Horticultural Oils – What a Gardener Needs to Know | Extension | University of Nevada, Reno (2023)

Introduction

As long as humans have been raising plants for food, they have been trying to control pests. Sulfur compounds were applied as far back as 2500 B.C. In 350 B.C., Romans used oil and ash for pest control. The Chinese used soap to control pests in 1100 A.D. By the 1880s in the U.S., horticultural oils were used regularly as dormant sprays on fruit trees. Horticultural oils continue to be applied today for pest control.

What Are Horticultural Oils?

Horticultural oils are pesticides that control insects, mites and some plant diseases. They are specifically designed to control plant pests. Commercially available horticultural oils are highly refined petroleum products that are filtered and distilled to remove compounds that can harm plants. They are 92 percent to 99 percent pure. After distillation and filtration, they are then formulated with a mixing agent (emulsifier) to blend with water for ease of application. In addition to petroleum-based products, plant-based horticultural oils are also available. These may contain soybean, cottonseed, sesame, neem or other oils. However, plant-based horticultural oils are less refined and may burn plants more readily (phytotoxicity).

List of Terms

Delayed Dormant Oil – Oil applied when buds are showing 1/16 to ½ inch of green.

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Dormant Oil –Oil applied to woody plants during dormant stage of growth (winter) before buds open in the spring at a higher concentration than summer oil. Refers to season of application.

Horticultural Oil –A pesticide used to control insects, mites and some diseases. May be petroleum- or plant-based.

Nonselective –Kills all insects, including beneficial insects.

Phytotoxicity – Plant injury due to a toxic effect by a compound.

Summer Oil – Oil applied when plants are in leaf at a lower concentration than dormant oil. Sometimes called superior oils. Refers to season of application.

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Superior Oil – Oil that doesn’t contain sulfur and can be applied when plants are in leaf. Also called supreme oil.

Supreme Oil – Highly refined oil. Similar to a superior oil.

Historically, horticultural oils were called “dormant” oils because they were sprayed only when plants, particularly fruit and shade trees, were in a dormant stage of growth before buds opened in the spring. As refining processes improved, “superior” oils were developed. These were lighter weight than the previous dormant oils and contained no sulfur. These were less likely to burn plants than traditional dormant oils. Because of this, superior oils mixed in the proper concentration can be applied, with some precautions, during the growing season when plants are in full leaf. They are also called “summer” oils, referring to the time of year for treatment. The term “dormant” no longer refers to the type of oil, but instead now refers to the seasonal timing of the application. “Supreme” oil is another term for superior oil.

Some common brand names of petroleum-based horticultural oils are Volck®, Sunspray® or Ultra-Fine®. An example of a plant-based oil is Garden Safe® Neem Oil Extract.

Neem Oil

Neem is a naturally occurring botanical pesticide found in seeds from the neem tree. It is made up of many components, but azadirachtin is the most active insecticidal ingredient. It reduces insect feeding, growth and egg laying. It also acts as a repellant. It is effective against immature stages of insects. Neem seed oil without the azadirachtin works as a protectant against insects, mites and fungi. It reduces fungal infection by preventing spore germination and penetration.

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Neem oil, with or without azadirachtin, is practically non-toxic to birds, mammals, bees (if applied late evening or early morning when bees are inactive) and plants, but is slightly toxic to fish and other aquatic animals. Neem products may be registered for fruits, herbs and vegetables in addition to woody plants. Repeat applications may be needed at seven- to 10 day intervals for fungi and more often for insects.

(Cranshaw & Baxendale, 2013; NPIC, 2012; Pundt, 2000; UC IPM, 2012.)

What Pests Are Controlled?

Oils are most effective against exposed eggs, immature stages and soft-bodied adult insects. These include scales, aphids, whiteflies, mealybugs, thrips, leafhoppers and arachnids, such as spider mites, on fruit or shade trees and on many ornamental plants. Oils are also used to control diseases such as powdery mildew, downy mildew, rust and leaf spot.

Besides being insecticides, horticultural oils can also be used as fungicides. The oil reduces the ability of the fungi to grow. By killing insects that spread viruses, viral diseases can be reduced.

How Do Oils Work?

Oils control insects with direct contact. The insect must be present for the oil to work. Complete coverage of the insect population is required for the treatment to be effective. The oil has no effect after it has dried. When horticultural oils are sprayed onto the plant, the oil covers any exposed insects or eggs and suffocates them by clogging their breathing tubes.

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This is a nonselective process, with the oil killing almost any insect it covers and reducing hatching success. In addition, the oil may disrupt how an insect feeds or interfere with cell membranes or normal metabolism. In the winter, dormant sprays only kill overwintering insects and exposed eggs.

What Are the Advantages to Using Horticultural Oils?

  • Oils are relatively safe for humans and wildlife.
  • While they are nonselective, smothering most insects they contact, including beneficial insects, oils evaporate quickly, degrade rapidly and leave no toxic residue. This makes them less disruptive to beneficial insect populations than chemical insecticides.
  • Since the mode of action is mechanical (smothering) rather than chemical, there is less likelihood of insects developing resistance to the oils.
  • Oils generally need no special equipment for application, other than standard garden sprayers.
  • Some formulations have been approved by the Organic Materials Review Institute and can be used by organic gardeners.

What Are the Disadvantages to Oils?

  • Horticultural oils can cause skin or eye irritation to humans.
  • They are toxic to fish and some are toxic to bees unless sprayed in early morning or late evening.
  • They can burn sensitive plants.
  • Spraying with oil at a dormant concentration after bud break when leaves have emerged may kill the young leaves, so the correct rate must be used at the appropriate time of year.
  • Since the oil does not work once it dries, it has little residual effect; new infestations are not controlled by a previous application.
  • Blue-colored evergreens can lose their blue color because the oil removes the bluish material from the needles.
  • Horticultural oils should not be used during temperatures in the 90s, or on drought-stressed plants or new transplants.
  • Oils should not be applied during freezing temperatures.
  • They should only be applied when plant surfaces are dry, but plants are well irrigated.
  • They should not be used on new growing shoots except as mentioned under “Dormant or Delayed Dormant Application.”
  • Fall treatments may cause winter injury.
  • Oils cannot be combined with sulfur products or sprayed within 30 days of a sulfur application.

Dormant or Delayed Dormant Application

There are two methods for dormant horticultural oil application. One is to apply it before the buds break or show any color. However, this can speed up spring bud development, making buds more susceptible to frost and cold temperature damage. The second method is the delayed dormant application. This occurs when buds are open at the tip showing 1/16-inch to ½-inch of green leaf tissue.

Dormant oil should not be sprayed 48 hours before or after a freeze occurs or is predicted.

Usually a higher rate of oil is mixed and applied during dormant or delayed dormant than with a summer application. The proper rate is listed on the label.

Always read and follow all label directions for proper timing and rates dependent on the stage of the life cycle of the pest. Apply only when the pest is present.

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Sufficient water must be mixed with the oil to cover all the bark cracks and crevices. According to University of Nebraska-Lincoln Extension, a 20-foot-tall tree will probably require 4 gallons of water for complete coverage.

Sensitive Plants*

  • Junipers
  • Cedars
  • Maples — particularly Japanese and red
  • Redbud
  • Smoke tree
  • Spruce — particularly dwarf Alberta

*Other plants may also be sensitive. Test a small portion of the infested plant prior to spraying the entire plant to determine if the plant is sensitive. Different plants exhibit sensitivity at different times of the year.

Conclusion

  • Horticultural oils work well to control insect pests and, if used properly, can be a less toxic approach than chemically based insecticides.
  • A dormant or delayed dormant application can kill many of the overwintering insect pests that would normally plague plants in the late spring, such as aphids. Treating in the winter or early spring can save time and avoid later plant problems.
  • Summer applications at the correct rate provide excellent pest control with the least amount of damage to beneficial insects or the environment.

Bibliography

  1. Chalker-Scott, L. 2008. Horticulture oils. Washington State University. [Online] Available at: WSU
  2. Cornell Cooperative Extension. 2005. Pest management around the home part II. Tips on using insecticidal soaps and horticultural oils. Cornell University Cooperative Extension Lewis County. [Online] Available at:CCE Lewis
  3. Cranshaw, W.S. and Baxendale, B. 2013. Insect control: horticultural oils. Colorado State University Extension. [Online] Available at: CSU
  4. Galloway, W. E. (undated). Horticultural oils explained. Organic Gardening. Emmaus, PA.
  5. Mason, S. (undated). Just what is dormant oil? University of Illinois Extension. [Online] Available at: Illinois Extension
  6. NPIC — National Pesticide Information Center. 2012. Neem oil general fact sheet. Oregon State University. [Online] Available at: NPIC
  7. Olkowski, W., Daar, S. and Olkowski, W. 1991. Common-sense pest control, least toxic solutions for your home, garden, pets and community. Taunton Press, Inc. Newtown, CT.
  8. Pundt, Leanne. 2000. Neem based insecticides. University of Connecticut. Integrated Pest Management.
  9. UC IPM Online. 2012. How to manage pests, pesticide information. Agriculture and Natural Resources, University of California. [Online] Available at: IPM
  10. University of Nebraska-Lincoln Extension. 2013. Using dormant & horticultural oil application for insect control. [Online] Available at: UNL
  11. Walsh, D., Zalom, F. and Grove, G. Adapted by Pscheidt, J.W. 2013. Horticultural spray oils. Pacific Northwest Disease Management Handbook. [Online] Available at: Pest management Handbook

FAQs

What is considered a horticultural oil? ›

Horticultural oils are pesticides that control insects, mites and some plant diseases. They are specifically designed to control plant pests. Commercially available horticultural oils are highly refined petroleum products that are filtered and distilled to remove compounds that can harm plants.

Will rain wash off horticultural oil? ›

How much drying time is needed before rain? Per the Bonide All Seasons Horticultural Spray Oil product label, you should apply when rain is not expected within 24 hours to ensure rain does not wash pesticide off the treated area. 167 of 176 people found this answer helpful.

Can I use mineral oil instead of horticultural oil? ›

Mineral oils and vegetable oils like canola oil, cottonseed oil, and soybean oil are all classified as horticultural and can be used in the garden to prevent pests and diseases.

What is the active ingredient in horticultural oil? ›

ORGANIC GARDENING - The active ingredient of All Seasons Spray Oil is mineral oil.

What temperature should I spray my horticultural oil? ›

If sprays are needed during summer, applications can be made in the cooler morning or evening hours. In cool weather, make sure temperatures during application are 50-60° F or above for several consecutive days (with nightly lows above freezing) to avoid phytotoxicity and so the oil maintains the proper viscosity.

Can you use horticultural oil on all plants? ›

Although safe to animals, horticultural oils may damage some plants, so landscapers and arborists should only use highly refined oil products and know what rates to use and which plants are sensitive.

How often do you spray horticultural oil? ›

Apply every 7-14 days as needed. Apply enough liquid per plant to wet entire surface.

Can I spray horticultural oil before rain? ›

Avoid spraying on windy days, before rainfall or irrigation, in high humidity or when plant leaves are wet. Horticultural oil will harm beneficial bugs as well as pests, so only spray plants with a confirmed pest problem or when beneficials are not present.

How long does horticultural oil take to dry? ›

It takes about a couple of hours for dormant oil to dry. You can repeat application in 2-3 weeks.

How much water do you mix with horticultural oil? ›

DIRECTIONS FOR USE: Mix 5-10 Tbsp per gallon of water, depending on pest levels and time of year. Mix thoroughly and apply enough solution per tree to wet entire surface of branches, limbs and trunk.

What plants are sensitive to horticultural oil? ›

There are also some plants, trees, and shrubs like beech, Douglas fir, white pine, and maple trees that are quite sensitive to horticultural oils, and you should avoid using them on these altogether.

How much do you dilute horticultural 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.

Can you spray horticultural oil on fruit? ›

Winter time is the time to spray fruit trees and small fruits species with horticultural oil (also called dormant oil) to decrease insect populations and minimize seasonal insect build-up. Horticultural oil is probably one of the most important sprays that can be applied to fruit trees.

Can I use horticultural oil in the summer? ›

The hot season does have an optimal time for using horticultural oils. You need to adjust the application time of horticultural oils to be during the evenings, after sunset or early in the morning before the temperatures rise above 85° F.

Does horticultural oil work on powdery mildew? ›

To eradicate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil or one of the plant-based oils such as neem oil (such as Green Light Neem Concentrate) or jojoba oil (such as E-rase).

Which is better horticultural oil or insecticidal soap? ›

Overall, horticultural oil killed a greater percentage of armored scales than soft scales, whereas insecticidal soap gave greater control against soft scales. We suggest that differences were driven by chemical properties of both insect integuments and insecticides.

How do you mix safer horticultural oil? ›

MIXING DIRECTIONS

1. Fill the spray tank or bottle 1/2 full with water. 2. SHAKE THE CONTAINER OF Safer® Brand Horticultural & Dormant Spray Oil VIGOROUSLY PRIOR TO MEASURING OUT THE SPECIFIED AMOUNT.

Can I use horticultural oil in the spring? ›

Horticultural oils are safe to use during the growing season. Early spring is a great time of year for horticultural oil applications, as they are safe and light enough to apply to plants that already have foliage.

Can horticultural oil burn leaves? ›

If you apply oil to the leaves when the temperature is too hot, you can burn the leaves of almost any plant, which may kill them if the damage is severe. It's important to avoid applying horticultural spray oils to drought-stressed plants.

Which oil is best for plant growth? ›

Vegetable oil helps increase plant growth by preserving the moisture in soil and by supplying the plant with certain nutrients.

How do you apply horticultural oil to plants? ›

For a dormant application, oils are sprayed in fall or early spring after leaves have dropped and before growth resumes. Thorough spray coverage is needed on all plant parts to ensure effective control. This allows the oil to smother pests and disease spores that overwintering on woody trunks, limbs, and branches.

Is neem oil same as horticultural oil? ›

At its core, neem oil is a type of pesticide known as horticultural oil, which is useful for controlling a wide range of immature and soft-bodied pests like aphids, white flies and spider mites.

Is insecticidal soap the same as horticultural oil? ›

Horticultural oil kills insects and mites primarily by coating them and clogging their spiracles, which are pores they use to breathe. With clogged spiracles the pests die of suffocation. Insecticidal soaps disrupt cell membranes in the pests but also suffocate them. Thus, these are strictly contact pesticides.

Is dormant oil the same as horticultural oil? ›

Unlike home remedies, they also contain an emulsifier to help water mix with the oil, which will provide more complete coverage of plant surfaces. Dormant oil may also be labeled as horticultural, superior or all-seasons oil; keep reading for more on this.

How late can you apply dormant oil? ›

Dormant oils should be applied in late March or early April before the plants show signs of breaking dormancy (before “bud break”). Dormant oils applied in February or early March are not effective as insects are not actively respiring at this time and, therefore, are not vulnerable to the oil's suffocating effects.

Can horticultural oil be used indoors? ›

Horticultural oils can also be used on Indoor plants to control scale insects, mealy bugs and spider mites. Some indoor plants, such as maidenhair ferns, are more sensitive to leaf damage so it is recommended to test any horticultural oil on a small area of foliage before using to avoid yellowing or leaf burn.

Can I mix horticultural oil and fungicide? ›

Keep in mind you should Never mix lime-sulphur spray with any fungicide or insecticide except dormant oil spray concentrate. It can react with other chemicals to release deadly hydrogen sulfide gas. Applying at the incorrect time can burn leaf buds that have started to swell or show any sign of green.

How much is horticultural oil per gallon? ›

Horticultural Oil Application Instructions

Mix 2 fluid ounces per gallon of water and spray as needed. Use enough spray solution to completely penetrate the leaf canopy and cover both top and bottom of all of the leaves until wet without significant runoff.

Can you spray dormant oil before it rains? ›

Do not spray when temperatures are below 40° F, or when there is a threat of frost for the next 36 hours. The oil works immediately, so a rain within 24 hours will not affect efficacy.

Can you use horticultural oil on tomato plants? ›

Spray the plants thoroughly so that the oil or soap spray drips or “runs off” from the upper and undersides of leaves and plant stems. It is best to spray horticultural oil or insecticidal soap when the temperatures are between 45 and 85 degrees, and always spray in the evening to slow the drying time for best control.

What plants do not use neem oil? ›

Neem oil shouldn't be used on herbs such as basil, caraway, cilantro, dill, marjoram, oregano, parsley, or thyme. Spraying neem oil on plants with delicate or wispy leaves, such as arugula, lettuce, peas, and spinach, should be done with caution because it can cause foliage burns.

Can I use horticultural oil on roses? ›

Roses can also benefit from the use of horticultural oil sprays in the control of black spot, powdery mildew and rust. Powdery mildew can be effectively controlled by using a mixture of 2 Tsp. of Baking Soda with 2 Tbs. of Horticultural Oil in 1 Gallon of Water.

Is there an organic horticultural oil? ›

Natural Guard Organic Horticultural Oil Ready-to-Spray controls insects, mites, and scale on roses, flowers, fruits, vegetables, houseplants, and trees. Horticultural Oil works in both the dormant and growing seasons.

Can olive oil be used for horticultural oil? ›

Olive oil can kill bug infestations on plants by blocking their pores and making it impossible for them to breathe. However, olive oil can also attract more bugs and pests due to its intense and distinctive aroma. Avoid using olive oil as your primary pesticide solution, as this is likely to damage your plant.

How much is a gallon of dormant oil? ›

DORMANT: Use 4 – 5 tbsp. of oil per 1 gal. of water. Apply before buds begin to swell in the spring.

What month do you spray fruit trees? ›

Spraying fruit trees during the cool seasons, November through March, can help control pests that take up residence in the cracks and crevices, according to Ross Penhallegon, horticulturist with the Oregon State University Extension Service.

Can I spray Epsom salt on fruit trees? ›

If you have some fruit trees, a boost in magnesium will do them a world of good. Epsom Salt is used on fruit trees or vegetables to help them yield larger, sweeter, and more fruits. It works great also for nut trees and fruit shrubs.

Can I spray fruit trees with vinegar? ›

Homemade fruit tree sprays can boost fruit tree health and productivity and they are easy to make from ordinary products like molasses, vinegar and yoghurt. Healthy fruit trees will produce an abundance of delicious, fresh fruit.

When Should I spray my roses for horticultural oil? ›

Late April is the time to apply dormant spray to fruit trees, to roses, and to certain ornamental shrubs that are susceptible to scale insects and spider mites.

Does horticultural oil expire? ›

Answer: The shelf life of Bonide All Seasons Horticultural Spray Oil is 3-5 years as long as it's stored in a temperature controlled environment and out of direct sunlight. 56 of 59 people found this answer helpful.

Will Epsom salt get rid of powdery mildew? ›

Simply mix one teaspoon of baking soda with one teaspoon of Epsom salts to create an effective treatment for powdery mildew.

Does soapy water get rid of powdery mildew? ›

THE CURE-ALL REMEDY FOR POWDERY MILDEW ON PLANTS

I recommend first spraying with a diluted solution of baking soda and dishwashing soap. Mix one tablespoon of baking and one teaspoon of liquid soap (not detergent) to a gallon of water. Spray on plants every one to two weeks.

How do you get rid of powdery mildew spores in soil? ›

Effective organic fungicides for treating powdery mildew include sulfur, lime-sulfur, neem oil, and potassium bicarbonate. These are most effective when used prior to infection or when you first see signs of the disease. Baking soda has been proved by many gardeners to be effective in treating powdery mildew.

Can vegetable oil be used for horticultural oil? ›

Today's horticultural oils include vegetable as well as mineral oil products. Recent studies have suggested that plant-based oils, such as soybean, are often just as effective as mineral oils and may be less phytotoxic.

Are neem oil and horticultural oil the same? ›

At its core, neem oil is a type of pesticide known as horticultural oil, which is useful for controlling a wide range of immature and soft-bodied pests like aphids, white flies and spider mites.

How often can you spray horticultural oil? ›

Fruit trees should only be treated with dormant oil when dormant; which is prior to bud swell. Applications may be repeated on fruit trees in 3 to 4 week intervals.

Does Dawn work as insecticidal soap? ›

Can you use other kinds of soap to make insecticidal soap at home? It's not recommended to use dish detergent (like Dawn), laundry detergent, or hand soap (even the “natural” versions), since these soaps contain abrasive ingredients that could harm your plants.

Can I use Dawn dish soap on my plants? ›

Dawn dish soap is safe for plants if you use a small amount diluted with water. It does contain chemicals that can be abrasive and harm plants if used in large amounts. I would also recommend using a Dawn variety that has minimal additives used for scent or cleaning.

What plants should not be treated with neem oil? ›

Neem oil shouldn't be used on herbs such as basil, caraway, cilantro, dill, marjoram, oregano, parsley, or thyme. Spraying neem oil on plants with delicate or wispy leaves, such as arugula, lettuce, peas, and spinach, should be done with caution because it can cause foliage burns.

Is neem oil similar to ivermectin? ›

48 Neem plant extract (Azadirachta indica A. Juss, Meliaceae) has been proposed as a viable 49 natural alternative to Ivermectin, due to its rapid degradation, low toxicity to mammals 16, and 50 effectiveness in controlling nematodes and ticks 17,18.

What is better than neem oil? ›

While neem oil has long been the go-to pest solution for organic gardeners, rosemary oil may offer a more effective solution.

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