# Horticultural oil

> A refined petroleum or plant oil sprayed to smother soft-bodied pests and eggs and suppress powdery mildew on contact; used only on foliage, never on flowers.

- Canonical: https://genetixbay.com/knowledge/glossary/horticultural-oil.html
- Also written: horticultural oils, mineral oil, paraffinic oil, summer oil, dormant oil, plant oil spray
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Horticultural oil is a highly refined oil, either paraffinic mineral oil or a plant oil such as canola, cottonseed or soybean, mixed with an emulsifier so it spreads in water. It kills mites, [aphids](https://genetixbay.com/knowledge/glossary/aphid.md), [whitefly](https://genetixbay.com/knowledge/glossary/whitefly.md) nymphs, scale crawlers and eggs by smothering them and disrupting their membranes, and it can suppress [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md). It has no residual action once dry.

## In practice

Oils are contact tools for veg and [mother plants](https://genetixbay.com/knowledge/glossary/mother-plant.md). Coverage of leaf undersides decides success, and repeat sprays are needed to catch hatching eggs. Risks: oils can burn leaves when applied in heat (above about 30 °C / 86 °F), under strong lights, on drought-stressed plants or close to [sulfur](https://genetixbay.com/knowledge/glossary/sulfur.md) applications. They should never touch flowers, where they coat [trichomes](https://genetixbay.com/knowledge/glossary/trichome.md) and leave residue. Many oil products are registered pesticides; check what is registered for cannabis locally, and test on a few leaves first.

## Don't confuse with

- **Neem oil**: may contain [azadirachtin](https://genetixbay.com/knowledge/glossary/neem-oil.md), an insect growth disruptor.
- **Insecticidal soap**: fatty-acid salts that break down cell membranes.
- **Essential oil sprays**: aromatic plant extracts; separate actives with their own rules.

## Related terms

[Neem oil](https://genetixbay.com/knowledge/glossary/neem-oil.md), [Insecticidal soap](https://genetixbay.com/knowledge/glossary/insecticidal-soap.md), [Contact vs systemic](https://genetixbay.com/knowledge/glossary/contact-vs-systemic.md), [Sulfur burner](https://genetixbay.com/knowledge/glossary/sulfur-burner.md), [Scale insect](https://genetixbay.com/knowledge/glossary/scale-insect.md)

## Read more

- [Spider mites on cannabis: life tables, ID and IPM](https://genetixbay.com/knowledge/pests/spider-mites.md)

## Sources

1. UC IPM. Pest Notes: Spider Mites (Publication 7405). University of California ANR. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7405.html
2. UC IPM. Pest Notes: Aphids (Publication 7404). https://ipm.ucanr.edu/PMG/PESTNOTES/pn7404.html