Dormant oil is a natural pesticide that can be sprayed onto plants to kill insects. It doesn't damage trees, plants, flowers, fruits or the environment. It is traditionally used on deciduous trees when they are dormant and is mixed with water and sprayed to control wintertime pests that make their home on tree trunks, branches, twigs and buds. Newer formulations can be applied when plants aren't dormant, and they will typically be labeled as all-season oil, horticulture oil or superior oil.
Making and Spraying Dormant Oil
Natural pesticide oils get sprayed onto trees and plants, covering exposed eggs and insects and suffocating them. They're relatively safe for wildlife and humans, but their chemical makeup means that they may also kill some beneficial insects. These oils evaporate and degrade quickly, so you'll need to apply them more often than chemical products. You can make a DIY horticultural spray by mixing 1 tablespoon of canola oil, a quart of water and several drops of detergent into a spray bottle.
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You can spray this concoction onto the entire surface of branches, the top and underside of leaves and trunks. You can mix larger quantities and apply it with a hose-end spray or a pump sprayer; make sure to cover the surfaces thoroughly. Keep in mind that certain trees are sensitive to horticultural oil sprays, so if you use dormant or horticultural spray products, read the labels before using them.
Horticultural Oil vs. Neem Oil
Is neem oil the same as horticultural oil? While they are both natural pesticides, there are key differences. Horticultural oils have oil bases and emulsifiers and may have other insecticidal ingredients to increase their effectiveness. They suffocate insects and their eggs, and some are made to use during dormant seasons. You should never use the dormant-use products after buds open. Also, avoid these sprays when temperatures are above 100 degrees Fahrenheit or below freezing, in damp conditions or within 30 days of applying sulfur-containing products.
Neem oil is extracted from Azadiractha indica trees since these contain the powerful natural insecticide azadirachtin. After someone extracts the oil, the azadirachtin is diluted, leaving trace amounts of the chemical. You can add raw neem oil as a soil soak or use it as a foliar spray. When insects ingest it, the azadiracthin affects their ability to feed as well as their reproductive ability and growth cycle. It's best to apply it at dawn or dusk but avoid doing so in sweltering and freezing temperatures, on wet foliage, close to fish ponds or beehives (it can be mildly toxic for them) and on young people or tender plants.
Tips for Using Natural Pesticides
Manufacturer labels detail how often you can use horticultural and neem sprays, and if you make your own, you'll want to be careful. If you have whiteflies, aphids or two-spotted mites on vegetables, ornamental plants or strawberries, apply two sprays three to five days apart. Don't apply this more than three times over a four- to eight-week period. For citrus leaf miners, spray new summer growth right away and repeat every five to 14 days; stop when most new leaves start to harden.
To use cold-pressed neem oil, add 1 or 2 tablespoons to a gallon of water and then drop in a teaspoon of liquid soap or insecticidal soap. Mix well in a spray bottle and thoroughly mist indoor and outdoor plant leaves. You can reapply it every two weeks as long as the plants thrive. If the plants appear damaged or are experiencing difficulty growing, you may want to try something else.