Horticultural Oil
A highly refined mineral or plant-based oil applied as a spray that smothers soft-bodied insects, scale insects, and overwintering pest eggs by blocking their air supply.
What it is
Horticultural oils are highly refined petroleum-based or plant-based oils formulated specifically for plant use. They are approved for organic production and work through a purely physical mode of action with no chemical toxicity. Modern formulations are far lighter and more refined than older types, dramatically reducing the risk of phytotoxicity when used correctly.
What it does
When sprayed onto an insect or egg, the oil forms a film that blocks the spiracles (breathing pores) through which the pest breathes, causing suffocation. This mechanical action means there is no risk of resistance development. It is particularly effective against scale insects (crawlers and settled adults), aphid eggs, spider mite eggs, and the overwintering stages of many pests.
When to use
Apply a heavier dormant-season rate to trees and woody shrubs before bud break to smother overwintering scale, mites, and pest eggs. During the growing season, use lighter in-season formulations at first signs of scale crawlers, aphid infestations, or spider mite colonies. Apply in calm, dry weather when temperatures are between 45 and 86°F.
When not to use
Do not apply when temperatures will fall below freezing within a few hours of application, or above 90°F, as phytotoxicity risk increases at extremes. Do not apply within 2 weeks of sulfur sprays as the combination can cause severe leaf burn. Avoid applying to drought-stressed plants. Check for sensitivity on cucurbits and some tender ornamentals.
How to apply
- 1
Choose the appropriate rate: dormant-season applications are typically 2 - 4% oil in water; in-season applications use 0.5 - 2%.
- 2
Add the oil to water and emulsify by shaking vigorously or using a small amount of castile soap as an emulsifier (a few drops per quart).
- 3
Apply in calm conditions using a pump sprayer, thoroughly coating all plant surfaces including bark, stems, and the undersides of leaves.
- 4
For dormant tree applications, ensure the entire branch and trunk surface is coated evenly to maximise egg and scale coverage.
- 5
Reapply after 7 - 14 days if pest pressure persists. Monitor for phytotoxicity (yellowing or leaf scorch) in the first 48 hours after application.
Garden, by Willowbottom recommends only organic, wildlife-friendly solutions. No synthetic pesticides, no harmful chemicals - ever.