In the Extension office this time of year, we often receive calls from concerned people wondering if they had missed the timing for dormant oil application to fruit trees. When asked why they can’t apply dormant oil, the answer is always “because it’s not the dormant season now!” They are relieved to hear the answer I give them of “have no fear and confidently spray away”! Then the discussions begin, “then why is it called a dormant oil spray if that doesn’t matter!”
All Seasons Horticultural & Dormant Spray Oil by Bonide
Let’s clarify some confusion and preconceived notions about using dormant oil sprays. First and foremost, what is dormant oil? Traditionally, these oil sprays were derived from paraffinic crude oil and were a heavyweight or more viscous than other petroleum-based oil sprays. Initially, these oils contained various sulfonated residues, which could cause damage to foliage or young, green leaf tissue. For this reason, products like the classic “Volck oil”, were given the moniker of “dormant oil” due to application timing being the greatest factor for ensuring safe usage. Consequently, dormant oils were ONLY applied to deciduous plants, specifically most fruit and shade trees, during the dormant season when plants were not growing!
Nevertheless, as oil refining processes and techniques improved, dormant oils could be more highly refined to create “superior” oils. Another popular term used for superior oil is “supreme” oil. Superior oils were less viscous and contained little to no sulfur residues compared to dormant oils. These oils were “superior” for use during the growing season when plants were in full leaf, if they were mixed and used in the correct concentration. They are also often called “summer” oils, again reflecting the time of year for usage of the product for treatment.
All these oils now fall into a commonly used category of “horticultural oils”. Horticultural oils are pesticides that are specifically designed to control plant insects, mites and some plant diseases. Our commercially available horticultural oils are highly refined petroleum products that are filtered and distilled to remove compounds that can harm plants. They are 92%-99% pure. After the distillation and filtration processes are finished, the oils are formulated with an emulsifier, which allows them to mix/blend with water for ease of application.
Captain Jack's Fruit Tree Spray is NOT a Dormant Oil but an insecticide by Bonide - NOTE: Active Ingredient: 70% Neem seed oil
In addition to petroleum-based products, there are now plant-based horticultural oils available. These may contain soybean, rosemary, cottonseed, sesame, neem or other oils; however, plant-based horticultural oils are less refined and may burn plant foliage more readily, causing what we term “phytotoxicity”. Some common brand names of petroleum-based horticultural oils are Sunspray® or Ultra-Fine®. An example of a plant-based oil is Garden Safe® Neem Oil Extract.
Please ALWAYS read the label of the product and be sure to follow the instructions. Certain species of plants, like arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur), are sensitive to dormant oil sprays applied to the branches or the trunk. Also be aware that the characteristic and prized needle colors of conifers, like Colorado blue spruce, Picea pungens, can be discolored or change from blue to green following an oil application!
Dormant oils are applied to the overall plant structure and specifically to the bark with all of its cracks and crevasses. For dormant oil to be effective, the target insect or pest to be controlled must have an overwintering life stage located on the plant. However, not all insect and/or mite pests overwinter on plants. Dormant oils have contact activity and suffocate the insect or mite pest overwintering life stages by blocking the breathing pores (spiracles) or directly penetrating and disrupting cell membranes. However, dormant oils have minimal residual activity (persistence) once residues dissipate; consequently, thorough coverage of the entire plant and all of its structure is critical for success.