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Gardening Articles: Care :: Pests & Problems

Horticultural Oil (page 2 of 2)

by Whitney Cranshaw

Petroleum-based Oils

Most horticultural oils contain naphthene and paraffin compounds. Paraffins are valuable to gardeners because they're more toxic to insects and less toxic to plants than other oil compounds. In contrast, oils containing naphthene are less pesticidal and more likely to injure plants than paraffinic types. Oils high in naphthene also contain more impurities such as phytotoxic aromatic and unsaturated hydrocarbons. However, the newest horticultural oils contain only tiny amounts of those compounds.

Another plant-damaging compound in oil is sulfur, and oils sometimes have a "UR" (unsulfonated residue) rating that indicates sulfur content. The higher the rating, the lower the sulfur content. Most horticultural oils have a UR rating of 90 or above.

Viscosity or thickness is another labeled measure of an oil's effectiveness and safety. (Oil viscosity is measured by how long it takes a given amount of oil to pass through a hole or ring.) For example, a lighter oil that takes 60 seconds to pass through a ring is a 6E oil; thicker oil that takes 80 seconds is an 8E oil. Lighter or thinner oils are more desirable. The UR rating and evaporation range are more reliable plant-safety predictors.

Over the years, horticulturists have coined several, sometimes confusing, terms for various oils.

Dormant oil is used on woody plants, especially fruit trees, during their dormant seasons. New, refined, lightweight oils have replaced older heavy dormant oils. Today, the name refers to the time and rate of application.

Mineral oil is a light, petroleum-derived oil gardeners can use to control corn earworm.

Narrow-range oil is a light oil graded according to the range of temperatures over which it evaporates. Lighter oils evaporate over a narrower range of temperatures than other oils, and thus this term is synonymous with superior or supreme oil. If an oil evaporates quickly, as light oils do, plants have a greater margin of safety.

Spray oil includes soaplike emulsifiers that allow water and oil to mix for spraying.

Summer oil is used on leafy plants during the growing season. Generally, it's the same as narrow-range, superior, and supreme oils.

Superior oil describes new, more refined oils that can be applied safely-at lower rates-to green leaves. Today, all horticultural oils are superior-type oils, and label directions specify varying application rates for use during dormancy or the growing season.

Supreme oil is one brand name for a superior or narrow-range oil.

Neem oil comes from the seeds of the neem tree (Azadirachta indica) and is used as both an insecticide and a fungicide. Neem oils such as Rose Defense or Trilogy (formerly NeemGuard) are effective at killing insect eggs and immature insects, notably small soft-bodied pests such as whiteflies and aphids. It has been shown that neem oil kills certain mite eggs, too. Neem oil also prevents powdery mildew and black spot. Use it on roses, fruit trees, and vegetables.

Neem-derived insecticides contain azadirachtin. Sold under trade names such as Azatin, Bioneem, Margosan-O, and Neemazad, they control whiteflies, aphids, and other soft-bodied insects. Neem oils are largely azadirachtin-free.

Other neem-seed compounds inhibit insect feeding, repel pests, disrupt insect growth, and kill fungi.

Vegetable oil is the term for any oil that's derived from oilseed crops such as soybean, rapeseed (canola), or cottonseed. Stoller's Natur'l Oil is cottonseed oil (the most insecticidal vegetable-seed oil) with an emulsifier. Soybean oil provides good control. Canola and sunflower oils are less effective, and corn oil shows mixed results in tests.

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