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Spraying Trees with Dormant Oil

Q. Are there advantages to routinely spraying my trees with oils?

A. Routine spraying of any product on healthy trees really doesn’t make much sense. If the tree has been under attack in the past from scale, mites or aphids, then an application of dormant oil is warranted. Dormant oil may be applied when the temperature is at least 40 degrees and there is no chance of freezing within 24 hours. The tree must truly be dormant and not have broken bud yet, and the weather should be calm with little wind. There are a few trees sensitive to dormant oil that should not be treated: maple, hickory, redbud, blue spruce, black walnut, beech, Douglas fir and several other evergreens. For these trees, a lighterweight version, sometimes called horticultural or summer oil, is recommended. This type of oil is applied during the growing season to combat the same type of insect problems.