Horticultural oil is a low-toxicity pest management tool used by home gardeners to control insect populations. These refined, oil-based products are mixed with water and sprayed onto plants, offering an alternative to traditional chemical insecticides. The primary mechanism of action is physical: the oil coats target pests, blocking their breathing pores and causing suffocation. Horticultural oils are considered safe for people and pets when used as directed, and they leave minimal toxic residue since they dissipate quickly.
Understanding the Types and Targets of Horticultural Oil
Modern horticultural oils are highly refined petroleum products, often called superior or ultra-fine oils. Impurities are removed to make them safer for use on plants with leaves. Historically, “dormant oil” was a heavier, less-refined product used only when plants were leafless in winter. Today, “dormant” and “summer” primarily refer to the timing and concentration of the application, not the oil’s composition. The same superior oil can be used for both by adjusting the dilution rate.
These oils target a range of soft-bodied insects, eggs, and mites. The physical smothering effect is highly effective on small, exposed life stages. Common pests controlled include scale insects, aphids, spider mites, whiteflies, and mealybugs. Because the oil must directly contact the pest to work, it is a non-selective, contact pesticide. Any insect coated by the spray, including beneficial ones, may be affected, but beneficial insects can safely return once the spray has dried due to the oil’s limited residual effect.
Determining the Correct Timing and Dilution Rates
Successful application depends heavily on seasonal timing and precise dilution to avoid damaging the plant. Dormant applications use a higher concentration of oil and are typically made in late winter or early spring. This occurs before the plant’s buds swell and show green tissue. This timing kills overwintering insect eggs and immature stages, such as scale and mites, without harming sensitive new growth.
Summer applications, often called foliar applications, use a lower concentration to manage active pest infestations on plants with leaves. For both timings, temperature is a limiting factor. Application should occur when temperatures are above 40°F, but not exceeding 85°F to 90°F. Applying oil during high heat increases the risk of phytotoxicity (plant burn). Low temperatures can cause the oil emulsion to break down, making the application ineffective or damaging.
Dilution rates are provided on the product label and must be followed exactly, as they differ between dormant and summer use. A dormant application may call for a 2% to 4% solution, which is a higher concentration than the 0.5% to 2% solution used for summer application. The oil concentrate must be combined with water and an emulsifying agent, already present in commercial products, to form a stable emulsion that looks like skim milk.
Technique for Effective and Safe Application
Effective physical application requires thorough coverage since the oil only works upon direct contact with the pest. The spray must completely coat all plant surfaces, including the undersides of leaves, stems, and bark crevices where pests may be hiding. Incomplete coverage leaves pockets of pests that can re-infest the plant later.
Maintaining the stability of the water-oil mixture requires constantly agitating the spray tank during the entire application process. If the mixture sits undisturbed, the oil and water can separate. This separation results in either an ineffective, watery spray or a concentrated oil spray that could damage the plant. Before full application, perform a spot treatment on a small, inconspicuous area of a sensitive plant and observe it for one or two days for any signs of phytotoxicity.
To minimize the risk of phytotoxicity, avoid spraying plants that are under drought stress, wilting, or experiencing excessive heat and humidity. Certain plants are known to be more sensitive to oil.
Sensitive Plants
- Japanese and red maples
- Smoke trees
- Blue spruces
- Blue junipers (may temporarily lose their blue color due to oil removing the waxy coating)
Always wear protective gear, such as gloves and eye protection, and ensure the spray mist is not inhaled.