Landscape and Nursery - Horticultural Oils

Horticultural oil is a major tool that is used for the control of scales, aphids, mealybugs, and other similar piercing/sucking insects in nurseries and landscapes. The following is an article on the subject.

HORTICULTURAL SPRAY OILS are great tools to use in the fight against pests and diseases. With the “dormant” season fast approaching, the subject of spray oils is an important one to review. A “dormant” oil is one that should only be applied while plants are in their dormant phase. Many evergreen plants are not dormant during the fall, winter, and spring and can be severely injured by dormant oils. In contrast, “horticultural” oils (also called summer or verdant oils) were developed to be used when plants were not in the dormant phase. Most horticultural spray oils are used at rates as low as 1% by volume (1.28 fl. oz./gal water) or 2% by volume (2.56 fl. oz./gal water). However, they can be used at dormant rates (3-4% by volume), depending on the application site. Oils are mixed with emulsifiers to allow the oil to mix better in water. Since oil and water do not thoroughly mix even with the best emulsifiers, keep the solution agitated to avoid spray burn. Other key terms to know when selecting a spray oil are the UR Number the amount of unsulfonated residues - these residues are what contribute to spray burn. A UR number/percent of 92-96 is a good value. Another value to note is the “distillation range” of your oil a value in between 412F-468F is a higher quality spray oil with less impurities. Don't apply spray oils below freezing or when humidity is high. Always avoid using them on plants with a grayish cast or waxy coat (e.g. Colorado Blue Spruce). While oil may be tank mixed with some pesticides, this increases chances of spray burn particularly if the tank mixed product is an EC formulation. NEVER TANK MIX OIL WITH SULFUR this is an unsafe and dangerous practice for plants and people. A great web resource for horticultural oils can be found at a Cornell Cooperative Extension Website: http://www.ccenassau.org/hort/fact_sheets/d110_hort_oils_apr00.pdf

Note: The good thing to know is that most horticultural oils now on the market that are sold by reputable suppliers are very highly refined and meet the standards above. - Gordon Johnson

Article by Casey Sclar, IPM Coordinator, Longwood Gardens