Using Horticultural Oils in Winter

by Jeanette Stehr-Green, WSU Clallam County Master Gardener

Garden Pest cartoon Although small in size, some insects and mites can affect the health and productivity of your fruit trees and berry bushes. If you have had past problems with these pint-sized pests, consider using horticultural oils this winter to control them.

Horticultural oils, typically applied to your plants as a spray, are used during the dormant season (that is, in the winter when deciduous plants shed their leaves and stop actively growing). Horticultural oils kill overwintering soft-bodied and slow-moving insects and mites. They block the air-holes through which insects and mites breathe. They also penetrate the shells of insect and mite eggs, decreasing the number that hatch.

Run-of-the-mill oils can harm plants because they contain toxic impurities and block carbon dioxide exchange which is important for photosynthesis. But horticultural oils are highly refined. When used as directed, horticultural oils rarely cause plant damage.

There are a number of reasons to use horticultural oils instead of other forms of pesticides on your fruit crops. Horticultural oils have little if any effect on birds, humans or other mammals. The excess oil evaporates quickly, so there is no toxic residue. Susceptible insects or mites are unlikely to develop resistance to these oils. Use during the dormant season has limited effects on beneficial insects that are unlikely to be active in winter. Horticultural oils are relatively inexpensive compared to other pesticides.

On the downside, horticultural oils do not control all pests and diseases that affect fruit crops. They will not control apple maggot, codling moth, or pear slug. They will not control apple scab or brown rot of stone fruit. They are effective, however, in decreasing populations of most aphids, leaf rollers, tent caterpillars, scale and mites.

If horticultural oils sound right for your orchard, here are some tips for applying them during the dormant season.

  • Purchase those that are at least 95% pure oil.
  • Apply horticultural oils as leaf and flower buds begin to swell, that is right before they break open, when insect pests become more active and are breathing.
  • Be sure to spray all parts of the tree or bush including cracks and crevices, limb stubs left behind from pruning, and the v-shaped crotch where limbs branch out from the trunk.

Do not apply horticultural oils during freezing weather because oil coverage will be uneven and you will miss some of the targeted pests. Temperatures between 40-80 degrees for 24 hours are recommended for application of dormant oils. Do not apply horticultural oils if it is wet or rain is likely because these conditions inhibit oil evaporation and can result in plant injury.

Avoid unintended spray drift onto other plants since many herbaceous and some woody plants are sensitive to horticultural oils. These include:

− Annual flowers − Black walnut − Cryptomeria − Douglas-fir − Hickories − Junipers and cedars
− Maples (particularly Japanese and red maple) − Redbud − Smoke tree − Spruce (particularly dwarf Alberta spruce)

Use of horticultural oils on these plants can result in blackened branches, yellowing foliage, and even plant death.

Even though horticultural oils are safe, they are pesticides. If you use horticultural oils in your garden, be sure to read the label before you use them and follow the instructions!

 
To download a list of horticultural oils recommended by WSU, click here. And remember: WSU advises homeowners to consult a professional for treatment of trees or shrubs above 10 feet tall.

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