Dormant Oil spray, a type of Horticultural oil, is applied after leaf drop in Autumn and again before the buds break in spring. They are used to kill pests and their eggs that have over wintered on and in the trees and woody plants and are effective against small, soft bodied pests such as white-flies, aphids, scales, leaf-hopper nymphs, and mites.
Commercially they are frequently used in conjunction with pesticides such as malathion, or permethrin. They are also sometimes used to curtail the spread of some plant diseases such as powdery mildew and some blights.
Because you can apply "dormant oils" throughout the year, the term "dormant" more correctly refers to the time at which the oil is applied.
Dormant oils sprays are either petroleum or plant-based oils at lower oil concentrations for spraying when plants are actively growing. Oil sprays can can actually damage plants, particularly the petroleum based ones so it's vital that you follow particular product label instructions when using them.
Neem Oil, is a naturally derived plant oil that is effective for this purpose, it has characteristics in common with petroleum based products and is used as an insecticide as well as a fungicide.
Summer oils are used on plants when foliage is actively growing . As with dormant oils, the term 'summer oil' most correctly applies to the timing of the application rather than to the physical properties of the oil.
Supreme Oils and superior oils are terms used to describe highly refined oils for use on plants. It is summer-use oils that meet certain criteria which includes a paraffin[wax] and hydrocarbons and purification that allows for year-round use with minimal or no toxicity to plants or produce.
Do not spray plants that are moisture-stressed. Avoid spraying under wet conditions , spray only when the air is dry and temperatures are not below 40 or above 90 degrees F.
Homemade do it yourself horticultural oils
Home made horticultural oil concoctions can provide benefits comparable to commercially produced oils and if done properly are certainly more cost effective.
Household dish soaps often damage plants when they are substituted for insecticidal soap and they generally are a bad choice. The damage consists of browning edges or spots on foliage, scalds, as well as reduced and delayed yields of fruits and vegetables. Dish soaps for household use are detergents that contain an extensive list of chemical moisturizers, fragrances, degreasers, and dyes they are not eco-friendly and not intended for human [or plant] consumption
A light, fine vegetable oil can at times be used instead of horticultural oil. The ultra-refined horticultural oils are better from a practical point of view, they will stay mixed into the water long enough for you to spray. Many Vegetable oils are also formulated to be water soluble and can be easily forced through a sprayer. I recommend using refined products manufactured for horticultural use for home gardeners.
There is however a dormant oil formula developed at Cornell University that controls overwintering insects as well as fungal diseases. If you have a small plot or only a few plants to spray, this is possibly more feasible.
2 tablespoons of ultra fine canola oil
1 tablespoon of baking soda mixed
1 gallon of water.
I've used a similar recipe in the past - which contains ...
4 tablespoons of hydrogen peroxide
2 tablespoons of baking soda
2 tablespoons of *castile soap
mixed in one gallon of water.
* Castile Soap is Olive Oil and sodium hydroxide
For horticultural oil to be most effective it must come in physical contact with the pests. Horticultural oils are safe to use around people, pets and wildlife but is toxic to fish.