This is the dormant season for most of our favorite perennial plants as they have shut things down for the winter. Many of us gardeners have more or less gone dormant too, especially this past week. We’ll just wait for rises in temperature and day length.
One dormancy-related horticultural term we’ve all heard is “dormant oil.” I suppose it is self-explanatory; oil used on plants during the dormant time of year. Oils of various types have been used for plant pest control for thousands of years. Modern dormant oils are mostly refined from petroleum.
Of the few plant-derived dormant oil products, cottonseed oil is the primary source. The oils are refined to remove impurities like sulfur and other natural chemicals that could harm plants. A generation ago, most dormant oils were heavier than today’s. They came with instructions to use only during cool weather since heat and oil can combine to damage plant tissue.
Now we can choose from lighter oils that go by the names “superior oil” or “summer oil.” Essentially, these newer oil products have been further refined and are lighter in weight. The lighter the oil, the less the threat of plant damage at higher temperatures.
Dormant oils do not work in freezing weather. And they wash away with rain and evaporate with no rain. They are best used during warm sunny periods of winter. The lighter weight products can be used after spring green-up, but be sure to read the label for temperature limits.
I read the instructions on one summer weight oil, and it claimed to be plant safe up to eighty-five degrees. That’s before summer around here.
The oils are officially listed as pesticides, but they do not kill insects by any chemical means. For the most part, oils suffocate insects. In some cases, repeated use of oils will weaken or “melt away” the bug’s outer covering, allowing the weather to finish the job.
The truth is there is not a whole lot of dormant oil use by home gardeners in these parts. The main targets of oils are overwintering scale, aphids and mites. Backyard fruit trees, especially peach trees, can benefit from dormant oil sprays to battle the white peach scale.
Like most scale, the adult has a hard outer covering that repels water-based insecticides. So the smothering oil is useful. Aphids are prime for two-season organic controls. Insecticidal soap mixed in water will smother them in warm weather and dormant oil will work on their eggs in winter.
I’ve always figured the most numerous scale around here is the euonymus shrub scale. The euonymus is an evergreen shrub, meaning oil should be used on it on cool, sunny days. Don’t spray oil on the green shrubs once it gets hot.
For bug fun, I read there are a couple of species of ants that carefully carry aphids to their nests to care for them during winter. The ants then release the aphids come spring to go suck plants and secrete sugary honeydew which ants love.
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Terry Rector writes for the Warren County Soil and Water Conservation District, 601-636-7679 ext. 3.