Highly refined horticultural oils are sprayed onto plants as a pesticide. They are used to control mites and insect pests, and to prevent some fungal infestations.
One big advantage of horticultural oil over other insecticides is that pests are not poisoned, they are suffocated. Chemical pesticides often use deadly chemicals to kill bad bugs, which can then affect human and pet health, as well.
When oil was first used to combat garden and agricultural pests, heavier “dormant oils” were used in the winter, while trees were dormant and leafless, and lighter “summer oils” were used when leaves were present. While both varieties are now much more refined, believe me when I tell you that using dormant oil on leaves in summer is a really bad idea.
Types of horticultural oil
Horticultural oils can be petroleum or plant based. Currently, the petroleum-based horticultural oils provide better coverage without damaging leaves. Most over-the-counter varieties are made from petroleum. Vegetable oils, such as sunflower, soybean, and cottonseed, provide good insect control, but phytotoxicity is a still a problem. Phytotoxicity is anything that is poisonous to plants. Research is continuing. At this time, only Neem oil is a consistently safe vegetable-based horticultural oil, but it acts differently than other oils by adding a naturally occurring chemical to the mix that makes insects ‘forget’ to eat or breed.
How to apply horticultural oil
Horticultural oils are best used when pests are vulnerable. Soft-bodied pests, such as aphids, can be sprayed as populations become troublesome. Scale insects are most vulnerable in the crawler stage, while caterpillars and spider mites are best managed as eggs, in winter and early spring. Heavy infestations of scale insects may require repeated sprayings through June. Once pests have reached a protected stage of growth, such as the hard shell covering over adult scale insects, horticultural oils are not effective. Also, the oil must coat pests completely to work.
WARNING: When applying horticultural oils, it is important that sulphur is not present. Horticultural oils should not be applied 2-4 weeks before or after a sulphur treatment, depending on who you ask. Also, horticultural oils should not be used on water stressed plants. Many horticultural oils are mixed with other ingredients, such as fungicides or insecticides, so it is very important that you read the label and follow the directions exactly, to avoid causing more harm than help.
Most horticultural oils are considered appropriate for organic farming.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.