Do Your Dormant Oil Diligence

— Written By Hannah Lepsch
en Español

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Applying dormant oil to backyard fruit trees and small orchard plantings.

Months after annual crops are “in the barn”, cucurbits preserved, tomatoes canned, and root veggies stored away, the orchardist is still busy. Fruit trees require care and attention year-round to grow well and produce high-quality fruit. Orchard management continues in the winter while trees are dormant – without leaves and not actively growing. The dormant season is time for essential tree pruning and orchard clean up (removing branches, leaves, and mummified fruit). Combined with these cultural practices, applying a dormant oil spray can help ward off pests before they attack.

Reasons oil and other dormant sprays are effective:
  • Selective products kill specific pests; rarely toxic to beneficial insects
  • Sprays applied early in the season prevent pest outbreaks later on
  • Dormant sprays are low risk to bees which are not yet active
  • Overall pesticide use is reduced by suppressing pest populations before they buildup

The in-season fruit tree spray program is intensive: applications of fungicides and insecticides every 10-14 days. Backyard growers find it easiest to use a combination fungicide-insecticide often listed as “Multi-purpose Fruit Tree Spray.” Local suppliers carry Bonide, Ferti-lome, and Ortho brands of this product. In contrast, dormant oils are applied one to two times during winter months when lighter orchard management is needed.

White peach scale

White peach scale on a peach tree. Photo by Debbie Roos

Target Pests

Dormant oils target dormant insect eggs overwintering on the tree branches and trunk, and are most effective on soft-bodied insect pests such as scale, mites, and aphids. These insects are vulnerable to pesticides before the onset of warm weather. San Jose scale and white peach scale, major pests of apple and peach, can be effectively managed with dormant oil applications. Monitor for scale insects that are covered with a protective armor looking like tiny, circular bumps on tree bark. “Crawlers” (emerged nymphs) can be detected with a 10x hand lens. If left unchecked, these pests significantly lower fruit quality making it unmarketable and can result in tree death.

White peach scale injury

White peach scale injury. Photo by Mike Parker

Closeup of scale injury

Closeup of scale injury on peach fruit. Photo by Mike Parker

Mode of Action and Products

Dormant oils work by smothering pests with a thin film of oil; the oil essentially clogs breathing pores of these soft-bodied insects. Every orchard can benefit from 1-2 applications of dormant oil, also referred to as superior or horticultural oil. These are high-quality petroleum-based oils specifically designed for agricultural use (such as damoil). Vegetable-based products, neem oil, and Monterey horticultural oil are sometimes used. Horticultural oils are deemed minimum risk pesticides and are approved for use in organic production.

Application

Apply dormant oil when trees are dormant but before leaves start to emerge. This period is typically from December to late February/early March in the Piedmont region, but timing varies from location to location and year to year.

Mix a 2-3% solution of oil with water for dormant applications, follow label directions for your specific product. For orchards that have a heavy scale infestation two applications applied two weeks apart may be most effective. Because oil kills by smothering, it is important to thoroughly cover the tree, to the point you see spray dripping from the branch tips.

Dormant oil sprays are temperature sensitive, and should be applied when temps are above freezing for more than 24 hours, both before and after application. Horticultural oils can cause phytotoxicity (severe leaf burn and defoliation) when applied to green tissue. Thus, we would not recommend applying oils after the green tip stage (when ½” of green is showing on buds). Note that sulfur-based fungicides are not compatible with oils and cannot be mixed with or applied within 30 days of applying dormant oil. Refer to product labels for specific application guidelines.

While there are many other components of pest management, many “little hammers” as we say, dormant oil sprays are an especially important tool for orchards of any size from 1 tree to 100 acres. Getting ahead of pests with dormant oil can save the orchardist labor, money, and environmental harm. The payout for doing your dormant diligence is abundant, high-quality fruit the following season.

For further questions about dormant oil or other fruit tree queries, contact the N.C. Cooperative Extension – Yadkin County Center at 336-849-7908 or your local Cooperative Extension office.