Horticultural oil (aka Dormant oil) is an ideal remedy to deter overwintering pests. The best time to apply it to woody plants and shrubs is before buds break and leaves open but you also want the little pests to be active, not hiding, so wait until temperatures warm up to above zero for at least 24 hours.
Why apply Dormant oil (aka Horticultural oil)?
To reduce overwintering pests such as aphids, mites, and scale on fruit trees, deciduous trees, and ornamental woody shrubs.
What is Dormant oil – An oil used on woody plants during the dormant season. This term originally referred to heavier weight, less well-refined oils that were unsafe to use on buds or foliage. However, these oils have been replaced with more refined, light-weight horticultural oils and the term ‘Dormant oil’ now refers to the time of application rather than to any characteristic type of oil.
When to apply – Be sure the temperature will remain above freezing for at least 24 hours and there isn’t any precipitation in the forecast. Spray on a dry, sunny morning to facilitate fast drying.
Diagnose first
Oils are indiscriminate and will destroy beneficial insects too. Don’t use it unless you need to!
Also, some pests are resistant as some overwinter as tolerant adults or eggs protected under the dead females, eg: euonymus scale, oyster shell scale, pine needle scale, golden oak scale, two-spotted spider mites. You will have less success using oil for these issues.
Check these databases to help with the diagnosis
Precautions
- oils can stain siding or painted surfaces.
- Do not apply to broadleaved evergreens when freezing temperature may be expected within 3 weeks after application.
- Avoid spraying during or immediately prior to hot weather (over 30C).
- Do not apply if frost is expected before the spray dries completely.
Avoid contact with plants that tend to be sensitive such as:
- Beech
- Black walnut
- Butternut
- Douglas-fir
- Hickories
- Junipers and cedars
- Maples (particularly Japanese and red maple)
- Redbud
- Smoke tree
- Spruce (particularly dwarf Alberta spruce)
- White Pine
More Information
Colorado State University – Dormant oil for scale control
University of Nebraska, Horticultural Oils for Pest Control
University of Missouri – Dormant Oil application in Fruit Trees
University of Nevada – Horticultural Oil – What a Gardener Needs to Know
Scale on Fruit trees can be battled with Horticultural oil
About The Author: Armstrong
As an author and editor, Linda directs her lifelong love of nature and plants to concerns about our environment and how we can do better. In addition to decades of gardening experience, and training as a master gardener, Linda focuses on learning from leading science-based educators and writers, and in the process finds that she uncovers many gardening myths.
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