Be Creative,   Be Inspired,   Be Selective   |   M-F: 7-7,  Sat: 7-6    |   937-845-0093

Horticultural Oil & Insecticidal Soap

June 20, 2022

ARE A SAFE, EFFECTIVE, AND LOW TOXICITY ALTERNATIVE

To Pesticides!

Horticultural oil is especially formulated for use on plants. It is a petroleum-based oil with an emulsifier added that allows it to mix with water. It kills scales, mites, aphids, whiteflies and other soft-bodied pests that are present by suffocation, plugging up the insects breathing pores.

Horticultural oils are classified as dormant oils, summer oils or superior oils. Dormant oils are the heaviest of the horticultural oils. Apply these oils during winter dormancy and before plants begin spring growth. Do not use dormant oils during the growing season unless the label specifically states that such use is safe.

Summer oils are lighter than dormant oils and are formulated for use during spring and summer. Superior oils are the most refined of all petroleum oils and are excellent products for horticultural pest control in any season. Superior oil products allow greater flexibility in their use and have been tested at temperatures in the mid-90s with no damage to shrubs.

Insecticidal soaps basically are made from potassium salt of oleic acid, which is present in high quantities in olive and other vegetable oils. Insecticidal soap causes insects to dehydrate by physically breaking down the insect’s outer protective layer (cuticle).

Thorough spray coverage is essential when using oils and soaps. Since these products are not poisons and must contact the pest, all plant surfaces, tops and bottom sides of leaves and stems must be coated with spray for best results. Horticultural oils are sold under various descriptive names such as “dormant oil,” “oil emulsion,” “pesticide oil,” “summer oil” and “superfine oil.”

Aphid Damage

Oils

Horticultural oils are generally classified as having a physical mode of action, rather than a chemical one like synthetic insecticides. The most common way that horticultural oils work is to coat and clog the spiracles, which are pores that insects use to breathe. Additional effects such as disruption of membrane functions and interfering with the sucking or piercing actions that some insects use to eat.

Fungal diseases like powdery mildew and sooty mold can be controlled with horticultural oils in three ways:

Controlling the insects that carry them, trapping fungal spores, which prevents them from spreading, and coating plant surfaces, making it difficult for fungi to adhere to the host plant.

Oil Application

Horticultural oils are applied as sprays directly to plant parts; they cannot be applied to the soil. They also cannot be used as preventative treatments since they do not provide residual control if the pest is not present. Most of the time, you will find pre-mixed products at garden and hardware stores; follow the label instructions for application rates and timing.

In some cases, you may find concentrated products that need to be diluted. Application rates are usually 2%-5%. Always check the label for instructions about mixing.

Since horticultural oils are volatile (likely to evaporate) and can degrade rapidly, it is important to protect yourself by wearing appropriate gloves and eyewear and to make applications when environmental conditions are right. Always read the label for temperature and humidity limitations before making applications to avoid damaging plants. High temperatures during oil application can kill plants and high humidity levels can slow the evaporation of the oil.

Here are some do’s and don’t for applications:

Do not apply to heat or drought-stressed plants or when leaves are wilted.

Do not apply in humidity over 90%.

Do not apply within 2 weeks (or as the label states) of a sulfur-based application.

Do not apply if leaves are wet since oil will have a difficult time adhering to the foliage.

Do not apply when temperatures are below freezing.

Be aware of any harvest time limitations.

If oils are mixed with other pesticides, consult the other pesticide labels for any additional limitations or pre-harvest intervals.

Aphids
Echinacea Sombrero Baja Burgundy

Soap

Soaps are made when the fatty acid portion of either plant or animal oils are joined with a strong alkali. They are potassium salts of fatty acids. Commercial insecticidal soaps are a highly refined version of liquid dish soap. While you could make your insecticidal soap mixture, there is a substantially increased risk of plant injury with them. Dry dish detergent and all clothes-washing detergents are far too harsh to use on plants because of all the additives in them. Some soaps and detergents are poor insecticides, and other additives in these products may be phytotoxic (i.e., they may damage the plant).

Some plants are sensitive to soap sprays and may be seriously injured by them. Read the label to make sure your plant is not one of them.

Sensitive plants include portulaca (Portulaca grandiflora), hawthorn (Crataegus spp.), sweet pea (Lathyrus odoratus), cherries (Prunus spp.), plum (Prunus spp.), horse chestnut (Aesculus hippocastanum), mountain ash (Sorbus spp.), Japanese maple (Acer palmatum), bleeding heart (Lamprocapnos spectabilis), maidenhair fern (Adiantum spp.), crown of thorns (Euphorbia milii), lantana (Lantana camara), nasturtiums (Tropaeolum spp.), gardenias (Gardenia jasminoides), and Easter lilies (Lilium longiflorum). Conifers may be sensitive under drought conditions. Plants with a bluish color caused by a waxy leaf coat may lose their desirable color as the wax is washed away.

Other somewhat sensitive plants are azaleas (Rhododendron spp.), begonias (Begonia spp.), fuchsias (Fuchsia spp.), geraniums (Pelargonium spp.), and impatiens (Impatiens spp.). Rinse plants with a clean water spray if they show signs of wilting or leaf edge browning within a few hours of treatment.

Soap Application

As with anything applied to plants, it is important to read the entire label and carefully follow the directions. Insecticidal soaps are usually used as a 1 to 2% solution (2½ to 5 tablespoons per gallon). Always follow the label for the product you are using. Do not attempt to use it in higher concentrations, as this may be very harmful. Mix the soap concentrate in a clean sprayer. Do not apply the soap in full sun or at temperatures above 90 ºF as this may damage the plants. High temperatures and high humidity may increase plant stress and, therefore, sensitivity. It is best to treat your plants in the early morning or late in the day. Since the soap spray is only effective as long as it is wet, the slower drying conditions favor better insect or mite control.

It is important to spray both the top surface and, especially, the underside of the leaves as many of the pests will be found there. Because of the relatively short residual action and the fact that the insects must be in contact with the soap to be effective, repeat applications may be necessary every 4- to 7-days (follow the label directions) until the pests are eliminated. Avoid excessive applications as leaf damage may accumulate with repeated exposure. Always follow the directions on the label.

The quality of the water you are using should be considered when using insecticidal soaps. Hard water reduces the effectiveness of the insecticidal soap. Calcium, magnesium, and iron cause the fatty acids to precipitate out of the solution causing the soap to be ineffective. It is important to use the purest water possible. You can determine if your tap water is compatible by mixing the recommended concentration of soap that you want to use with the appropriate amount of water in a glass jar. Agitate and let the mixture stand for 15 minutes. If the mix remains uniform and milky, the water quality is fine for the spray. If there is scum on the surface, you should use distilled or bottled water.

Summer Wave Large Blue
Red Elf Coreopsis

Soap Tips

The only disadvantages of insecticidal soaps are associated with the limitations of their nature.

The soap solution must wet the insect during application.

There is no residual effectiveness because soap dries or is washed away.

There is a potential for phytotoxicity when the soap residue is affected by high temperatures.

Insecticidal soap is a great tool for any gardener. It provides a safe and effective way to grow plants naturally, controls many soft-bodied pests safely, and reduces the number of harsh chemicals needed to keep your garden lush, lovely, and healthy.

Source: Clemson Cooperative Extension: https://hgic.clemson.edu/factsheet/insecticidal-soaps-for-garden-pest-control/

Original Author(s)
Joyce D. Ubl, Master Gardener
Carlin Munnerlyn, Retired Horticulture Extension Agent, Clemson University

Revisions by:
Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University




X