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Leave the 10W-30 for the guys down at the auto service center. We’re talking about the horticultural oils lots of us use to control scales, spider mites, aphids, whiteflies, mealybugs and other critters.
Ever wonder what the difference is between “summer oil” and “dormant oil”? How about “superior” and “supreme”? Let’s try to clear up some of the confusion.
In the old days, dormant oils had the highest viscosity (heaviness or thickness) and were used during the dormant season. The lighter summer oils were used as spray adjuvants when plants were actively growing. Today, these names really don’t mean anything other than referring to time of application.
Superior and supreme refer to oils conforming to a set of specifications including a high content of paraffinic hydrocarbons. All horticultural oils today meet this standard.
The accepted name is now “narrow-range oil” that refers to some rather complicated chemistry concerning the distillation temperature. The goal is to get oil molecules to be as uniform as possible. Read the label on your oil product. Some will suggest higher rates when plants are dormant. Other formulations recommend the same rate winter or summer.
Horticultural oils work primarily by interfering with respiration–shutting down the breathing. Arthropods such as spider mites are very sensitive to anything that interferes with respiration, so oils work well on these pests.
As the temperatures continue to moderate, the numbers of cool-weather spider mites will build, especially if the dry conditions continue. Check your conifers such as arborvitae, juniper and Leyland cypress as well as camellias, azaleas and hollies by shaking some of the foliage over a light-colored piece of paper. Dark red to black dots moving around on the paper may indicate a spider mite problem. Later in the fall, you’ll notice a bronzing of the foliage and maybe even some fine webbing.
Injury to plants from oil sprays is difficult to predict. Impatiens are just as tolerant as privets while silver maple may be injured at high oil rates. When using high rates of horticultural oil, wait until the daytime temperatures are below 65 degrees. Oils may clog the stomates and lenticels through which your plants respire. During warm times, this may cause overheating and death of smaller twigs.
Many old labels recommended suspending oil sprays at temperatures below 40 degrees, but newer evidence suggests this is not a problem for plants. It may be a problem for the poor guy who has to be out there spraying in those temperatures, though.