Dormant oils

This is time of year to protect woody ornamentals and fruit trees from insect pests by spraying the trees with horticultural oil. There are two kinds of horticultural oils. Summer or foliar oils are used on plants when foliage is present; or dormant oils, for when tissue is dormant.

Apply dormant oils now. The window to apply dormant horticultural oils is when buds are still dormant. Dormant oil is a safe preventive for overwintering pests if sprayed while buds are dormant before opening and are effective if they come in contact with the insect; therefore, apply in the winter months when the insects are sessile (stationary). Pests controlled by horticultural oils are soft-bodied insects like scale, aphids and mites.

The reason buds need to be dormant when the oil is applied is because the oil smothers insects or their eggs when a layer is applied. To be most effective the insects have to be fully coated when the oil is wet. The oil has no effect after it dries on the plant and thus loses the ability to control pests once dried.

Horticultural oils are among the safest chemical treatment of insect control. The oils that are used are petroleum-based oils that dissolve the protective waxy coatings of pest eggs. Thoroughly coat when the fruit tree is dry; never apply when below freezing.

All horticultural oils are nonselective, meaning they kill both pests and susceptible beneficial insects. Dormant oils have several advantages over summer applied oils. Dormant oils are applied when plant systems are dormant

Dormant oils are applied when plant systems are dormant is that beneficial insect populations are not usually around during the dormant season. Dormant oils also have low toxicity to humans and pets and are a safe option to control pests.

The armored and soft-bodied scales are the most common pest targeted with a dormant oil application. A close inspection is needed to determine if there is an infestation of scale. Scales blend in well with branches; they look as if they are just bumps in the wood. But scales are smooth, rounded objects. Eggs are under the female shell and when hatched they quickly crawl over the wood. They are tiny and can be blown around by the breeze. Thus, it is critical to smother them before the eggs hatch.

Warning: Dormant oils can harm cedars, spruce, and junipers.

Ellen Peffley Harp, a retired professor of horticulture at Texas Tech University, writes about gardening for several Texas newspapers.