Products made from various oils have been used in the garden for centuries. Ancient Romans used mineral oil to control plant pests, while the American colonists relied on whale oil.
Beginning in the 1930s, the oil of choice was a petroleum product known as dormant oil, so named because it was applied during the dormant season. It was used primarily to control scale insects and mites on dormant trees and shrubs.
Today new superior oils – also called highly refined oils or simply horticultural oils – have replaced dormant oil. They’re effective, low in toxicity and much lighter (that is, not as viscous). These oils can be applied just about any time, even during the growing season, and control not only pests but various diseases too, in particular fungal diseases.
The key to using horticultural oils during the growing season is to make sure the plants you’re spraying aren’t water stressed; if they are, some damage could occur. So simply water the plant well the day before you spray.
Neem oil is an excellent product for controlling pests and diseases. "It’s derived from the tropical neem tree," says Paul James, "and it works better than any other oil-based formulation I’ve ever used."
Within the category of herbicides, new products seem to pop up every year, says Paul, including one introduced in 2005 that is made from clove oil. "In my experience, these oils control weeds best when temperatures are warm, say in the 80s or 90s," he says.
horticultural oil (Concern/Necessary Organics) - Woodstream Corporation
clove oil - St. Gabriel Laboratories
neem oil - Safer Brand