What Exactly Is Horticultural Oil?

Oil-based pesticides commercially labeled as general-purpose horticultural oils are an effective and ecologically friendly way to handle many garden insect pests and even some diseases. The majority of pest control oils are a type of mineral oil, a refined petroleum product. Vegetable oils, such as cottonseed and soybean oil, are also effective pesticides. The oil is usually combined with an emulsifying agent so it mixes with water and is applied with a spray bottle.

Types of Horticultural Oil

Application rates will vary according to plant and season, but “horticultural oil” has become the common term for all types of oils used for this purpose. That being said, depending on the species of plant needing treatment, you may encounter different terms or types recommended.

Dormant Oils

The first dormant oils were heavy, and you couldn’t safely use them on actively growing plants, or you could damage the foliage. Dormant oils were further refined to produce lighter-weight oils that can be applied during the growing season, without harming many plants. When the term dormant oil is used now, it generally refers to the application timing during the dormant season rather than a type of oil.

Summer Oils

Summer or All-Season oils are lighter versions of dormant oils that can be applied to plants during their growing season. You still need to use caution when applying summer oil. Oil-treated plants will burn in the hot sun, and many plants can’t handle oil at all. However, summer oil is now being combined with additional insecticides for even broader control with less possibility of plant damage.

Superior Oils

Although there's a difference between summer and dormant oils, most products are generally labeled as all-purpose horticultural oil. In addition, the term “superior oil” was once used to describe improved, more highly refined oils that are safer when applied to leaves during the growing season. Today, all horticultural oils are technically superior oils.

How Horticultural Oils Work as Insect Control

The primary way horticultural oil kills insects is by suffocating them. The oil blocks the spiracles through which insects breathe.

Horticultural oils also disrupt the metabolism of insect eggs and the ability of some insects to feed, causing them to starve to death. Not a pretty picture, but remember that insects, like aphids, carry diseases from plant to plant by feeding.

How Safe Is It?

Horticultural oil is a labeled pesticide and should always be used according to the label’s instructions. Both good and bad bugs may be affected by horticultural oil so use only when necessary or when plants are dormant.

Horticultural oils must be sprayed directly on the pests, to be effective. Excess oil evaporates and dissipates quickly, leaving no toxic residue, and making the horticultural oil safe around humans and pets.

When to Apply

If dormant oil is recommended, apply just before either leaves or flowers show signs of swelling or breaking open. Don’t apply it too early, though, since the insects need to be active and breathing to be affected.

Summer applications are easier to time. Horticultural oils are most effective on young, soft, and slow-moving insects and less on mature insects. You will need to keep watch for when the insects first appear.

When Not to Use

• During extremely high temperatures: Do not apply horticultural oil when temperatures are about 100 degrees Fahrenheit. Drought-stressed plants are more susceptible to horticultural oil damage.
• During freezing temperatures: Do not apply horticultural oil in freezing temperatures because the emulsion doesn’t hold together and coverage is uneven.
• When plants are wet or during high humidity: Applying horticultural oil in damp conditions decreases the evaporation rate and puts the foliage at risk of burning.
• During the fall: For an unexplained reason, spraying with horticultural oil in the fall puts plants at increased risk for winter damage and dieback.
• Anywhere sulfur or pesticides containing sulfur are applied: If sulfur has been added to your garden or yard in the past 30 days you must wait to apply horticultural oils. Combined with sulfur, it is a toxic combination for plants.
• On plants known to be sensitive to oils: You will usually find a comprehensive list of plants sensitive to horticultural oil on the back label of the ​package. Consult this list before spraying your plant in case it is included.
• On open blooms: Oils may cause petals to spot and become otherwise damaged.

What Pests Are Controlled With Horticultural Oil?

Adelgids, aphids, caterpillar eggs, leafhoppers, mealybugs, scale insects, spider mites, thrips, and whiteflies are the most common targets of horticultural oil. Horticultural oil is also effective as a fungicide to combat powdery mildew, leaf spot, and rust.

In addition, since horticultural oil is effective against aphids, which spread viruses by feeding on plants, horticultural oil is also something of a virus control.