated products that need to be diluted. Application rates are usually 2%-5%, and they can be mixed with other products like surfactants, if the label allows it, to extend their performance. In some cases they can also be mixed with other pesticides. Always check the label for instructions about mixing.

Since horticultural oils are volatile (likely to evaporate) and can degrade rapidly, it is important to protect yourself by wearing appropriate gloves and eye wear and to make applications when environmental conditions are right. Always read the label for temperature and humidity limitations before making applications to avoid damaging plants
. High temperatures during oil application can kill plants and high humidity levels can slow the evaporation of the oil.

Here are some do’s and don’t for applications:
  • Do not apply to heat or drought stressed plants or when leaves are wilted
  • Do not apply in humidity over 90%
  • Do not apply within 2 weeks (or as the label states) of a sulfur-based application
  • Do not apply if leaves are wet since oil will have a difficult time adhering to the foliage.
  • Do not apply when temperatures are below freezing.
  • Be aware of any harvest time limitations.
  • If oils are mixed with other pesticides, consult the other pesticide labels for any additional limitations or pre-harvest intervals.

Other safety precautions

Petroleum-based products can cause skin irritation or other reactions when they come in contact with skin or eyes. They can also harm your lungs, because they are volatile, or likely to evaporate, which makes you more likely to inhale them during application. Always use the personal protective equipment listed on the label to protect yourself. Oils are flammable and should be stored away from heat sources, other chemicals, and combustible materials. Clothing used during application should be washed separately from other laundry and allowed to air dry. Do not use a heat dryer since any remaining residues can ignite. And as always, keep these products away from children at all times.

Authors: Jolene Hendrix, Tana Haugen-Brown, and Jeff Hahn, UMN Extension Educators


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