Get It Growing: Horticultural oils for pest control

If your fruit trees and berry bushes have had past problems with insect pests and mites, horticultural oils, applied this winter, could be a safe and effective approach to control.

Horticultural oils are refined petroleum products from which most plant-damaging impurities (sulfonated residues) have been removed. They are used during the winter (“the dormant season” when fruit trees and blueberry bushes lose their leaves and stop actively growing) to kill overwintering soft-bodied and slow-moving insects and mites.

Horticultural oils block the air-holes through which these pests breathe. They also penetrate the shells of insect and mite eggs and interfere with their metabolism and respiration.

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.

Horticultural oils have many benefits. They have little, if any, effect on birds, humans, pets and other mammals. They have limited impact on beneficial insects because beneficial insects usually aren’t active in winter. In addition, they do not have an objectionable odor during application and evaporate quickly, leaving no toxic residue.

On the downside, horticultural oils do not control all fruit tree and small fruit pests and diseases. They will not control apple maggot, codling moth, or pear slug. They will not control apple scab or brown rot. They are effective, however, in decreasing populations of overwintering aphids, mites, scale, mealy bugs and lace bugs.

If horticultural oils sound right for your orchard or blueberry patch, here are some tips for applying them:

• Because horticultural oils are most effective if pests are active and breathing, apply them just before leaf and flower buds show signs of swelling or breaking open (sometimes called a “delayed dormant stage”).

• Be sure to spray all parts of the plant including cracks and crevices, the top of the plant, the v-shaped crotch where limbs branch out from the trunk and any limb stubs left behind from pruning. Insects and mites can escape suffocation if oil coverage is poor.

• Apply dormant oils when temperatures are above 40 degrees. If applied during freezing weather, coverage will be uneven.

• Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation and can result in plant injury.

Avoid spray drift onto unintended plants when applying horticultural oils. Many herbaceous and some woody plants are sensitive to horticultural oils. (See sidebar.)

Spraying these plants with oils can result in blackened branches, yellowing foliage, leaf loss and even plant death.

Even though horticultural oils are very safe, they are pesticides. Therefore, always read the label before you use them and follow the directions for greatest success and safety.

Horticultural oil applied during the dormant season is a slick way to control aphids, mites and

other soft-bodied bugs. If you have had past problems with these pint-sized pests, consider using horticultural oils this winter.

Jeanette Stehr-Green is a WSU certified Clallam County Master Gardener.

Plants sensitive to horticultural oils

Avoid spraying horticultural oils on sensitive plants including the following:

Azaleas (selected varieties)


Black walnut



Douglas fir*


Japanese holly*


Maples (particularly Japanese and red maple)


Plume cedar


Smoke tree


* — Tendency toward sensitivity to horticultural oils

More in Life

Port Angeles Community Players set to stage ‘Miss Bennett’

The Port Angeles Community Players will present “Miss Bennet — Christmas at… Continue reading

Peninsula College’s ‘Jazz in the PUB’ concert set for Nov. 30

The 12-piece Peninsula College Jazz Ensemble will present their first indoor concert… Continue reading

Milestone: Master Gardener quartet earn golden trowel honors

Audreen Williams, Laurel Moulton, Jan Danford and Teresa Bibler were awarded 2021… Continue reading

Community Calendar — Nov. 24, 2021

Editor’s note: Is your group meeting once more and wanting to get… Continue reading

Parenting In Focus: New level of success for your growing child

By the time your little one has reached 18 to 24 months,… Continue reading

Milestone: Sequim Soroptimists pick Girls of the Month

Soroptimist International of Sequim recently named their Girls of the Month for… Continue reading

Renne Emiko Brock offers hand-dyed superhero capes and scarves during the Art Beat Small Business Saturday event on Nov. 27. Submitted photo
Flurry of Art Beat events, activites set for Saturday

Celebrate creativity and collaboration by supporting local artists and arts organizations with… Continue reading

A&E briefs — Nov. 24, 2021

Strummers set concerts Olympic Peninsula Ukulele Strummers (OPUS) will perform holiday concerts… Continue reading

Right: Pieces of Civil War veteran Moore Waldron’s headstone can be seen in the right-hand corner of this photograph. Historical preservationist Mick Hersey, left, and the Taylor family of Gig Harbor returned the pieces to the Pioneer Memorial Park of Sequim for their friends the Englands (Moore’s descendants). The Englands read in the Sequim Gazette about the Sequim Garden Club’s preservation efforts at the park and decided to return these pieces for restoration. Moore now will have two markers in the park, as the Veteran’s Administration commissioned a new stone for Waldron in 2017 — an article about which can also be found on the Sequim Gazettte’s website. Moore moved to Sequim with his family in 1905 and died in 1908. Moore had five children and has descendants in Sequim and Pierce County as well as other places. Moore’s great-grandson is the founder of the Waldron Endoscopy Center in Tacoma, according to Cheryl England. Sequim Gazette photo by Emily Matthiessen
Historic headstone returns to Sequim

Right: Pieces of Civil War veteran Moore Waldron’s headstone can be seen… Continue reading

Most Read