To protect our precious plants and our environment we look for safe and natural ways to garden. Oil-based pesticides are an effective and ecologically friendly way to handle many insect pests and even some diseases. Various oils have been used for centuries. They remain an important tool to manage certain pest problems (e.g. scales, aphids, mites) on fruit trees, shade trees and woody ornamental plants. Several recently developed oils extend this usefulness to flowers, vegetables and other herbaceous plants. Oils also can control some plant diseases such as powdery mildew.
Usually called dormant oil or horticultural oil, they pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests. This allows oils to integrate well with biological controls. Toxicity is minimal, at least compared to alternative pesticides, and oils quickly dissipate through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment and can be mixed with many other pesticides to extend their performance.
Dormant Season Applications to control some of these pests:
- Aphids that curl leaves in spring
- Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)
- Mites that winter on the plant (e.g. conifer-infesting species)
- Scale Insects (e.g. pine needle scale, striped pine scale, Kermes scale, cottony maple scale)
Diseases
- Powdery mildew
- Some aphid-transmitted viruses
Here are four tips to maximize the effectiveness of an application of dormant oil:
- Timing is everything. Dormant oils should not be applied until a tree has gone completely dormant, but also before new growth occurs. Applications in late winter or early spring are ideal, when temperatures are above freezing (over 40 degrees is ideal), but also below 70 degrees. Applying oil too early may expose your tree to winter damage. Applying too late will damage new growth.
- Coordinate with other applications. If you also apply a sulfur-containing pesticide, be sure to space out your applications. A combination of dormant oil and a sulfur-containing pesticide can create a compound poisonous to plants. To play it safe, wait 30 days after a sulfur pesticide application before using dormant oil.
- Check the label. When choosing a dormant oil, look for a low-viscosity oil, which will spread more easily. Make sure the product you choose is specifically marked as a dormant oil, as other oil types can damage trees and shrubs. Also, be aware of the types of trees and shrubs your dormant oil is formulated to protect. A dormant oil for your apple tree might be harmful for a woody ornamental elsewhere on your property.
- Apply with caution. Follow all instructions on the label when applying a dormant oil and be aware of your tree or shrub’s surroundings. If you aren’t sure how or when to apply, contact a licensed professional. Nearby annual flowers, in particular, can be damaged.
Plants Sensitive to Oil
- Black walnut
- Cryptomeria
- Douglas-fir
- Hickories
- Junipers and cedars
- Maples (particularly Japanese and red maple)
- Redbud
- Smoke tree
- Spruce (particularly dwarf Alberta spruce)
Neem oil is very useful in the garden. It can be used on ornamentals, vegetables and herbs. It helps get rid of aphids, mealybugs, scale and whiteflies. Neem oil fungicide is useful against fungi, mildews and rusts when applied in a 1% solution. It is also helpful for other issues such as root rot, back spot and sooty mold.
Sources:
Davey Tree Company
https://www.thespruce.com/what-is-horticultural-oil-1402736
https://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/