Neem oil is often mentioned in plant care articles as a naturally occurring, non-toxic pesticide. For gardeners, it is a relatively effective and inexpensive way to rid yourself of common pests.
If you’re battling with an infected plant and are wondering how to manage this, this guide is for you. Here’s everything you need to know about neem oil, whether it’s an appropriate solution for you, and how to use it.
What is Neem Oil?
Cold-pressed neem comes from extracting oil from the seeds of the neem tree (scientific name: Azadirachta indica), a tropical evergreen native to India. The resulting oil contains a high concentration of the active ingredient Azadirachtin, which repels and kills many common houseplant pests. Because of its ability to impact many types of bugs, it is considered a medium to broad-spectrum pesticide.
It even works as a miticide (acting against mites, which are not technically insects) and fungicide against powdery mildew.
While it is often used to combat already-infected plants, neem oil is safe and effective to use as a preventative measure too. It can form part of your routine plant maintenance. To keep pests at bay, utilise this solution on your plant’s stems and leaves every seven days.
For information on the type of spray or solution to use, read the How to Use Neem Oil section.
How does Neem Oil work?
Neem oil works in many ways, making it effective against several pests.
- In small sucking insects such as spider mites, aphids and whiteflies, neem oil restricts breathing by forming a layer of oil on breathing pores.
- At the same time, the oil coats soften and dissolve the exoskeletons of some pests, causing desiccation and death. Exoskeletons are a protective layer for insects’ internal organs. Without this, insects dry out and die.
- Lastly, the active ingredient in neem oil interferes with several physiological processes, such as:
- Preventing larvae from maturing. This helps control population growth, remembering that rapid reproduction is one of the key ways these pests take over your plant. (As an example, a female spider mite will typically lay up to three hundred eggs over two weeks. And she will only be around five days old when this happens!)
- Feeding and mating behaviour.
According to the University of New Hampshire, neem oil is also effective against fungi like that which causes powdery mildew in houseplants. It acts by preventing fungal germination and penetration into plant tissue. While this will not reverse the effects of already-damaged tissues, it can help limit the fungi’s spread to healthy parts.
One of the benefits of this naturally occurring oil over synthetic insecticides is that the former does not kill pests by poisoning them. Therefore, insects do not develop resistance against Azadirachtin.
Which Pests are Neem Oil effective against?
Because of the multiple ways it can work against pests; neem oil is documented to be effective against almost 200 insect species. These include:
- spider mites
However, do not that neem oil spray is ineffective for use on plants with needles or fuzzy foliage. Insects can escape from the oil sitting on the surface by crawling deeper into the leaves.
At the same time, neem oil must be used carefully for plants that have sensitive foliage, like Calatheas. These plants are prone to “neem overdose” leading to sunburn or yellowing of leaves. Ensure you apply neem oil only in the evenings when it’s cooler, allowing time for the oil to be absorbed into leaves rather than sitting on the surface. Also use it sparingly.
Do also note that neem oil is most effective against immature insects. If you notice adults present, we recommend using a synthetic insecticide that will prove more effective for the pesky issue at hand. Our trusty favourite is the Bonicide Insecticidal Soap spray that is effective against most houseplant pests.
We always have a bottle around just in case!
How do I use Neem Oil?
One option is to purchase a pre-mixed neem oil spray. We recommend this brand.
Alternatively, make your own by mixing one tablespoon of 100% raw cold-pressed neem oil and one teaspoon pure castile soap per 1-gallon water. Castille soap helps neem oil mix with the water rather than float on top of it.
Remember to start treatment at the earliest sign of infection.
To use neem oil on your infected plant,
- Put on gardening gloves. This is because neem oil may cause an allergic reaction in some people.
- Test the oil on a small part of your plant. Wait 24 hours to identify any adverse effects.
- Wait till the cooler hours of the day before applying. Applying in harsh sunlight may burn foliage as the oil heats on the surface of the plant.
- Prepare your purchased or pre-mix spray.
- Shake before using, as neem oil may separate from its mix.
- Spray affected plant stems and leaves thoroughly until the solution drips from the leaves. This includes the undersides of the leaves where pests may hide.
- Reapply once a week until pests are eliminated. Note that it may take some time for pests to reduce in number visibly.
For severe cases, also use the same mixture as a soil drench. Thoroughly soak your plant’s soil with the oil solution until excess water runs off the drainage hole. This allows Azadirachtin to be absorbed into plant tissues directly.
Note that there is no need to rinse off neem oil on plants. Also, use a fresh batch each time.
Frequently Asked Questions
Are there any plants that shouldn’t use neem oil?
Yes. Besides plants that do not have smooth surfaces (mentioned above), seedlings less than a month old are not suitable for neem oil.
Some plants that have very sensitive foliage are also not suitable. This is why we recommend testing on a small area of the plant before thorough application!
What happens if I apply too much neem oil?
If you apply neem oil multiple times a day or forget to dilute your neem oil (see recipe above), this is usually too much for your plant. When overapplied, neem oil may burn your plant’s foliage and cause leaves to turn yellow.
In particular, harsh sunlight and oversaturation of neem oil is a bad combination. Neem oil will form a layer over foliage, heat up in the sun and cause leaves to burn. A thick layer of neem oil may even suffocate leaves and prevent them from carrying out photosynthesis.
Luckily the solution is easy. If you’re guilty of overzealous application, wipe it off with a damp cloth.
Is neem oil ecologically friendly?
Yes, neem oil is ecologically friendly. Azadirachtin breaks down on plant leaves with a half-life of 1 to 2.5 days. This is why there is no need to “rinse away” the solution after use.
According to the US Government’s Environmental Protection Agency, neem oil will not cause adverse effects to humans and other non-target organisms.
Does neem oil harm your plants?
Neem oil is thankfully not toxic to plants when used properly.
As mentioned above, in some plants with sensitive foliage, it is best to test before applying.
Are there any other alternatives to neem oil?
It depends on the insect or pest you are trying to ward off. Insecticide soap spray can work effectively for most household pests. If you are planting outdoors, an option to deter insects as a preventative measure is using cedar mulch as a broad-based pesticide.
Deborah is a plant enthusiast and founder of Gardening Collective.