Soon it will be time to think about planting your garden. Did you know that some oils can be sprayed in the late winter, early spring to limit pests later? Learn about horticultural oils and choosing the right one for pest management.
Photo credit: University of Nevada Extension
There are many types of oils that can be used in your garden, however knowing which one to use at the right time, is important. Horticultural oil is considered a practice of Integrated Pest Management (IPM) for controlling insect damage to plants. Horticultural oils are 92-99% pure: highly refined petroleum products, that go through a distillation and filtration process. They are then mixed with water and an emulsifier to keep the oil homogenized and ready to use. Plant-based horticultural oils are less refined and may burn plants more readily. It is best to apply horticultural oil on plants late evening or early morning. Use the lowest possible concentrations in the summer (1-2%) and winter (3-4%).
Types of Garden Oils
Delayed Dormant Oil or Pre-Emergent Oil- oils have a high distillation temperature, therefore, evaporate more slowly and stay on the plant longer. Oils are more viscous. Options to when to apply:
- Method One Application - apply before the buds break or show any color, however, this can speed up spring bud development, making the buds more susceptible to frost damage.
- Method Two Application- Apply when buds are showing 1/16th to 1/2 inch of green growth.
- Apply when temperature remains above 40°F for 24 hours. If sprays are applied prior to temperatures dropping below freezing, the emulsion breaks down, causing the oil to adhere to the bark/leaves instead of the targeted insect stage.
- Apply on the first warm day in February or March.
Summer Oil (sometimes called Superior Oil) - refers to the time of year of application. It is applied when plants are in leaf. The oil is in a lower concentration than Dormant Oil.
Supreme Oil - Highly refined oil: similar to Summer Oil.
Vegetable Oil - 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 has provided somewhat protection against insects and mites.
Neem Oil - is a naturally occurring botanical pesticide found in the seeds of the Neem tree. One compound in neem oil called aza has attracted special interest because of its effect as an insect anti-feedent and as an insect growth inhibitor. Aza remains the most researched component of neem oil against insects, mites, and fungi. Neem oil is non-toxic to birds, mammals, plants, and bees. Apply late evening or early morning when bees are inactive.
How do the Oils Work?
Complete coverage of the insect is required for the treatment to be effective. The oil has no effect after it has dried. When horticultural oils are applied to a plant, it covers any exposed insects or eggs by suffocation. With the oil killing almost any insect it covers, it reduces hatching, disrupts insects from feeding or metabolizing normally. In the winter, dormant sprays only kill overwintering insects and exposed eggs. Carefully read the instructions, and mix well before using to ensure it is homogeneous.
Article by Master Gardener Janet Finnerty
Resources
Penn State Extension - "Less Harmful Pesticides"
Michigan University Extension - "Plant Phototoxicity in the Greenhouse"
University of Nevada Extension - "What a Gardener Needs to Know About Horticultural Oils"
Kaufman, Cseke, Warber, Duke, and Brielmann. "Natural Products From Plants"
Rutgers Cooperative Extension - "Plant and Pest Advisory, A Primer on Dormant Oil Applications"
Colorado State University Extension - "Insect Control Horticultural Oils"
Iowa State University Extension - "Using Horticultural Oil Sprays for Pest Control"
Washington State University Extension - "Myth, Miracle or Marketing? Horticultural Oils"
North Carolina State Extension - "Horticulture Oils for Ornamental Plants"
