Get It Growing: Spring Spray with Horticultural Oils

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Although small in size, insects and mites can affect the health and productivity of your fruit trees and berry shrubs. If you’ve had trouble with these pint-sized pests, consider using horticultural oils this winter and/or spring to control them.

Horticultural oils are used during the dormant season (i.e. winter, when deciduous plants shed their leaves and stop actively growing) to control overwintering insect pests, eggs and mites.

Horticultural oils, usually applied to your plants as a spray, kill soft-bodied, slow-moving insects, overwintering eggs, and mites by suffocation. They also penetrate the outer skin of insect and mite eggs, thus decreasing the hatching number.

Ordinary oils can harm plants because they contain toxic impurities such as sulfur and block the exchange of carbon dioxide, which is important for photosynthesis.

However, horticultural oils are highly refined and contain at least 95% pure oil. When used as directed, horticultural oils rarely cause damage to plants.

There are a number of reasons to use horticultural oils instead of other forms of pesticides on your fruit crops.

• Horticultural oils have very little effect on birds, humans or other mammals. Excess oil evaporates quickly, so there is no toxic residue.

• Sensitive 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 at that time.

• Horticultural oils are relatively inexpensive (compared to other pesticides).

In contrast, 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 stone fruit brown rot.

They are, however, effective in reducing populations of most aphids, leafrollers, tent caterpillars, mealybugs and mites.

If horticultural oils are right for your orchard, here are some tips for applying them during the dormant season. Apply horticultural oils just before leaf and flower buds show signs of opening. This is when insect pests become more active and breathe.

Be sure to spray all parts of the tree or bush, including cracks and crevices, branch tips left over from pruning, and the V-shaped crotch where branches branch out from the trunk .

Do not apply horticultural oils in freezing weather as oil coverage will be uneven and the oil-water mixture may separate. Temperatures between 40 and 80 degrees for 24 hours are recommended for the application of dormant oils. Do not apply horticultural oils if it is humid or raining, as these conditions inhibit oil evaporation and can damage plants.

Avoid unintentional spray drift to other plants since many herbaceous plants and some woody plants are sensitive to horticultural oils. (See box.)

Although horticultural oils are safe, they are pesticides. If you use horticultural oils, be sure to read the label before using them and follow the directions!

Bob Cain is a master gardener from Clallam County.

Plants sensitive to horticultural oils

Do not use horticultural oils on the following plants. Using horticultural oils on these plants can lead to blackening of branches, yellowing of foliage and even plant death:

annual flowers

black walnut

Cryptomerism

Douglas fir

hickories

Junipers and cedars

Maples (especially Japanese maple and red maple)

redbud

smoke tree

Spruce (especially dwarf Alberta spruce)

“Permaculture and the garden”

Be sure to join us for the next Green Thumb presentation, “Permaculture and the Garden,” presented by Delvin Solkinson, from noon to 9 p.m. Thursday, April 28, on Zoom. Solkinson is a community gardener, student and teacher from British Columbia. Presentations cover basic gardening topics relevant to most home gardeners. Seminars are free, but donations to help support the WSU Clallam County Extension Master Gardener Program or the Master Gardener Foundation of Clallam County are appreciated. Get the Zoom link at extension.wsu.edu/clallam/master-gardener-calendar.

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