Organic Disease Control, Organic Pest Control

Horticultural Oil Spray: How Dormant Oil Works


Person spraying horticultural oils

You may have heard of horticultural oil spray, but what is it? It’s a type of natural pest control that can control insect pests and diseases in your garden, whether they’re on your landscape plants, edible plants, or fruit trees. There are many different types of oils, and making sure you use the right one can be tricky if you don’t know the difference between them.

Some oils can be applied all year long, while others are meant to be applied at certain times in the year, like summer oil or dormant oil. If you’re an organic gardener, you won’t be able to use every kind of oil because some of them have been processed in a way that doesn’t make them qualify as organic. Some plants are sensitive to horticultural oils, so you’ll need to be careful of that, too.

Every insect control method has its caveats, but that doesn’t mean you can’t use them! Horticultural oil sprays are effective and easy to use since they don’t require any tools other than a sprayer, which you may already have on hand. If you add multiple kinds of oils to your cabinet of insect control supplies, you can take care of common garden pests all year long! Let’s dive into what this spray can do and how you can use it in your garden.

What Is Horticultural Oil Spray?

Person spraying horticultural oils
Using a horticultural oil spray can help reduce pests. Source: LiuTao

Horticultural oil spray is a naturally-derived product that’s used to control diseases and pests. It’s more useful when used for insect pests, but it’s great to use for disease prevention. It’s super easy to apply to your plants—all you need is something to spray it with! Depending on the quality and type of oil you buy, you can use it as an organic pest control method to keep your plants safe.

Not all horticultural oils are created equal, so we’ll look at each type so you can determine which one to use for your plants.

Types of Horticultural Oil

These are the major types of horticultural oil you’ll be able to find in most garden centers. Each one functions somewhat differently, so you’ll need to make sure you get the right one for the purpose you need.

Nonselective Oil

All horticultural oils are nonselective, meaning that they’ll kill any kind of pest, including some types of beneficial insects. Chemical pesticides may be selective and may not affect beneficial insects, but that’s not the case with natural products like horticultural oil! On the bright side, most impacted beneficial insects are predatorial, which means they move regularly. With the proper timing, you can spray the plant without harming your good guys.

Horticultural oils control pests by suffocating them. If an insect is on the plant you spray, it’ll get covered and die. You can avoid harming beneficial insects by spraying early in the morning or late in the evening when the pollinators aren’t out. The oil sprays can also suffocate insect eggs and disrupt insect feeding, causing the population of critters to dwindle.

Superior Oil

Just because they’re called superior oils doesn’t mean that they’re the best, but hey, they might be! Superior oil is lightweight, highly refined, and doesn’t contain sulfur. Most oils are considered to be superior today, so you’ll probably end up with this kind.

Summer oil, which we’ll talk about more in a bit, is a type of superior oil that can be applied while the plants are in bloom and have all their leaves. Because the oils are highly refined, you don’t have to worry as much about the oil burning the plant. Some oils can only be applied when plants are dormant, so having a summer oil to use while your plants are growing is a must.

Supreme Oil

Supreme oil isn’t much different than superior, and the term is often used interchangeably with superior. Just like superior, supreme oils are highly refined and can be used while your plants have leaves.

Mineral Oil

Mineral oils are derived from petroleum and are made of hydrocarbons that are either saturated or unsaturated. The refinement of mineral oil is usually listed as a percentage of UR, or unsulfonated residue, which tells you how much unsaturated hydrocarbon is in the oil. 

The UR is important because unsaturated hydrocarbon can cause plant injury. The higher the percentage, the safer it is for your plants! Dormant oil is usually 50-90%, and superior oil is 92-96%.

JMS Flower Farms has a mineral oil called stylet oil that has a UR of 99%. It’s food-grade and can be used as a fungicide, pest control, or to prevent viruses on plants.

Even though these highly refined petroleum products don’t come from plants, some brands may be suited for organic gardening.

Other Plant Or Vegetable Oil Types

Vegetable oils are derived from plants. Some examples are neem oil, cottonseed oil, soybean oil, and sesame oil. These oils may occasionally be listed as superior or supreme oils and depending on where they came from and how they were processed, they may also be listed as suitable for organic gardening. While these oils come from plants, they can still burn plants if they’re applied incorrectly, so you’ll still need to be careful with them. 

Some oils work better than others for specific pests, even though they’re all nonselective, so it’s smart to keep several kinds on hand to target specific insects as needed. Neem oil functions as a repellent and is very effective against the larval stages, so it’s good to use early in the year even though it’s safe to use all year long. Soybean oil is good to use against mites, and cottonseed oil is typically considered one of the best all-purpose veggie oils.

Seasonal Formulations

Certain oils work best at specific times of the year. It’s crucial that you remember when you can and can’t use certain oils because incorrect applications can harm your plants.

Dormant Oil

Dormant oil sprays should be applied over winter when plants don’t have any leaves on them and have temporarily stopped all of their growth. Dormant oil is typically labeled as superior or supreme these days, but you may occasionally find one that is an older, more viscous formulation. 

Traditional dormant oils are ideal for killing overwintering insect pests to prevent outbreaks in spring. A dormant oil will kill mature insects that are hiding from the cold, as well as insect eggs that may have been laid inside of plants.

Horticultural oils aren’t effective in freezing temperatures, so you should apply them at least 48 hours before and after a freeze. It’s best to apply them at the beginning or end of winter when the temperatures are cooling down or warming up, and there isn’t any danger of frost. You also need to make sure there aren’t any buds that are about to open if you’re using it near spring because the buds will be sensitive to the oil.

Delayed Dormant Oil

Delayed dormant oils work similarly to dormant oil, but you can use a delayed dormant oil when buds show up to half of an inch of green. You’ll need at least 1/16 an inch of green before you can start applying it, though; it’s not as effective when the plant is still completely dormant. These are slightly less viscous than other forms of dormant oils, but not as thin as a summer oil.

Summer Oil

Summer oil is meant to be applied when plants are in bloom and have all of their leaves. It doesn’t necessarily have to be summer to use it! Summer oils are almost always superior oils since they’re highly refined and thinner in formulation, and the lack of viscosity makes them less likely to cause plant damage as they don’t linger on the plants as long.

Since horticultural oils kill pests by suffocating them, beneficial insects will be at risk since they’re more likely to be out during the time of year you can use this oil. Be sure to apply it before or after the pollinators have come out.

Year-round Oil

Year-round oils, also called all-season oils, are safe to use all year long as long as the temperature range is suitable. You’ll still need to take precautions regarding pollinators, but you won’t have to worry as much about burning your plants.

Neem oil and many superior oils are good to use all year long. Since the superior oil category is large and contains many different kinds, be sure to read the package label before you use it in your garden to make sure that it will be safe to use the way you intend to use it.

How Does It Work?

Horticultural oils can control insect pests and prevent future outbreaks by smothering eggs, larvae, and adult insects. In fact, horticultural oils work by clogging the spiracles (breathing pores) along the sides of pest abdomens. Sometimes these oil covers can also disrupt their feeding habits and interfere with their metabolism, a benefit if your pests don’t die of oxygen deprivation. Regular application of the sprays can disrupt the lifecycle and eliminate pests like spider mites, aphids, whiteflies, and other insects from your garden.

Horticultural oils control viral and fungal infections, such as powdery mildew, in a similar manner. The oils trap fungal spores and prevent the diseases from spreading. Infected leaves may then be removed to prevent further spread. Many insects are responsible for the spread of diseases, so getting rid of the pests will also slow the spread viruses.

Unfortunately, oils can’t cure diseases that are already affecting your plants. If you spot a disease, remove infected plants and spray the rest to slow down the spread. Caught early on, you may be able to simply remove the infected portion of the plant and rescue the rest of it if you manage to get complete coverage while spraying the plant tissues. 

Benefits Of Using Horticultural Oil Sprays

Someone in protective suit spraying
You don’t have to be this protected when spraying horticultural oils! Source: Apionid

There are so many benefits to using horticultural oil sprays in your garden. If you don’t already use them, you probably will soon!

Perhaps one of the most important benefits of horticultural oils is that it’s generally considered safe to use around humans and pets. It can cause irritation when you come into contact with it, so be sure you don’t inhale or eat it, and keep it away from your skin and eyes. The same rules apply for protecting children and pets. Other than mild irritation or the slippery feeling of oil on the skin, there aren’t any major side effects to worry about.

Horticultural oils degrade rapidly and don’t leave behind a toxic residue, so you can time the applications just right and avoid harming beneficial pollinators. Since the oils coat the insects and prevent them from breathing, it’s not likely that pests will develop a resistance to them. If you use chemical pesticides and you notice that the insects are starting to build resistance to your pesticide treatments, try switching to horticultural oils for a bit.

If you like to use organic methods to keep chemicals out of your garden, there are formulations that are certified by the Organic Materials Review Institute (OMRI) that will be safe to use in your garden. Just look for their symbol on the packaging.

Horticultural oils are relatively inexpensive. The oils are usually sold as concentrates that can be mixed with water, so one container will last longer than you might expect. Plus, you won’t need any tools other than a sprayer, and you don’t have to get anything fancy!

Pests/Diseases Treated

Generally speaking, horticultural oils can be used to treat aphids, arachnids, mealybugs, scale insects, spider mites, thrips, and whiteflies. They can also prevent viral and fungal diseases such as downy mildew, powdery mildew, leaf spot, rust, and other fungi.

Not every oil will work for all pests and diseases, so it’s good to keep a few on hand. Neem oil is typically treated as a good general-purpose option since its effective against many pests due to its azadiractin content, including aphids and mites, and it can also prevent fungal infections. Soybean oil is predominantly used for mites, but it can also be used for other insects. Many people turn to cottonseed oil for most pest problems.

Drawbacks Of Using Horticultural Oils

As with most pest control methods, there are a few downsides. The biggest issue is that there are some sensitive plants you can’t use these oils for. Many trees like cedars, maples, redbuds, junipers, and spruce, among others, can’t be treated with the oils due to the damage they can cause. Similarly, some species of roses cannot be treated with some horticultural oils. There have been some signs that excessive use of horticultural oils may reduce fruit set in wine grapes, as well.

Temperature limits are also a downside. Oils can’t be sprayed when the temperature is 90°F (32.2°C) or when you get below freezing temperatures. 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. With drought-stressed plants, they’re already under a lot of stress, and adding the potential of scorching damage on plant surfaces may become a major problem. Similarly, if your plant is showing any signs of wilted leaves, avoid applying during that time as it can create a phytotoxic situation for the plant that can result in harm to the weakened foliage.

Horticultural oils are only effective when they’re wet. Once they dry, they no longer kill pests. You’ll have to continually apply the product to your plants to control huge infestations. Remember, these oils are capable of harming beneficial bugs, so be careful when you spray!

How To Use It In The Garden

Spraying oils
Depending on the spray type, you may apply horticultural oil year-round. Source: edgeplot

Applying horticultural oil spray is super easy! Most oils are sold as concentrates that are meant to be mixed with water before you use them. Mix some oil with water in a sprayer. The package label will tell you what ratio to use, and you can adjust according to how much water you have.

Once you have your sprayer ready to go, apply the spray to your plants. Get the tops and bottoms of all the leaves, as well as the stems. Try to avoid flowers if you can to prevent potentially harming butterflies and bees. Make sure to shake the sprayer regularly to make sure the spray solution is mixed well and the oil is evenly distributed throughout the water. 

The best time to apply is early morning or in the evening when pollinators aren’t out so you can avoid accidentally harming them. Choose a time of day that isn’t below freezing or above 90°F (32.2°C) for oil applications, and make sure it’s not windy, so it doesn’t land on sensitive plants.

Be sure to carefully read the label of your product, so you don’t accidentally harm your plants.

Frequently Asked Questions

Q: When should I spray my horticultural oil?

A: Spray the oil in the morning or evening when the pollinators aren’t out. Some oils can only be applied at specific points in the year. Dormant oils are intended for when plants are bare and don’t have leaves, while summer oils are applied when plants have all their leaves.

Q: How often can you spray horticultural oil?

A: You can apply horticultural oil once a week as long as you have pest infestations.

Q: Can you spray horticultural oil in summer?

A: Summer oil can be sprayed in summer when the plants have all of their leaves. Other superior oils should be okay, too, but be sure to read the label before applying them.

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