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Plant Answer Line Question

Keywords: Horticultural oil, Apples--Diseases and pests

I have a dwarf Braeburn apple tree that gets spotted apples every year. The leaves drop off and the apples are stunted and not edible. I am spraying with dormant oil spray per the instructions and it looks beautiful right now. I need to know how often to spray it and how long into the season. The instructions aren't clear on this. Also, does the dormant oil spray make the apples unsafe to eat at all?


Here is what Michael Phillips says in his book, The Apple Grower: A Guide for the Organic Orchardist (Chelsea Green, 1998): "Oil sprays smother the overwintering eggs and emerging nymphs of a number of foliar feeders. Use of a highly refined oil is tolerable in an organic orchard, but generally not necessary." He recommends encouraging beneficial insects to control aphids. Aphids may be a sign of a deeper imbalance that needs addressing.

Whether the dormant oil spray makes the fruit unsafe to eat depends greatly on what the oil is made of: many such sprays are petroleum-based and would therefore not be safe. See the following information formerly available online from BeyondPesticides.org:


"Most horticulture oils used today are petroleum based (Grossman 1990), yet a growing number of horticulture oils are being made with vegetable oils, which are considered a least toxic pesticide. Carefully read the label or ask your pest control service provider to determine if the horticulture oil is vegetable or petroleum based."

From Washington State University Extension agent Mary Robson:

"How Do I Use Dormant Sprays?"

"Neither the spray nor the applicator is dormant in a 'dormant spray': the plants to which it's applied are. The term refers to winter-applied sprays for insect pests and diseases, put on before foliage begins to leaf out.

"To use dormant sprays, first identify the reason for the spraying. They are often used on fruit trees to control over-wintering insect pests such as scale and aphids. (The aphids over-winter as eggs, and the spray smothers the eggs, preventing spring hatching.) A dormant spray isn't an all-purpose winter splashing of pesticide around the garden: it's a specific spray chosen for a specific pest. The dormant spray used on fruit trees is often horticultural oil (sold as superior-type oil), and it may be mixed with lime-sulfur depending on the pest to be controlled. It's sprayed thoroughly to give good coverage on the trunk, branches, small limbs and shoots.

"Because dormant sprays are generally applied early in the season, they tend to be less disruptive to beneficial insect predators and parasites which aren't in active life stages in mid-winter. While generally used in fruit tree maintenance, dormant oil sprays are helpful for landscape plants with similar aphid or scale problems. Ornamental plums (purple-leaf plums) often suffer from infestations of aphids or scale; if that's been the case, a dormant oil spray may help reduce the populations."

The following two links are from the Colorado State University Cooperative Extension. Insect Control: Horticultural Oils and Horticultural Oils. Excerpt from a version of this page no longer available online:

"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."

Have you determined what the cause of the spotting on your apples is? Might it be apple scab? In case that is what you have been seeing, here is what Washington State University Extension says:

"Apple scab is caused by a fungus which also causes scab on crabapple and hawthorn. The first infections occur during wet weather in the spring. Initially, the disease causes tiny, pale, chlorotic, water-soaked spots on the leaves. The spots enlarge and darken to a dark, velvety, olive-green then to black. Leaves may become distorted, puckered, and mottled. Leaves may drop, sometimes resulting in severe defoliation of susceptible trees. Scab can also affect fruit. Fruits infected early in development show olive-green to brown, roughened or corky spots which may develop deep cracks. These apples are often misshapen. Fruits infected at later stages develop small black "pinpoint" scab spots while in storage. The disease is favored by cool, wet conditions and overwinters in infected plant debris.

"Management Options:

"Select Non-chemical Management Options as Your First Choice!!

  • Avoid overhead irrigation.
  • Plant in full sun.
  • Plant scab-resistant varieties such as 'Akane', 'Chehalis', 'Liberty', 'Paulared', 'Prima', or 'Tydeman Red'.
  • Rake and destroy (do not compost) fallen leaves, or cover them with soil.
  • Space plantings and prune to provide good air circulation and light penetration.
  • The application of nitrogen to the leaves in the fall will enhance the decomposition of the fallen leaves."

The following website is for large-scale growers, but may have information of interest to you:
Apples: Organic Production Guide by Tammi Hinman and Guy Ames, 2011

Date 2019-04-06
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Plant Answer Line Question

Keywords: Azadirachta indica, Horticultural oil, Laurus nobilis, Insect pests--Control

I have some insects on my bay laurel, which we use for seasoning. Someone told me to spray it with horticultural oil. I wondered, though, if it would still be safe to use the leaves in cooking. Is horticultural oil petroleum based?


As you suspected, horticultural oil is petroleum-based. U.C.Davis provides information on Integrated Pest Management for the plant I am assuming you mean, the usual source of culinary bay leaves, sometimes called sweetbay, or Laurus nobilis. (Just to be clear, I don't think you are referring to the plant known as California bay laurel, Umbellularia californica, which has leaves that are aromatic and reportedly edible, but not commonly used for seasoning.) They also note that horticultural oils are "...specially refined petroleum products, often called narrow-range, superior, or supreme oils. Some botanical (plant-derived) oils are also available."

When referring to plant-derived oils, I believe they are referring to Neem oil, as described in this information from U.C. Davis Integrated Pest Management. Additional information from the Cornell University Resource Guide for Organic Inesct and Disease Management about Neem and human health is excerpted here:

Studies of azadirachtin mutagenicity and acute toxicity have shown that it likely does not pose a significant risk to human health. However, some people have exhibited skin and mucous membrane irritation from neem seed dust (Weinzierl and Henn 1991). Note that most studies have been done on azadirachtin, and may not show the effects of a whole neem product. Neem is used in some commercial human hygiene products.

Another long excerpt on horticultural oil from Colorado State University Extension suggests the following:

Essentially all commercially available horticultural oils [...] are refined petroleum products also known as mineral oils. Impurities in the oil that are associated with plant injury, such as aromatic compounds and compounds containing sulfur, nitrogen or oxygen, are removed. Filtration, distillation and dewaxing complete the production of the finished base oil. Final formulations of horticultural oils are normally combined with an emulsifying agent that allows the oil to mix with water. This mixture usually is used at about a 2 percent dilution.

Vegetable oils also can be used as insecticides, although the type of oil can greatly affect its activity. Cottonseed oil is generally considered the most insecticidal of the vegetable oils. Soybean oil, the most commonly available vegetable oil used in cooking, has often provided fair to good control of some insects and mites.

Extracts from seeds of the neem tree, Azadirachta indica, have recently attracted attention as a source of pest management products. Several neem-derived insecticides have been developed. A number of compounds found in neem seeds, notably azadirachtin, have proven useful as insecticides. However, the oil fraction of neem seed extracts, which is mostly free of azadirachtin and related terpenoid compounds, also has demonstrated effects as a fungicide and insecticide. At least one product currently on the market, TrilogyR, consists of a largely azadirachtin-free oil fraction of neem seed extracts. It is formulated with an emulsifier and mixed with water at a concentration similar to horticultural oils (0.5 to 2.0 percent). Many over-the-counter products sold in nurseries that mention neem contain the oils of neem seed extracts.

If you know the insect on your bay tree, Peaceful Valley Farm Supply is one commercial supplier which carries less toxic, of not organic, products to control specific pests. That does not necessarily mean they are safe enough to spray on leaves which will be harvested for cooking.

Date 2018-09-29
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