Dormant Spraying: A Winter To-Do That Pays Off in Spring

What is Dormant Spraying?

If your trees have had significant pest issues or diseases in prior years, dormant spraying may prevent or reduce severity next year.

Dormant spraying is an often overlooked but highly effective winter garden task that can make a big difference in the health of your trees once spring arrives. When trees are dormant, after leaves have dropped and before buds begin to swell, it’s the ideal time to address overwintering pests and diseases before they have a chance to become active. Dormant spraying allows you to target these issues early, reducing potential for infestation/disease growth on your trees during the growing season.

Why Winter is the Perfect Time

Keep in mind that sprays may also affect beneficial insects. Winter is the perfect time for dormant spraying because pollinators such as bees are not active and beneficial insects are not foraging. Pests and disease spores, on the other hand, are often hiding on trunks and branches or in bark crevices where they overwinter. Treating trees during dormancy makes it easier to reach these problem areas while minimizing impact on beneficial organisms, making dormant spraying both effective and environmentally responsible. 

Horticultural Oil - For Overwintering Pests

One of the most common tools used in dormant spraying is horticultural oil, which works by smothering soft-bodied insects and their eggs. This makes it especially effective against pests like aphids, scale, mites, and mealybugs that overwinter on trees. For gardeners with younger or smaller trees, a ready-to-use option such as Monterey Horticultural Oil RTU provides a convenient solution that requires no mixing. For larger trees or multiple trees, Monterey Horticultural Oil Concentrate can be diluted with water and applied using a hose-end sprayer or pump sprayer to ensure thorough coverage of trunks and branches.

Liquid Copper - For Fungal & Bacterial Diseases

Liquid copper fungicides are another important component of dormant spraying, helping to control fungal and bacterial diseases that overwinter on tree surfaces. Dormant applications of copper are commonly used to manage issues such as peach leaf curl, fire blight, powdery mildew, and various leaf spot diseases. For small-scale applications, Monterey Liquid Copper Fungicide RTU offers an easy, ready-to-use option. For larger trees or small orchards, Monterey Liqui-Cop Fungicide Concentrate can be mixed and applied with a sprayer, making it more practical for treating larger areas or multiple trees at once.

Avoiding Phytotoxicity 

Dormant sprays, including horticultural oil and liquid copper, are powerful tools for managing overwintering pests and diseases, however both can injure your plants if applied incorrectly. Phytotoxicity may appear as leaf burn, fruit blemishes, or reduced plant vigor. 

Here are a few ways to reduce the risk of phytotoxicity:

• Read Labels Carefully: Copper and oil products vary by formulation and concentration. Always follow the manufactures instructions for application rates and timing. 
• Watch Water Quality & Spray Conditions: Copper becomes more likely to cause injury in acidic spray solutions (or when your water has a low pH level), while horticultural oils can damage plants if applied in freezing temperatures or when plants are not fully dormant.
• Avoid Over-Application: Focus on targeted applications and avoid broad, repeated applications. Copper can accumulate on plant tissue and in the soil, and horticultural oils can block gas exchange if applied too heavily. 

How & When to Apply Dormant Sprays

When applying dormant sprays, timing and conditions are important. Applications should be made on a dry day when temperatures are above freezing, generally at or above 40°F, and when no rain is expected for at least 24 hours. Thorough coverage is essential, as sprays need to coat trunks, branches, and bark crevices where pests and disease spores may be hiding. Always follow label directions for mixing rates, application methods, and safety guidelines. Depending on pest and disease history, many trees benefit from one or two dormant applications during the winter months.

In our garden in Oregon, we applied dormant sprays in early January at temperature highs of 60°F. 

A Head Start on a Healthy Season

Dormant spraying is a simple yet powerful way to get ahead of common tree problems before spring growth begins. A quiet winter doesn’t mean inaction, it means opportunity. A little prevention now can lead to healthier trees, stronger blooms, and better harvests when spring arrives.

Sources:
https://ucanr.edu/blog/hort-coco-uc-master-gardener-program-contra-costa/article/it-time-use-dormant-sprays-your

https://www.dpi.nsw.gov.au/agriculture/horticulture/citrus/citrus-connect/2020-citrusconnect-articles/58-steven-falivene-and-andrew-creek-using-copper-sprays-to-control-diseases-in-citrus