Dormant oil is a horticultural oil applied to plants during their dormant season (late fall to early spring). It is made from vegetable oil (e.g., soybean or canola oil) mixed with water. The oil works by smothering overwintering pests like aphids, mites, and scale insects, blocking their respiratory openings.
This non-toxic pest control method is popular among organic gardeners. When applied correctly, dormant oil is safe for most plants and also helps prevent certain fungal diseases by coating plant surfaces.
How Dormant Oil Works
Dormant oil works primarily by smothering pests that overwinter on plants. When applied, it forms a thin film over the surface of the plant, coating insects and their eggs. This coating blocks the pests’ spiracles, the small openings they use to breathe, effectively suffocating them. This physical mode of action makes dormant oil a highly effective, non-toxic pest control method.
Dormant oil targets a range of pests, including aphids, mites, and scale insects. By interrupting their life cycles, it prevents these pests from causing damage when the growing season begins. Additionally, the oil can help control certain fungal diseases by creating a barrier that limits spore germination and growth.
For the oil to be effective, it must be applied during the dormant season when plants are not actively growing. This period ensures that the oil does not harm new growth or sensitive foliage. Proper weather conditions, such as temperatures above freezing and no impending rain, are crucial for optimal application.
DIY Dormant Oil: How to Make and Use It
Homemade dormant oil is an effective, budget-friendly solution for controlling garden pests.
Materials Needed
Ingredients:
- 1 cup of vegetable oil (e.g., soybean or canola oil)
- 1 tablespoon of liquid dish soap
- 1 gallon of water
Tools:
- Measuring cup and spoon
- Mixing container
- Garden sprayer
- Protective gloves and goggles
Step-by-Step Guide to Making DIY Dormant Oil
Prepare the Oil Mixture:
- Pour 1 cup of vegetable oil into a mixing container.
- Add 1 tablespoon of liquid dish soap to the oil. This helps the oil emulsify with water.
- Mix thoroughly to combine the oil and soap.
Dilute with Water:
- Pour the oil and soap mixture into 1 gallon of water.
- Stir or shake well to ensure the mixture is evenly distributed.
Applying DIY Dormant Oil
Check the Weather: Ensure temperatures are above 40°F (4°C) and no rain is expected for 24 hours.
Inspect Your Plants: Identify areas with visible pests or previous infestations. Prune any dead or diseased branches to improve oil coverage.
Application Process: Wear protective gloves and goggles. Pour the diluted dormant oil mixture into a garden sprayer. Spray the mixture evenly over all surfaces of the plants, including branches, twigs, and trunks. Ensure thorough coverage, especially in crevices where pests may hide.
Post-Application Care: Monitor plants for any adverse reactions. Some leaf drop or discoloration might occur but should be minimal. Reapply if necessary, especially if pest activity persists.
Safety Precautions and Potential Risks
When using dormant oil, follow these safety precautions to protect your plants and environment.
Protective Gear: Always wear gloves, goggles, and a mask to protect yourself. Long sleeves and pants are recommended.
Application Conditions: Apply on a calm day to avoid drift and ensure temperatures are between 40°F (4°C) and 85°F (29°C). Test on a small area if unsure about plant sensitivity, and avoid applying to stressed or newly planted vegetation.
Potential Risks: Over-application can cause phytotoxicity, leading to leaf burn, drop, or stunted growth. To protect beneficial insects, avoid applying when they are active. Prevent runoff into water sources to protect aquatic life, and dispose of leftovers according to local guidelines.
Plants Suitable for Dormant Oil Application
Dormant oil is versatile and can be used on a wide variety of plants to control overwintering pests. Here’s a list of plants that benefit from dormant oil application:
Fruit Trees
- Apple Trees: Effective against scale insects, mites, and aphids.
- Pear Trees: Controls psylla, scale, and aphids.
- Peach and Nectarine Trees: Targets scale, aphids, and mites.
- Cherry Trees: Manages scale insects and spider mites.
- Plum Trees: Effective against aphids and scale insects.
Nut Trees
- Walnut Trees: Controls scale and mites.
- Pecan Trees: Targets scale insects and aphids.
Berry Bushes
- Raspberry Bushes: Effective against cane borers and mites.
- Blueberry Bushes: Controls scale insects and mites.
Ornamental Trees and Shrubs
- Maple Trees: Targets scale insects and mites.
- Elm Trees: Effective against scale insects.
- Azaleas: Controls lace bugs and scale.
- Rhododendrons: Manages aphids and scale insects.
- Hollies: Effective against scale insects.
Evergreen Trees and Shrubs
- Pine Trees: Targets pine needle scale and aphids.
- Spruce Trees: Controls spruce spider mites.
- Junipers: Effective against spider mites and scale.
Roses and Other Flowering Plants
- Roses: Controls aphids, mites, and scale insects.
- Hydrangeas: Effective against aphids and scale insects.
Tips for Safe Application
- Test First: Always test dormant oil on a small area of the plant if you are unsure about its sensitivity.
- Avoid Sensitive Plants: Some plants like Japanese maples and certain evergreens may be sensitive to oil sprays and should be treated with caution.
FAQs About Dormant Oil
What is the best time to apply dormant oil?
The best time to apply dormant oil is during the dormant season, typically late fall to early spring, when temperatures are between 40°F (4°C) and 85°F (29°C).
Can dormant oil be used on all plants?
Dormant oil is safe for most fruit trees, berry bushes, ornamental trees, shrubs, and some flowering plants. However, it is best to test a small area first, as some sensitive plants may react adversely.
How often should dormant oil be applied?
Dormant oil is usually applied once during the dormant season. However, in areas with high pest pressure, a second application may be necessary in late winter or early spring.
Is dormant oil safe for fruit trees?
Yes, dormant oil is safe for most fruit trees, including apple, pear, peach, cherry, and plum trees. It effectively controls pests without harming the trees when applied correctly.
What are the signs of over-application of dormant oil?
Signs of over-application include leaf burn, discoloration, and potential leaf drop. It is essential to follow the recommended dilution and application guidelines to avoid phytotoxicity.
Can I use dormant oil on newly planted trees and shrubs?
It is best to avoid applying dormant oil to newly planted trees and shrubs, as they can be more susceptible to damage. Wait until they are well-established before applying.
How does dormant oil affect beneficial insects?
Dormant oil can harm beneficial insects if applied when they are active. To minimize impact, apply the oil during the dormant season when beneficial insects are less active.
Can I make my own dormant oil at home?
Yes, you can make a DIY dormant oil using 1 cup of vegetable oil, 1 tablespoon of liquid dish soap, and 1 gallon of water. Mix thoroughly and apply as you would commercial dormant oil.
What should I do if it rains after applying dormant oil?
If it rains within 24 hours of application, the oil may wash off and lose effectiveness. You might need to reapply when the weather is dry and suitable.
Are there any plants that should not be treated with dormant oil?
Some sensitive plants, such as Japanese maples and certain evergreens, may react poorly to dormant oil. Always test a small area first or consult specific plant guidelines before application.
