Which Plants Do Not Like Neem Oil?

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As gardeners look for ways to encourage the growth of their plants, there is one consistent problem they often encounter pests. With many people looking to avoid the use of harsh chemical pesticides, many are increasingly turning to alternatives such as neem oil to keep certain pests at bay. Before using neem oil, though, it should be carefully reviewed, as some plants will not react well at all.

Plants with light and wispy leaves, such as herbs like cilantro, thyme, parsley, oregano, and basil, do not like neem oil as they are more likely to experience burns on their leaves. Even plants that can tolerate neem oil can be at risk if neem oil is not applied appropriately.

The best approach to knowing the right (and wrong) situations in which to use neem oil is to understand what role neem oil plays and what happens when it is applied to a plant. Read on to understand more about the role of neem oil in the garden.

Table of Contents

Why Do Some Plants Not Like Neem Oil?

Neem oil, as the name suggests, is a type of oil derived from the fruit and seeds of the name tree, whose scientific name is Azadirachta indica. This tree is native to both India and across the continent of Africa and is grown most frequently in tropical and semi-tropical climates.

Neem oil is a yellowish-brown color and has been reported to smell like a mixture between sulfur and garlic. It’s used to extract certain compounds for use in many products but has been used as a natural pesticide for hundreds of years. This in itself goes to show the effectiveness of neem oil in specific situations.

There are two challenges associated with neem oil:

Neem oil burn
Toxicity of neem oil

Let’s consider each of these issues to round out our understanding of the context for which neem oil might not be a perfect solution.

Neem Oil Burn

The main issue with using neem oil is that it increases the reaction of the leaves on which it is applied to sunlight — an effect called an increase in photosensitivity. This means that plants with the neem oil applied can be prone to foliage burns, so it is important to take care when looking to use neem oil in your garden.

Another way of looking at it is that, as an oil, heat absorption is increased on the surfaces to which it is applied, similar to the process used when cooking with oil. This is why the plants which are most at risk are those which are particularly delicate, with light and wispy leaf structures that require much less exposure to the sun in order to suffer from burns.

This does mean that provided adequate care is taken, neem oil is a good solution to most pest issues in the garden.

Toxicity of Neem Oil

Another issue with the application of neem oil is that it’s not always a good solution for those in the garden who are sensitive to immune responses like dermatitis. In fact, in large quantities, the ingestion of neem oil can pose significant risks, such as:

Metabolic acidosis

Seizures
Kidney failure
Encephalopathy

Severe brain ischemia

This is why although neem oil is a natural pesticide, it is not without risks and should be used carefully. In reality, you’re very unlikely to be using neem oil in the sorts of quantities that will cause issues. It does, however, mean that you should be especially careful with applying neem oil to plants that are going to be eaten, applying at least 72 hours prior to the harvesting.

It’s also been well-documented that neem oil on the skin often causes skin irritation, and so in most cases, it’s strongly recommended that hand protection be worn when going through the process of applying neem oil to your plants.

Similarly, as the negative impacts of neem oil are much more pronounced in infants and small children, care should be taken if there is a risk of exposure to the young.

Why Is Neem Oil So Effective Against Garden Pests?

Neem oil is very effective at pest control by impacting a wide range of plant pests, such as:

Flies

Termites
Mosquitoes
Aphids

Nematodes
Mealybugs
Cabbage worms

Whiteflies
Snails
Gnats

Moths
Cockroaches
Mites

It does this in a number of ways, but the most effective is through suffocation. Many garden pests breathe through openings in their body, and so when they are covered with neem oil, they are no longer able to respire — meaning it’s only a matter of time before they die.

There is also another method through which neem oil impacts insect populations. Neem oil has been shown to have an effect on the way that a specific insect hormone works, a hormone called ecdysone. Ecdysone plays an important role in helping insects regulate their ability to molt effectively, and without this, they are unable to continue their required metabolic cycle, leading to their death.

One other related benefit of using neem oil is that it has been shown not to impact insects which, as a broad rule, do not feed on plant leaves. This means that there is little concern about the impact it can have on mammals, butterflies, and ladybugs in the garden.

This quality is especially important because were it not the case, then using neem oil as a method of pest control would have a knock-on effect on pollinators in the garden. The role of pollinators is, of course, extremely important to the long-term health of a garden ecosystem, and so any negative impacts here could outweigh the benefits in helping remove pests.

How Should Neem Oil Be Applied?

Neem oil for the garden comes in a variety of different forms. The most typical is a prepared solution that is ready to use and available in a spray bottle. This is straightforward to use, and if in doubt, reference should be made to the instructions on the packaging.

Another form that neem oil can be acquired in is a cold-pressed form. This is much more concentrated and not suitable at all for application in the garden in this state — a number of steps to prepare an appropriate solution are required. These are:

Preparing a detergent and water solution
Adding the raw or organic neem oil to the solution
Testing and the application

Preparing A Detergent and Water Solution

The first step is to prepare a solution of water and detergent. This is because we are trying to water down the neem oil, reducing its concentration to a much lower level in order for it to be appropriate for our garden use cases.

Unfortunately, neem oil will not mix with water. This is because water is a polar molecule and oil is not; however, we are able to get around this by adding another substance known as an emulsifier. In this case, the detergent is our emulsification substance and will allow us to water down the oil with the active ingredient contained in the neem oil distributed relatively evenly throughout the solution.

The best type of detergent to use in this case is mild dish soap, and we don’t need a lot of detergent to utilize its emulsifying effects. If we apply a squirt or two of dish soap to a gallon of water, that should be more than adequate.

An important consideration at this point, though, is the temperature of the water. In order to facilitate the maximum level of distribution, the water should be warm. Both hot and cold water temperatures will reduce the effectiveness of this step.

Adding The Raw Neem Oil to The Solution

Once the detergent and water solution have been prepared, the next step is to add the neem oil. As raw neem oil is highly concentrated, we do not need to add much of it to get the balance right. Carefully add 1–2 teaspoons of neem oil and gently mix into the solution until there is no visible trace of the neem oil.

Over time, certain compounds within the neem oil will sink to the bottom of the solution as a result of gravity. As a result, once the solution has been added to a spray bottle, it should be shaken before each use to help ensure a more balanced distribution of the active ingredients.

You can see another method to create an effective neem oil solution here.

Testing, And the Application

With both a commercially prepared neem oil solution and a homemade one, testing is a sensible first step before applying broadly to the plants you have in mind. This allows us to ensure that the mixture is not going to damage the plants ahead of wider application.

One crucial consideration when applying neem oil to plants is the time of day that it is done, especially in warmer climates. This is because of the risk of burning that comes from the photosensitivity that neem oil increases for the plants it is applied to. Therefore, we would always recommend applying as the sun is setting in order to minimize the immediate levels of sun that the plants will receive during the time period when the concentration of neem oil on the plants is the highest.

The application of the solution should aim to reach all leaves of the plant and across both sides. This will ensure that any insects in place will be exposed, reducing their impact on the plant.

As a general rule, neem oil applications are conducted every 7–14 days, with the frequency higher where it is being applied in order to remove an existing infestation of a pest that neem oil has been shown to be effective on.

How Long Does Neem Oil Take to Work?

Neem oil has been shown to be very effective at both killing and repelling insects in the garden, but due to the way it works, expectations should be managed appropriately.

There are three main factors that will impact the amount of time it takes for you to see the results you’re hoping for from an application of neem oil.

The amount of neem oil applied
The types of insects that you are trying to get rid of

The level of infestation you have

The Amount of Neem Oil Applied

It might go without saying, but the amount of neem oil that makes its way onto the plant leaves will make a difference. A higher concentration of neem oil will increase the chances of an effective amount of the active ingredients being applied to the insect population; however, this is a balance to ensure that the plant is not at a heightened risk of foliage burn due to increased sensitivity to light.

This also accounts for the coverage of the plan itself. If the application is thorough, you can expect all surfaces to benefit from the effects of the natural pesticide. If your application is varied, it may not impact some areas as effectively as others.

The Types of Insects

While neem oil will be very effective against a wide range of insects, it is also worth bearing in mind that each bug’s metabolism will operate slightly differently. This means that a hormone imbalance caused by neem oil might impact an aphid very quickly but a snail much slower and so some consideration of what pests you are up against should also be considered.

The Level of Infestation You Have

If you have a very large population of pests that you are trying to resolve, it will naturally take a longer period of time for all of them to be exposed to the neem oil. In these circumstances, provided that some progress is identified within the first seven days post-application, continual spraying of the neem oil solution should yield positive results over time.

Conclusion

It’s clear that neem oil can play an extremely useful role in the garden, warding off a wide range of pests without needing to rely on harsh artificial chemical pesticides. With a little understanding of neem oil and the role it plays, you can ensure you are using it effectively without risking the high-quality growth of your plants that every gardener strives for.

Sources

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