Dormant spraying is the application of a pesticide to a plant when it has no leaves or isn't actively growing. These types of sprays control a wide range of soft-bodied insects such as aphids, immature whiteflies, scales and true bugs, psyllids, thrips, aphid and caterpillar eggs and mites along with major fungal diseases such as powdery mildew, rust, blackspot, fireblight, peach leaf curl and even downy mildew. Spray solutions consist of horticultural oil with either sulfur or copper and are generally safe to use on fruit trees, roses and deciduous trees and shrubs.
Written records of the use of oils as pesticides date from as early as the first century A.D. when the Roman scholar Pliny the Elder wrote that mineral oil controlled certain plant pests. In the late 18th century, petroleum oil and turpentine were in common use as insecticides, followed by whale oil, and in the mid-19th century, a mixture of kerosene, soap and water.
Horticultural oil is considered a "contact insecticide" - it kills only the insects present at the time of application. These oils break down quickly in the environment and are more toxic to pests than to beneficial insects. Sulfur is effective in preventing development of many fungal disease-causing organisms. Copper is a powerful, nonspecific fungicide and bacteriocide that adheres to plants through rainy weather, making it an excellent choice for a winter fungicide.
Older types of horticultural oil, considered "heavy" petroleum oil, can be toxic to plants with leaves, so should be used only on truly dormant plants. Summer oils, also referred to as superior or supreme oils, are lighter petroleum oils that contain fewer impurities and can be used throughout the year. The most common form of dormant spray is a Bordeaux mixture, a powder or liquid containing copper and agricultural lime. Dormant spray products are readily available in concentrate and ready-to use formulations at nurseries and home improvement centers.Ê
Sprays can be applied with a pump sprayer or hose-end sprayer that is sized appropriately for the number of plants you need to spray. The sprayer should be clean, in good working order and not been used for any herbicides. When using any chemical, make sure to follow the manufacturer's directions. Mix only what you can use - you can't save the prepared solution for use later. The oil is suspended in water, and as oil and water don't mix, a third material such as a surfactant or soap can be added to produce an emulsion. Thoroughly blend the mixture by shaking it. Wear appropriate protective clothing, and wash hands and face after using any garden pesticide.Ê
Select a mild, clear day in mid-winter (January through February) when there is little or no breeze. The ideal temperature for application is between 40 and 70 degrees with the temperature remaining over 50 degrees for at least 24 hours (the temperature is more critical for your own comfort than the effectiveness of the spray). Don't spray if there is any chance of frost overnight.
Spray the entire dormant plant plus the soil around it, taking care to saturate every branch, stem or cane as insects and the tiny dust-like spores of fungal diseases hide in the smallest nooks and crevices. Don't use a dormant spray on any plant that has any leaves or is actively growing. Leaves, especially tender new growth, may be damaged by the spray from the impurities in the oils or the reflection of the sun off the oil. A Bordeaux product colors the sprayed plants a bluish-green (from the copper) and may discolor house paint; the color will disappear with time and exposure to rainy weather.
If your garden is plagued by diseases or insects, you may choose to do dormant spraying twice, about four weeks apart, being careful that the last spraying is still while the plants are dormant.Ê
So take some time now to give your garden a dose of preventive medicine, and you'll be rewarded by fewer unwanted invaders in the coming growing season.Ê
The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato