Spray in Winter to Control Pests and Diseases

Many garden pests and diseases can be controlled by using dormant sprays or horticultural sprays on dormant plants during winter. These compounds work by smothering (in the case of oils) or desiccating (drying out) eggs and larvae.

When to Spray

Trees and shrubs are dormant after their leaves fall and before you see new growth in spring. This is typically November through March, depending on where you live.

The daytime temperature needs to be above 35 F to avoid potential damage to bark. It also helps if any snow on the ground is not covering the tree or shrub.

Which Trees or Shrubs to Spray

Dormant spray is suitable for deciduous plants. Many evergreens can be damaged by dormant sprays, as can euonymus and rhododendrons. One of the dangers is burning bark or leaves when the sun shines on the oil and concentrates the heat on the plant. Do not use dormant spray on an evergreen until you have checked about its effectiveness on the specific plant.

Spray only plants that had an insect or disease problem the previous year. Dormant sprays can also kill beneficial insects and, when falling on the ground, can affect any emerging or growing annual plants. Cover the ground under any tree or shrub before you spray.

Choosing Which Dormant Spray to Use

• Horticultural Oil: Oil sprays are used to control insects. Insects (See Image 2 below), eggs, and larvae are smothered by the oil. In the organic garden, use a soybean-based oil. Most horticultural oil is petroleum-based, so check the label before you purchase a product.
• Lime-Sulfur: Lime-sulfur (Calcium Polysulfides) sprays work to control fungal and bacterial diseases such as cankers, black spot, peach leaf curl (See image 3 below), fire blight, scab and anthracnose. It is often sold as a mixture of Calcium Polysulfides and oil. The oil acts to keep the lime-sulfur on the plant.
• Fixed Copper: Use fixed copper on for a wide range of plant diseases such as powdery mildew, rusts, black spot, leaf and fruit spot, downy mildew, fruit rot and late blight.. To help the cooper adhere to the plant surface, you need to add something to help it stick. For the organic garden, look for a copper product mixed with true soup (a natural fatty acid). Both ingredients break down naturally. Allow two weeks between applications of copper and any sprays containing sulfur.
• Bordeaux Mixture:This mixture of fixed copper and lime has long been used to control fungus and disease. You can buy it already mixed, but many experts say the mixture is more effective when mixed just before use. You can find details on how to do this at the UC Davis Integrated Pest Management site.

Applying a Dormant Spray

Complete coverage of any spray product is required for effective control of overwintering pests. A pressure pump is most effective as an applicator (See image 4 below for examples), but you have to make sure it covers every surface, including the undersides of leaves and branches.

Protect yourself and others parts of the garden when you spray. Wear a mask, gloves, and long sleeves when spraying your plants. Even organic products should not be inhaled, swallowed, or left on your skin. Spray when the wind is still. If feasible, cover surrounding plants that may be damaged by the spray or do not need treatment.

Important: Always read the entire label and follow all directions when applying any pesticide or fungicide to plants.

Reading More

Jamie McIntosh writes more about dormant spraying.

Anne Spiegelman discusses dormant spraying for fruit trees.