How to Use Neem Oil on Plants to Control Pests

Gardeners have used neem oil to control pests for hundreds of years. Neem oil is an organic pesticide derived from the seeds of the neem tree (Azadirachta indica), a species native to tropical forests in Burma, India, and Sri Lanka. It's effective against a wide variety of insect pests, mites, and certain fungal diseases. While maintaining plant health is the first defense against pest problems, sometimes control measures are necessary. As with any pesticide, it's important to understand how to use neem oil on plants to control pests, as well as the benefits and limitations of the available products.

How Does Neem Oil Work?

Neem tree seeds contain several compounds, but the main one responsible for pest control is azadirachtin. This compound reduces insect feeding and disrupts the growth and development of immature insects. It also acts as a repellent and, like other horticultural oils, can suffocate soft-bodied insects and mites by coating their bodies with oil. Neem oil is most effective against the immature stages of insects, however, it also impacts adult feeding. Some target pests include whiteflies, thrips, caterpillars, aphids, mealybugs, spider mites, and immature scales.

Neem can also be used to manage some fungal diseases by preventing the germination of fungal spores and impeding their penetration into leaf tissues. It will not cure already infected plant tissues but helps limit the spread to healthy tissues. In this capacity, it's most commonly used against powdery mildew.

Neem Oil Benefits

Neem oil is a safe and effective pesticide that can be used both indoors and out. It's considered safe for humans and pets, so you don’t have to worry about using it to treat aphids or fungal gnats on your houseplants. Outdoors, neem oil has very low toxicity to beneficial organisms, such as bees, birds, and mammals. However, it's always best to spray pesticides early in the day when bees and other pollinators are less active to avoid any direct contact.

Neem oil is biodegradable and rapidly breaks down in soils, water, and on plant tissues. It's not known to have negative impacts on earthworms and can be used as a soil drench to manage certain pests, such as root nematodes. Many neem oil products are listed as certified organic by OMRI.

Which Plants Should You Use Neem Oil On?

Neem oil products are labelled for a wide variety of crops including houseplants, vegetables, fruits, and ornamental plants. Check individual product labels for a complete list of approved plants.

Though neem oil is generally safe to use, some plants can be sensitive to horticultural oils and soaps. So it's always a good idea to test products on a small area before treating the entire plant. Wait 24 hours after application to assess the treated area for signs of damage. If you don't notice any discoloration or leaf burn, then you can move forward in treating the entire plant.

Neem oil isn't recommended for recent transplants, certain thin-leaved herbs or leafy greens, and plants exhibiting stress, since they can be particularly sensitive.

Best Neem Oil for Pest Control

Several formulations of neem oil are available at garden centers, but they do not all work the same way. To determine which type of neem oil you're purchasing, look at the active ingredients listed on the bottle. These are the three main types of neem oil formulations: clarified hydrophobic extract of neem oil, azadirachtin, and cold pressed neem oil.

It's important to note that the most widely available formulations, those containing hydrophobic extract of neem oil as the active ingredient, do not contain azadirachtin. Without the insecticidal properties of azadirachtin, these products work by suffocating insects and coating foliage with oils to combat foliar disease. But they don't disrupt insect growth and development. However, some of these are mixed with other pesticides, such as pyrethrin, to provide greater control.

Products containing pure extracts of azadirachtin are also available. These tend to be tailored toward commercial use and are often more expensive. So, the best products for home use are those containing cold pressed neem oil as the active ingredient, which contain azadirachtin. 

How To Apply Neem Oil

Neem oil is typically sold in a concentrated formula that requires dilution prior to application, as described below. The substance is then commonly applied as a foliar spray in an oil-based suspension. Foliar sprays should be applied by misting the entire plant, covering both the undersides and topsides of leaves as well as stems. Make sure to spray the bottom of leaves thoroughly, as this is where many pests hide. A second application is usually required 7 to 10 days after the first.

Though neem oil is generally safe to use, some people are sensitive to direct contact. Wear gloves and a mask to protect sensitive skin and lungs.

Another way to apply neem oil is in a soil drench, which involves pouring the diluted solution directly on the soil. This is a good method for treating fungus gnat larvae, nematodes, and other soil-borne pests. When applied as a soil drench, the compound azadirachtin is also taken up by plants through roots and works as a mild systemic pesticide against certain insects (such as aphids) that feed on the foliage by sucking plant juices.  

How To Dilute Neem Oil for Application

Cold pressed neem oil is highly concentrated and needs to be diluted for use. You will need either a one-quart spray bottle for small applications, or a one-gallon pump sprayer for larger applications. Always follow specific mixing instructions listed on the product label. In addition to water, a small amount of horticultural soap or plant-safe liquid soap should be added to the suspension as an emulsifier to help the water and oil mix.

Begin by mixing the water and soap. For a gallon of water use one teaspoon of soap, and for a quart spray bottle, reduce the amount of soap to ¼ teaspoon. After the water and soap are mixed, add a small amount of neem oil, one to two tablespoons per gallon of water, or one to two teaspoons per quart of water. Apply the mixture to a small test area on plants before treating the entire plant.

The solution for soil drenches should be prepared in the same manner as for foliar sprays, however, you will likely need a larger volume depending on the intended use. Remember that neem oil breaks down rapidly. Do not mix more than you intend to use in a single application and mix a fresh batch for repeat treatments.

While you may see some initial knock down of pests due to the suffocating effects of oil itself, the impacts of azadirachtin on insect feeding and development are not immediately evident. So, don't get discouraged if the issue doesn't go away overnight. Instead, watch for a decrease in pest numbers over time and reapply neem oil weekly until you notice an improvement.