Oils have been used by farmers and gardeners alike long before chemical pesticides came to be. The original oil used was called dormant oil because it was unsafe to use on a growing plant, but would not cause injury when it was dormant. Because of this tradition, oils designed to be applied to a dormant plant are now referred to as "dormant" oils, though they are much safer for dormant and active plants alike.
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When to Spray
Dormant oils are most effective when applied to a dormant tree, shortly before bud break. The temperature should be above freezing for at least 24 hours, though the optimal time is when temperatures remain between 40 and 70 degrees Fahrenheit. Applying oils during freezing weather will result in uneven coverage. Never apply dormant oil to evergreen trees.
Advantages of Dormant Oil
Because dormant oil is a mechanical means of insect control, it is unlikely that insects will develop a resistance to the oil. It is applied directly to specific trees when the target insects are dormant, thus reducing the impact on beneficial insects. Dormant insects are affected across the board and even eggs can be destroyed using dormant oils. Since they are highly refined, dormant oils have little impact on animals or humans.
Dormant Oil Chemicals
Dormant oils are no longer made of unrefined petroleum products, but they do still contain oils from varying sources. Most contain naphthene and paraffin in varying quantities. Generally, the paraffin is in larger portions and the naphthene in smaller portions because it is more likely to damage plants. Some dormant oils also contain sulfur, which will be indicated on the label as unsulfonated residue. A higher rating indicates a lower amount of sulfur.
Many Horticultural Oils
Dormant oil is only one kind of modern horticultural oil. The term "dormant" is used to refer to oils that are applied during the dormant season, rather than a specific type of oil. Superior oil is a description that applies to all modern horticultural oils -- these are more refined oils that can be applied to foliage as well as woody plant parts and are thus safe to use on growing plants. Neem oil is an oil derived from the seeds of the neem tree (Azadirachta indica), it has both fungicidal and insecticidal properties. Vegetable oils are somewhat effective for insect control, but the results are mixed.