Fungus gnats are not usually a major issue for your houseplants, if not in a large number. However, they can be rather unpleasant – we don’t really want insects flying around our homes. Then, what should you do to get rid of them?
Surface covering, castile soap, neem oil, cinnamon, physical traps, and biological solution are the most effective solution to get rid off fungus gnats from the soil of house herbs and plants. Each technique aims to break a different point in the fungus gnats’ reproductive cycle.
To learn how to avoid issues with these pests and get rid of them if you find them in your pot plants and herbs, read on.
For those of you in a hurry here a list of all the remedies that do work against those pesky fungus gnats.
For more details, check the rest of the article.
Table of Contents
- 1 What Are Those Black Flies Fungus Gnats?
- 2 Why Are There Fungus Gnats in Your Home? 2 Reasons
- 3 Getting Rid of The Fungus Gnats? 6 Organic Ways
- 4 Preventing Fungus Gnats: The 2 Mistakes To Avoid
- 5 Fungus Gnats vs. Fruit Flies
- 6 Takeaways
Fungus gnats (the most common are Bradysia coprophilia and B. impatiens) are small flying insects that look like mosquitoes. However, they don’t bite, and they do not carry any disease. Adult gnats are generally very tiny (from 1⁄16 to 1⁄8 inches, around 1mm, in length).
Differently from mosquitoes, these insects are always around houseplants potting soil. If you see a swarm of insects flying around every time you touch, come close or water your houseplants, then you know that they are fungus gnats.
The eggs fungus gnats lay very small (barely visible), shiny, white, and oval in shape. The problem is that they can lay, on the surface of the soil (or up to 3 inches deep), hundreds of eggs at once, making an infestation very hard to eradicate.
After the eggs open and the small larvae will start feeding on leaves on the soil and plant roots. After 2-3 weeks, they developed into full adults (able to fly) and will be around for another week in the adult form to then die.
Even though larvae (not adults) feed on the plant’s roots, hardly ever (if not in massive numbers) significantly affects plant growth. This can only be the case in case of a massive infestation or in the case of a small (seedling) plants.
Fungus gnats are in houseplants for two reasons.
One of the most common indoor gardening mistakes is to use outdoor gardening soil. This is because of the presence of pests, and, among them, fungus gnats are quite common.
However, this is not the only reason. Indeed, also normal potting mix bought from a qualified retailer might still host fungus gnats. Moist organic matter added to the potting mix when produced is very likely to be responsible in this case. Compost and peat moss, for instance, quite often added for the perfect potting mix, are the two main fungus gnats carriers. You might not be the first (neither the last) to receive an infested bag of soil (welcome to the club). Indeed, especially their larvae, are hard to spot.
Fungus gnats thrive in the presence of wet soil. The wet surface represents the best place for females to place their eggs. This is often due to overwatering the house plants, ignoring the soil conditions, the season, or the plant growth (on this more on the prevention part).
Getting Rid of The Fungus Gnats? 6 Organic Ways
Independently from the strategy, you are going to use, the only way to get rid of fungus gnats for good is to break their (otherwise) endless cycle. Indeed, this insect is so persistent (and looks omnipresent) not because it has a long life-span (on the opposite, they die pretty quickly) but just because they reproduce very quickly. Hence, you just need to either or:
- Capturing/killing adults and/or preventing them from depositing new eggs;
- Get rid of new eggs leaving only the current generation and wait until such will die off either naturally or artificially.
There are literally countless approaches to address both aspects. Here the best organic way to get rid of fungus gnats using a product that perhaps might already be hanging around your house unaware of their hidden powers.
Using soil cover will prevent fungus gnats from laying eggs in the soil as it is now out of reach. This will stop them from multiplying decimating their population in a week or less.
Hence, if you cover that moist surface area, fungus gnats can no longer place their eggs on the soil. There are a number of different soil covers that will work well to disrupt fungus gnats’ life cycle. For example, a 1-inch layer of sand, gravel, or other inert material can prevent them from reaching the soil and laying eggs.
If you want a soil cover, even more effectively go for a thin layer of diatomaceous earth. This will kill gnats, not only preventing them from accessing the soil.
Diatomaceous earth material is classified as an insecticide, and you should not use it on edible herbs. Moreover, avoid breathing it. Some brands might come as a very fine dust that should not be breath. No worry, it is totally harmless to plant. If you use more regular soil cover like sand and gravel, it might take longer to be effective (1-2 weeks).
Diatomaceous earth very effective and fast-acting. Indeed, diatomaceous earth is a superdry and sharp (to insect) powder. This allows it to wound the insect sking and, once ingested (or in contact with its internal organs), it sucks out all the moisture drying it out. It is organic as made from dry fossilized algae called a diatom. A great plus is its extensive applicability. Diatomaceous, it can be used to get rid of cockroaches, flies, centipedes, ants, and more around your house, not only on houseplants.
If you go for a more traditional soil cover like sand or gravel (avoid organic material like chip wood), it will take longer to act. This is because such soil covers are just physical barriers preventing the insects from laying its eggs, and it does not kill it actively.
If you see large numbers of fungus gnats, then one organic insecticide to try is a castile soap solution. For those of you who do not know, this is not a generic soap. Castile soap was made in the Mediterranean area before its use spread to Europe. Traditionally, castile soap was made of olive oil. It gets its name from the Castile region of Spain.
Castile soap is produced based only on vegetables with no animal fat or synthetic ingredients. This is a 100% organic soap that will not damage the environment when it ends up in the drain. It is not toxic, and it is usually sold both in liquid and solid form. These days, the soap is also made with coconut, castor, or hemp oils. Sometimes it’s made with avocado, walnut, and almond oils as well.
Castile soap is not meant to be used as an insecticide but mainly as a body cleaning product due to its lathering, moisturizing, and cleansing properties. However, there are more than 20 uses of this soap, from deodorant, dish soap, shampoo, body wash, and plant spray for bugs.
In case you want to use this soap as a fungus gnats killer, here what to do:
- Add 2 teaspoons of castile soap to 1 quart of water.
- Place this mix into a spray bottle, and shake well.
- Apply the mixture to just one leaf and wait a few hours. This is just a test as some plants might be damaged by the soap (although not common).
- If there is no damage, spray this mixture onto your pots, soil, onto the leaf of the affected houseplants, and all surrounding plants (fungus gnats might be developing there).
- If possible separate from the others the affected plants to avoid spreading
- Repeat after 3 days for 3-4 times
The key ingredient that is defeating the fungus gnats is the fatty acid contained in the castile soap, that, as discussed by the Colorado State University, is a chemical that is very effective on soft-body insects (like aphids for instance). However, as the majority of organic products, it might take multiple applications for success.
Remember to spray through the soil. If you are fast enough, you can also spray the fungus gnats directly (as you might have noticed, they are quite slow insects, and they prefer walking on the soil and saucer and occasionally fly around).
Tip: To increase the efficacy of this spray, you can also consider adding essential oils. Essential oils with insecticidal properties such as rosemary (here a good on Amazon), peppermint, eucalyptus, tea tree, and cedarwood are great additions. Simply add 2 drops of your chosen oil to the castile soap solution and apply as suggested. Another way is to use those oils in a diffuser placed close to the herbs to discourage fungus gnats and a range of other insects and create an amazing and relaxing atmosphere.
3 – Use Neem Oil
Neem oil is an effective organic insecticide that can be used, when diluted, to get rid off fungus gnat in the soil of potted plants.
As in the case of castile soap, I do not recommend using neem oil in pure form. It should be diluted. Here what to do:
Add 2 tablespoons of organic, cold-pressed neem oil and 1 tablespoon of castile soap (or dish soap) to 2 cups of water (a quarter of a liter) to make a relatively concentrated solution. Shake the solution vigorously to let the neem oil mix into the water. Remember, the soap here is used not for its insecticide properties but just to break the neem oil that otherwise will just float on top of the water (like kitchen oil) making the solution useless;
Don’t spray this directly onto plant leaves but instead spray or pour it liberally around potted plants to kill off larvae and eggs in the top layer of the growing medium.
Apply once every 5-7 days until the fungus gnats are gone
Avoid creating a batch of such liquid solution. Indeed, its effect tends to fade away over time. Also, I usually apply such solutions at night or once I wake up. Why? Some droplets might create a “lens” effect on the leaves and burn them (this happens in case of good sunny days).
To your surprise (and mine when Sher, a friend gardener) told me about the cinnamon technique. Indeed, cinnamon is shown to have come out that cinnamon has fungicidal properties.
Why is this important?
Fungus larvae feed mainly on fungi. These are present in most potting mixes, and most of them cannot be seen. Indeed, the fungi on which fungus gnats (and even other insects) feed on are more often than not within the soil, not visible at all.
Hence, if you manage to kill the fungi, the larvae into the soil will starve and die. Hence, there will not be new offspring to continue the (otherwise endless) fungus gnats cycle. What you need to do is to wait for the adult gnats to die either naturally after a week (or even less if you remove them yourself), stopping for good its reproductive cycle.
How to apply the cinnamon?
Just sprinkle the cinnamon on top of the soil as uniform as you can. No need to have a thick coating at all, of course!
You have to know that cinnamon also has other great properties! Indeed, cinnamon helps with issues such as damping off, and mushrooms growing around potted plants. Interestingly, it can also help cuttings to establish a rooting hormone to help the plant heal from superficial wounds (and avoid infections).
Do not be afraid! Cinnamon won’t damage plants when watering at all. In fact, as mentioned above, it can actually help roots grow stronger.
As you can guess, this approach breaks the fungus gnats cycle by getting rid of the adult gnats. Remember that getting rid of the adult gnats currently flying around the planter is only half of the task. You need to get also rid of the future adult gnats that are in the soil in the larva stage. Hence, whatever strategy you choose, be sure to apply it until 2 weeks after the last gnats disappear to be totally sure to not miss future larvae.
Despite there being quite a few ways to trap them, vinegar and sticky trap are by far the most successful.
You can leverage the passion of fungus gnats for vinegar (ideally apple vinegar, despite normal vinegar works). These insects, like many others, are attracted by such a smell (probably connected with the one of decaying fruit, think of fruit flies, for instance).
The trap is quite easy to do and works greatly:
- Take a glass or alternatively a plastic bottle (small one) cut in half. Place the second part upside down on top of the bottom part of the bottle (the lid pointing down).
- Place inside the container a solution of vinegar and dishwasher soap. In case vinegar is missing, you can opt for beer or wine. The soap helps to break the superficial tension making the fungus gnats drawn once they land on the solution.
- Cover the glass with some plastic film and make some small holes in it (a toothpick or a fork is good for the purpose). In case you used the plastic bottle with the half upside down is enough. The idea of the holes or plastic bottles is to make the fly reach the vinegar easily but making it difficult to escape and forcing them to touch the liquid at least once and drawn.
These are extremely effective in getting rid of fungus gnats and a large majority of other insects (centipedes, ants, and more).
The principle is very simple. Fungus gnats and many other insects, are attracted by the yellow color (probably because it is related to the sunlight color). Hence, placing a special yellow paper with glue on it is a death sentence for fungus gnats. They will just land on it and get stuck.
You can easily find them on Amazon for a few dollars. I do recommend the Garsum below. They are waterproof (important considering that the plant might need to have them applied for 1-2 weeks), they look quite good (due to their shape), and they can kill easily hundreds a day. Ths is a game-changer for many gardeners that used them.
A biological control is based on the introduction of another living organism (not a liquid, powder, or traps, as discussed before) that thrive on feeding on the unwanted insect (fungus gnats).
This is quite an effective method, although not very common, as does require some attention.
Three commercially available biological control agents can be purchased to control fungus gnats in houseplants. These are:
- A bacteria (called Bacillus thuringiensis subspecies israelensis): this bacteria creates a substance that kills larvae preventing them from developing. It is the best biological control for indoor applications as no further insects are involved. Usually, you need four tbs per gal of water applied every three days;
- Hypoaspis: mites that that prey on fungus gnats
- Steinernema feltiae (a nematode applied within a soil drench).
The first of this is likely to be the easiest for you to get your hands on. This is most commonly utilized by those who have a severe fungus gnat problem indoors.
The methods discussed so far were 100% biological. Hence, substances found in nature and not produced almost from scratch. However, if you are not very into bioproducts, there are a few man-designed pesticides that can be quite effective in getting rid of fungus gnats.
This product is a kind of biological control specially designed for indoor use. It comes in the form of small bits (check it here on Amazon if you do not know what they are) coated with the bacteria Bacillus thuringiensis subspecies israelensis. We discussed these bacteria before. It does produce toxins that kill larvae in the soil.
The best way is to drop around 2 tablespoons in a container with half a gallon of water. Stir it if and leave it there to soak for 20 minutes. The solution so obtained can be applied to water the plants whose soil is infested with fungus gnats larvae. After several waterings, the gnats were gone. This solution is not harmful to pets, humans, just to the larvae.
One note of caution here. Using insecticides to deal with insect infestations is not always the best policy. Remember, insecticides are indiscriminate and will kill insects you do want and those you don’t.
For example, if you grow tomatoes or other fruits or flowering plants indoors, pollinators may be required. Even if you take pollination into your own hands, letting in a few predatory insects (such as ladybugs, for example) can make it far easier to deal with aphids and other common pests organically. Often, it is best to simply live and let live.
Moreover, if you follow one (or at least 2) of the previously discussed techniques or products, believe me, the fungus gnats problem will be gone in no-time.
Here once more an overview of all the remedies seen so far that you can purchase online if needed.
Did you manage to get rid of those pesky fungus gnats? Well done. Now what? Well, you do not want to go through the hassle to fight them again. Hence, let’s start by avoiding some basic mistakes that might have brought them in the first place.
As discussed at the very beginning two are the causes that trigger a fungus gnats infestation:
- Contaminated soil
- Excess watering
Leave the soil open for a few days (just open the bag and leave it in the garage). If you see flies around, then you are certain that you have fungus gnats (or other insect problems). So you know that if you use that soil, you are going through lots of hassle. If you still want to give it a shot, you might consider sterilizing it.
Other actions to reduce the risk of fungus gnats are:
- Choosing a peat-free potting mix. Such as one containing coconut coir, for example, this one on Amazon. This will help make sure you don’t accidentally import eggs from elsewhere.
- Eliminating damp and dark spots in your home (drains, damp corners, etc..), which could harbor gnats that could spread to your plants.
- Quarantining plants, you purchase outside for a while, to check they are not harboring gnats that could spread to other plants.
Overwatering is the cause number two, after infested soil, for fungus gnats in potted plants (as well as root rot).
This is particularly true in autumn/winter. Indeed, what I noticed chatting with gardeners, is that overwatering is caused by lack of any seasonality in the watering pattern. Indeed, in cold and cloudy weather, when not much photosynthesis is going on, not much water is needed. Moreover, the low temperature also reduces the water that naturally evaporates from the growing medium. Hence, during winter, you should water way less (even less than half of the times) your plants.
People forget that, and they end up constantly dumping the plant-soil in the cold season. This is the perfect environment for fungus gnats to lay their eggs.
How to avoid overwatering?
First, to avoid overwatering, it is important to test the soil before applying water. Just stick your finger in the potting mix (1 inch is enough) Is the soil moist? If so, then it’s enough, no more water is needed. This simple truth is applicable to the vast majority of houseplants and especially herbs. Remember, it is quite often easier to recover a plant from lack of water than too much water.
Second, avoid letting stagnant water sit for too long in trays and saucer beneath pots. Indeed, that water might be closer than you think to the plant roots. Moreover, if dry, the soil will suck water from the bottom, making the situation worse (as you are essentially watering twice the plant).
Third, and this is related to the first tip, let the soil surface dry out before watering again. Why? Fungus gnats love moist soil as they hate dry ones. They will not deposit their eggs in dry soil. Creating drier conditions can help avoid an issue in the first place. Drying out houseplants can also help get rid of the problem once it occurs. It will not be a quick solution, but it will reduce the number of flies seen as the larvae dry out and die.
Fungus gnats and fruit flies might be confused with each other due to their small size and ability to fly. However, their similarities end here.
Fungus gnats differ from fruit flies due to their color, speed, size, and area where they reproduce. More precisely, fungus gnats are often dark, while fruit flies are yellowish. Fruit flies have a more spherical body compared, and they are slower than Fungus gnat. Moreover, fungus gnats have way larger wings. Finally, you find fruit flies close to decaying fruits (the famous overripe banana). At the same time, fungus gnats are more common around houseplants and animals.
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- Prevention is better than cure. Create an environment where fungus gnats simply can’t thrive through avoiding overwatering and covering the soil surface.
- Organic insecticides such as castile soap, certain essential oils, and neem oil can help get rid of fungus gnats.
- Cinnamon is an effective natural fungicide that will starve fungus gnats of the food they need in the larval stage. And trapping adult gnats can also disrupt the life cycle.
- Biological controls can get rid of fungus gnats if the problem becomes severe, and all else fails.
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