Horticultural Oil 101: Your Guide To Natural Plant Protection

Last Updated December 15, 2020 By Bella Zinti

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Horticultural oil is an oil-based pesticide used in horticulture and agriculture. It is an effective and ecologically friendly way to control and manage insects, mite pests, and even certain diseases. It is a valuable tool for managing insects and mite pests in the edible garden, flowers, fruit trees, and even houseplants.

Horticulture oil became a popular way of controlling and managing insect and pest problems with all types of plants. Especially on fruit trees because it is an edible plant. Unlike chemical insecticides that remove all insects, oils allow pollinating insects to stay and do their helpful jobs.

What Is Horticulture Oil?

Horticulture oil is a complex mixture of a vegetable (soybean and cottonseed), mineral, or petroleum-based oil. Typically, the oil is blended with an emulsifying agent, often water, to create a lightweight spray solution. Horticultural oil is considered safe and is the main form of organic pest control, as they are an effective and eco-friendly way to combat certain diseases, pests, and mites in the garden. Insects generally do not build resistance to horticulture oil.

Types of Horticultural Oil

Mineral Oil: Mineral oils are versatile horticultural oil that is effective against a wide range of pests, including aphids, scales, and mites. It creates a protective shield on the surfaces of plants, effectively suffocating and disrupting pest populations. It can be used safely on most plants throughout the growing season.

Neem Oil: Neem oil, extracted from the neem tree, is renowned for its dual-action advantages. It controls pests such as aphids, mealybugs, and caterpillars while also having antifungal properties for disease prevention. Neem oil can be used year-round but should be tested on sensitive plants before widespread application.

Petroleum-Based Oils: These oils, similar to mineral oil but derived from petroleum, are effective against pests like scales and mites. They are often used on fruit trees and woody plants during the dormant season to smother overwintering pests.

Soybean Oil: Soybean oil serves as a natural and environmentally friendly choice for pest management. It effectively manages pests like aphids, spider mites, and whiteflies and is safe for most plants throughout the year. This oil and other vegetable oils are effective pesticides.

Cottonseed Oil: Cottonseed oil is another natural horticultural oil that controls soft-bodied insects such as aphids, scales, and mealybugs. It can generally be used on most plants.

Jojoba Oil: Jojoba oil is added to horticultural oil formulations to enhance their spread and adhesion on plant surfaces. It improves coverage and effectiveness.

Summer Oil: Summer oil is a lighter-weight horticultural oil designed for use during the growing season. They control pests without causing harm to plants and are suitable for mild to warm weather conditions.

Dormant Oil: Dormant oil is a heavier oil used during the dormant season, typically in winter or early Spring when plants have shed their leaves. They smother overwintering pests and disease spores without harming active plant growth.

Ultra-Fine Oils: Ultra-fine oils are finely refined petroleum-based oils that provide excellent pest control while minimizing plant harm. They are suitable for various plants and weather conditions.

All-Season Oils: All-season oils are versatile horticultural oils designed for year-round use with reduced risk of causing harm to plants. These oils can be applied in various weather conditions, but it's crucial to always follow the instructions provided on the label.

Supreme Oil: Supreme oil is a highly refined and effective choice for controlling garden pests. It forms a protective barrier on plants, suffocating and disrupting the life cycles of insects like aphids, mites, and scales. When used responsibly, Supreme oil is a valuable tool for plant protection.

How It Works

Horticultural oil, often used in gardening and plant care, operates by suffocating and disrupting the life cycles of pests. When applied to plants, it forms a thin, protective film that blocks the breathing pores of insects like aphids, scales, and mites. This suffocation prevents these pests from feeding and reproducing. Additionally, horticultural oil can deter certain fungal spores from establishing themselves on plant surfaces, hindering the development of diseases. It's a safe and environmentally friendly option that, when timed correctly and applied thoroughly, helps maintain plant health by curbing infestations and infections, ultimately promoting the well-being and longevity of the garden or landscape.

How Safe Is Horticulture Oil

Horticultural oils, often called spray oils, are considered a safe and effective tool in gardening when used responsibly. Their safety extends to both plants and the delicate balance of beneficial insects in your garden ecosystem.

Horticultural oils are generally non-phytotoxic when applied correctly, meaning they do not harm plants. They are suitable for various plants, including ornamental flowers, fruit trees, vegetables, and shrubs. However, the timing of application is crucial to ensure plant safety. Avoid applying these oils during extreme weather conditions or when plants are stressed, as this can potentially lead to damage.

Horticultural oils are also considered environmentally friendly compared to some synthetic pesticides. Many formulations are biodegradable and break down relatively quickly in the environment. They also leave minimal to no residue on plants, further reducing their environmental impact.

When To Apply Horticultural Oils

If horticultural oil is needed during dormant months (Feb and March), apply it on the leaves or flower before the bud break in the Spring. This will help combat the pests before they arrive in the coming warm months. During Spring and Summer, you can use the oil again if necessary. By practicing both, it can help prevent and control insects at the different stages of their lives. 

Now, there is a difference between Summer and dormant oil. Summer or all-season oil represents a lighter variant of dormant oil. It is ultra-refined oil, so it is less likely to cause phytotoxicity because it is cleaner and purer. The more highly refined oils, the safer it is to apply to plant leaves during the growing season. On the other hand, Superior oil is less expensive and isn't as pure as summer horticulture oil, and it can only be used during the dormant period.

Read the instructions if you plan on purchasing your horticultural oil from a store. 

When Not To Use

During High Temperature: Horticultural oils are not recommended when temperatures are around 100 degrees Fahrenheit because the plants are already experiencing stress in the extreme conditions. This makes them more susceptible to damage. 

During Freezing Temperature: The same goes for freezing temperatures: the emulsion of oils won't hold together, and coverage would be uneven and spotty. 

Away From Sensitive Plants: Always ensure a plant isn't already known to be sensitive to horticultural oils. (Often, the oils have a list of plants to avoid on their labels.) Also, make sure no sensitive plants are nearby or have tender new growth. 

Pesticide Contains Sulfur: Avoid using horticultural oils if you've used a sulfur-based pesticide recently, as this makes for a toxic combination for the plant in question. 

When Plants Are Too Wet Or During High Humidity: Spring is always an excellent time for plant maintenance. However, avoid days with too much wetness and moisture because this increases the likelihood of evaporation and leaves the plant at risk of damage or burning. 

Benefits Of Using Horticultural Oil Sprays

Using horticultural oil sprays in your gardening practices has many benefits that contribute to healthier plants, reduced environmental impact, and more sustainable pest management. Here's a more detailed exploration of these advantages:

Effective Pest Control: Horticultural oils are highly efficient at controlling various garden pests. They work by forming a physical barrier on plant surfaces, suffocating and disrupting the life cycles of pests like aphids, scales, mites, and mealybugs. This targeted approach reduces infestations and the damage caused by these insects.

Minimized Chemical Use: Opting for horticultural oils over synthetic chemical pesticides reduces your garden's reliance on potentially harmful chemicals. These oils leave minimal residue and are known for their biodegradability, which means they break down naturally in the environment, reducing long-term chemical impact.

Safe for Beneficial Insects: Horticultural oils exhibit selective action, primarily affecting pests with soft bodies. This selectivity ensures that beneficial insects remain unharmed, such as ladybugs and lacewings, which play crucial roles in natural pest control. It's a win-win for pest management that doesn't disrupt the ecosystem.

Disease Prevention: Beyond pest control, horticultural oils create a protective shield on plant surfaces. This layer acts as a deterrent for fungal and bacterial diseases like powdery mildew and rust, reducing the risk of infections and ensuring plant health.

Compatibility: Horticultural oils can seamlessly integrate into your garden's integrated pest management (IPM) strategy. They work well with biological controls and other pest management methods, offering versatility in your approach to maintaining a pest-free garden.

Reduced Resistance: Alternating horticultural oils with different pest control techniques helps prevent the development of pest resistance. By using multiple methods, you can maintain the efficacy of your pest management efforts over the long term.

Environmentally Friendly: Many horticultural oil formulations are considered environmentally friendly. Their ability to biodegrade and leave minimal environmental impact makes them a responsible option for gardeners who prioritize sustainability.

Plant Health: By effectively controlling pests and preventing diseases, horticultural oils contribute to overall plant health. Healthy plants are more resilient and better equipped to withstand environmental stressors, leading to improved growth and higher yields in edible gardens.

Versatility: Horticultural oils are versatile and suitable for use on various types of plants, including ornamental flowers, fruit trees, vegetables, and shrubs. This versatility makes them a valuable tool in diverse gardening settings.

Reduced Risk to Humans and Pets: When used responsibly and according to the manufacturer's instructions, horticultural oils pose a lower risk to human health and pets compared to some chemical pesticides. This makes them a safer choice for gardeners and their furry friends.

Drawbacks Of Using Horticultural Oils

While horticultural oils offer many benefits, they also have drawbacks and limitations. Here are the drawbacks of using horticultural oils in gardening:

Selective Action: Horticultural oils primarily target pests with soft bodies, leaving hard-shelled insects largely unaffected. This selectivity can limit their effectiveness against certain pest species.

Effectiveness on Mature Insects: Horticultural oils are often more effective against younger stages of insects, such as nymphs or larvae, than mature adults. Fully developed insects with thicker exoskeletons may be less susceptible.

Temporary Solution: Horticultural oils provide a temporary solution to pest problems. They establish a protective barrier on plant surfaces; however, this shield may diminish over time, necessitating periodic reapplication to sustain their efficacy.

Weather-Dependent: The effectiveness of horticultural oils can be influenced by weather conditions. Extreme heat or cold can impact their performance, and rain or irrigation shortly after application may wash them away, reducing their efficacy.

Phytotoxicity: While horticultural oils are generally safe for many plants, some species, especially those with delicate or sensitive foliage, may exhibit phytotoxic reactions. Conducting a spot test on a small portion of the plant before widespread application is essential.

Impact on Beneficial Insects: While horticultural oils have a lower impact on beneficial insects than chemical pesticides, they can still harm certain beneficial species if not applied carefully or when pollinators are active. Timing and application practices must be considered to protect these beneficial insects.

Limited Residual Activity: Unlike some chemical pesticides, horticultural oils offer limited residual activity. They do not provide long-term protection, meaning pests can reinfest plants after the oil has worn off.

Plant Stress: Applying horticultural oils during periods of plant stress, such as drought or excessive heat, can exacerbate stress and potentially harm the plants. It's essential to consider the plant's condition before application.

What Pests Are Controlled With Horticultural Oil?

Horticultural oil effectively affects many insects, pests, and even diseases like powdery mildew. Horticulture oils are known to effectively control aphids, caterpillar eggs, leafhoppers, adelgids, mealybugs, scale, mites, spider mites, whiteflies, and thrips. Because of their effectiveness with aphids, horticultural oils help control the spread of viruses and diseases from one plant to another.

Symptoms Of Plant Damage

Damage to plants resulting from misusing or improperly applying horticultural oils can manifest in various noticeable symptoms. Understanding these signs is crucial for gardeners to identify and address issues to prevent further harm promptly. Here's an expanded explanation of the common symptoms of plant damage caused by horticultural oils:

Leaf Discoloration: One of the most prominent symptoms is the discoloration of leaves. Infected leaves often exhibit shades of yellow, brown, or bronze. Typically, this discoloration begins along the edges or tips of the leaves and gradually spreads to encompass larger areas. Discolored leaves affect the plant's aesthetics and indicate underlying stress.

Leaf Dropping: As a response to stress, affected plants may prematurely shed their leaves. This early leaf drop, known as defoliation, can significantly weaken the plant. A reduced leaf canopy also hampers the plant's ability to perform photosynthesis, impacting its overall health.

Stunted Growth: Horticultural oil damage can lead to slowed or stunted growth in affected plants. This may result in reduced plant size, fewer branches, and smaller overall stature compared to healthy specimens. The oil's impact on plant metabolism can hinder the plant's development and growth potential.

Leaf Curling: Leaves may exhibit abnormal curling or twisting patterns. This curling can occur as a response to the stress induced by horticultural oil. Curled leaves can interfere with the plant's ability to capture sunlight and exchange gasses during photosynthesis.

Necrotic Spots: Brown or black necrotic spots may develop on leaves or stems due to horticultural oil toxicity. These areas of dead tissue can further compromise the plant's ability to perform critical functions, such as transpiration and nutrient transport.

Deformed Growth: In severe cases of oil-related damage, plants may exhibit deformities in their growth patterns. This can include distorted leaves, stems, or flowers, which affect the plant's appearance and hinder its functionality.

Reduced Flowering or Fruit Production: Damage to flower buds or young fruit can impair the plant's reproductive capacity. This may lead to a reduction in flowering and fruit production, ultimately affecting the plant's capacity to yield seeds or fruits.

Phytotoxicity: Phytotoxic reactions can manifest as a general decline in plant health. Symptoms may include the scorching or browning of leaf edges, often called leaf burn. This signals that the plant is facing challenges in adapting to the impacts of horticultural oils.

Make It Yourself

Making horticultural oil yourself is a great way of controlling indoor and garden pests without bringing harmful chemicals into your home. Even household cleaners contain traces of horticultural oils in their ingredients.

To make your own horticultural oil spray:

  1. Begin by mixing one tablespoon of liquid soap with one cup of oil. You can use cottonseed oil, soybean oil, or safflower oil.

  2. Find a spray bottle and mix one and ½ tablespoons of your mixture per every 1 cup of water. (Remember that too much oil will be too strong a solution and can cause damage to the plant you are trying to save.)

  3. Thoroughly shake the bottle and apply the spray to the affected plants. Try to direct the spray directly onto the pests for optimal effectiveness.

Remember: Oil and water don't mix. They separate constantly. Therefore, constant agitation of the liquid is necessary when spraying. Keep shaking that bottle between applications. 

What is the difference between horticultural oil and neem oil?

Horticultural oil is a highly refined oil, often derived from petroleum or vegetable sources, valued for its efficacy against garden pests. Neem oil, classified as a vegetable oil, offers similar pest control benefits but stands out by serving as an insect growth regulator and providing superior disease prevention. The choice between them depends on specific gardening needs and plant sensitivities. Here are the major differences:

Composition: Horticultural oil is typically a highly refined oil, often derived from petroleum or vegetable sources. These oils undergo extensive processing to eliminate impurities, yielding a clear and pure product. In contrast, neem oil is extracted from the seeds and leaves of the neem tree, earning its reputation as a natural and environmentally friendly vegetable oil.

Pest Control: Horticultural oils are recognized as superior oils for their effectiveness against a wide range of garden pests, including aphids, scales, and mites. They work by suffocating and disrupting the life cycles of insects. Neem oil is also effective against many garden pests. Additionally, it acts as an insect growth regulator, inhibiting the development and reproduction of pests.

Disease Prevention: Although horticultural oils primarily target pests, they can provide some degree of disease prevention by creating a protective barrier on plant surfaces. Neem oil excels in disease prevention due to its potent antifungal properties. It effectively inhibits the growth of various plant diseases.

Plant Safety: Highly refined horticultural oils are generally safe for most plants when used as directed. They pose a lower risk of causing harm to plants. Neem oil is safe for many plants but should be tested on a small portion of sensitive ones to ensure compatibility.

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About the author

Bella has a Bachelors degree in interior design, is a master gardener. She designs nourishing outdoor & indoor spaces guided by the practice of Feng Shui.