Dormant Oil Sprays

February 13, 2015


Dormant oil sprays are applied to plants during mid to late winter to control specific insects, usually scales. Now is the time to inspect plants and prepare to apply dormant oils if scale insects are found.

The dormant season is a good time to inspect plants for scales. Without leaves on plants, these small insects are easier to see on branches and small twigs. Scales are oval or circular shaped and bronze, tan, brown or white in color. Some resemble small shells, others look like raised brown buds, and some even resemble plastic foam.

Scales usually overwinter in the egg stage beneath a waxy covering; hence the name scale. When eggs hatch, the almost microscopic crawlers move to a new location on a branch. They attach themselves, grow their own waxy covering for protection, and feed on sap from beneath the scale.

While feeding on plant sap, a heavy scale infestation can weaken plants, kill branches, reduce fruiting, and kill entire plants.

Some plants to inspect for scale insects are lilac, dogwood, Euonymus, apple, pear, other fruit trees, crabapple, oak, ash, maple, pine, spruce and yew. Scales are most common on these plants but can attack many others.

If scales are found, options for control include pruning and destroying infested branches; applying a dormant oil to suffocate the insect; or waiting until egg hatch and applying an insecticidal soap, summer oil, or an insecticide labeled for use on the plant to be sprayed.

Dormant oils may be the most effective option. Because scales are small enough to miss during an inspection, it would be easy to overlook removing an infested branch when pruning.

And it can be difficult to detect when scales hatch. Insecticidal soaps and most insecticides need to be applied when young scales are in the crawler stage. This is a very short period of time and the optimum time for a spray application could easily be missed.

If a dormant oil spray is used, read and follow label directions closely. Dormant oils need to be applied before plants break bud or tender plant tissue will be damaged. Thorough coverage of branches and twigs is needed for good control.

Dormant oils also need to be applied when temperatures are above 40 degrees Fahrenheit; and applied early enough in the day to dry before freezing. If the oil freezes before it dries, plants can be injured.

Pruning Shrubs

This reminder really hits home, because I've just been working with Pete, our custodian, here at the courthouse on managing shrubs that were outgrowing the place they were originally planted. The usual way of dealing with this is either shearing the shrub back to the desired size or cutting it clear to the ground. I really discourage shearing, and want to suggest there is an alternative to cutting it back to the ground.

If you have a large overgrown shrub that has not been pruned in years, it can be renovated without cutting the shrub to the ground. Now through March is the time to prune for shrub renovation. A good method for renovating large shrubs is to thin overgrown branches. Thin the shrub by removing one-third of the largest diameter branches back to another branch or near the ground.

Over a three year period, remove one-third of the older, large diameter stems. By the end of the third year, the shrub is renovated. Annual thinning of a few stems will then keep the shrub from becoming overgrown again. If the shrubs height needs to be shortened, cut it back by about one-third.

But the key to renovation is thinning out larger, older stems and not just reducing the shrubs height. This method of renovation is less of a shock to larger shrubs; allows blooming on spring blooming shrubs; and is less likely to result in straggly sucker growth after pruning.