So, you need to get rid of fungus gnats in your potting soil.
In this article, we’ll cover:
- Why you have fungus gnats
- Natural ways to get rid of them from soil
- Ways to get them out of your houseplants
- Natural predators that eat them
- How to prevent and keep fungus gnats away
- And more
You should have everything you need to know about fungus gnats and how to control, eliminate, and repel them going forward.
Bookmark this page for easy reference later.
Questions? Post a comment and ask!
Sound good? Let’s clean up your potting mix.
Last updated: 8/17/20.
What’s a fungus gnat?
A fungus gnat is a tiny black fly that’s usually around ⅛” as an adult.
They can easily be seen against a white surface or leaf.
These gnats have a long pair of antennae and very thin legs that are nearly translucent.
They have two main life stages that are distinct:
- A wormlike larvae sequence
- A flying adult sequence
Both are the exact same pest, but most people won’t know that they’re dealing with the same bug at all.
You’ll often come across fungus gnats next to your doors, windows, or other areas of entry.
They tend to hang around areas where there’s natural sunlight or artificial lighting like patio lights, desktop lights, lamps, etc
They can be found in both outdoor plants and houseplants. Fungus gnats can start to infect soil that’s never had any gnat problems. Or they can fly out of a new bag of potting soil you just bought!
Here are some other common names that people call these pests:
- Small black flies
- Black flies
- Tiny black flies
- Little black flies
Fungus gnats are easily noticeable with their long antennae and dark coloration.
They’re about 2-9mm in length and have a translucent wingspan.
There are 6 noticeable limbs with an extended abdomen. The head segment is darker than the rest of the body. The wings have noticeable muscles that span across the clear fiber.
The life cycle of a fungus gnat is simple.
The adult fly will deposit eggs that give birth to larvae. The larvae are worms and will continue eating the plant root under the soil and destroy the plant.
There are 4 different instars for the larvae until it turns into a pupa. After pupation, it emerges as an adult from the soil surface. The pupation lasts about 7 days.
Each egg takes about 3-6 days to hatch, depending on temperature. The larvae feed and develop for 14 days before pupating. The overall life cycle depends on conditions in the environment and temperature as well as food availability.
What do fungus gnats eat?
Fungus gnats primarily eat the many nutrients found in soil.
This is why you may find a bunch of larvae when you open a new bag of potting soil or garden soil. Because the soil contains many organic nutrients, the fungus gnats have plenty of food as they evolve into adult gnats.
These gnats will feed on a variety of things found in potting mix, garden mix, potting soil, and garden soil:
- Grass clippings
- Bat guano
- Root hairs
- Leaf mold
- Other fungi
These decaying organics will provide plenty of food for fungus gnats. And many soils will have a mix of these by default when you buy it from the store.
This is why you may come across adult gnats or larvae when you rip open a bag of Miracle Grow or any other leading brand. The gnats are often hard to spot until they grow in numbers. And the adults can deposit eggs during any part of the soil’s production from the farm to the factory.
If the soil is not sterilized and is all-natural or organic, this just makes it even more attractive to gnats and other bugs commonly found in the soil like caterpillars and worms.
Organic soil houses a lot more nutrients than attract a whole host of common garden pests like yard spiders and pill bugs. Sterilized soil is sterile, so there’s no organic matter that attracts nutrient-seeking bugs.
Most of the pests that are found in sterile soil are there just to lay eggs and use it as a substrate rather than feed on it because there are no nutrients found.
This is why you’ll find those tiny black flies in organic soil.
Why are there gnats in my bag of potting soil?
You may find fungus gnats in your newly opened bag of soil because they’re attracted to the micronutrients found in soil.
They may fly right out of the bag as soon as you open it, or you may start planting the soil and you’ll come across them buried within the mix.
The gnats are feasting on the organic matter commonly found in soil, such as manure, guano, fertilizer, mulch, clippings, plant matter, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, etc.
They may have found their way into the bag during any part of the soil’s production.
This means the gnats could’ve infested the bag from the factory or production plant, farm where it was harvested, or even during transport on a trailer or within a storage warehouse.
Rips, tears, and other damaged soil bags can make way for adult fungus gnats to find their way through the packaging and start depositing eggs.
This brings up another important point- you should NEVER buy discounted soil because of this exact reason. Some department or hardware stores mark down their damaged soils because of lost product or damaged packaging.
This is just asking for a pest problem to infest your yard. The money you save is not worth the trouble of getting rid of a bug problem once you bring them into your yard. Don’t risk it for a few dollars.
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Does potting soil have bugs?
Microscopic bugs and bacteria are nearly present in virtually all bags of potting soil.
Depending on how carefully protected the soil was during its journey from harvest, production, transport, storage, and retail, there may be bugs present. Most leading brands have good production chains and are pest-free.
However, bugs can still find their way into a bag of soil during any part of the production to the consumer cycle. This makes it hard to answer this question.
But if you go for a leading brand from reputable hardware or nursery, you probably don’t have to worry about anything because you can just return it for a refund or exchange.
Avoid buying cheap, discounted, or marked down soils that you’ve never heard of. They could be the remnants of good soil that are being sold as a discounted brand or all the less desirable portioned trimmings that would otherwise be discarded.
Even the top brands such as Miracle Grow, Scotts, and Fox Farms, Ace, Dr. Earth, Black Gold, Happy Frog, and Earthgro may have bugs.
There are reports from people online in gardening forums that complain if you look hard enough. No soil is completely bug-free unless it’s sterilized.
Are fungus gnats bad for plants?
Fungus gnats are harmless towards plants as adults.
But the gnat larvae have been known to chew on plant roots under the soil surface and stunt plant growth. The larvae are the destructive part of the fungus gnat life cycle, as with most other larval pests like worms and caterpillars.
You may notice your younger seedlings and leaflets growing slower, wilting, or drooping. This may be the result of the fungus gnats damaging the plant’s root system.
When there are many larval gnats, they can quickly overtake a young plant and slowly kill the plant over time.
Just a few won’t hurt, but when there are many of them eating away at the root system, the plant will have difficulty absorbing nutrients from water and soil. Thus, the plant may be killed when there are many fungus gnats.
Do fungus gnats bite humans?
No, fungus gnats don’t bite, sting, or transmit any known diseases to humans. Both adult and larvae gnats aren’t known to harm humans.
Do fungus gnats carry diseases?
Fungus gnats have no known transferable disease to humans.
However, they can carry diseases like pythium which can spread between plants.
This disease can cause damping-off and kill small seedlings. It’s spread by touching other plants that have pythium and sticks to their feet. They can also pollinate and transfer mushroom spores.
What attracts fungus gnats?
If you find these gnats start to appear in your houseplants, this is a common sign of overwatering or over-fertilizing. They may be eating the roots that have been in the water for too long.
This can cause plant roots to rot. Some fungus that grows on topsoil can also be an attractant to them. Reducing humidity, moisture, or water can help reduce their numbers.
Signs of fungus gnat damage
Fungus gnats leave minimal damage that’s noticeable.
But over time, you’ll start to see that the plant will show these signs:
- Stunted growth
- Damaged roots
- Strange leaf growth patterns
- Yellowing of leaf tips
- Damaged or distorted leaves, especially the tips
These are common signs to tell you if you have fungus gnats.
This depends on the plant’s overall health and how many gnats there are.
They rapidly breed, so you’ll start to notice the gnat damage to your plant bumps up exponentially.
Note that the gnats that are adults are harmless. Only the nymphs (worms) will eat your plant roots.
Will fungus gnats go away on their own?
Not until the plant has no more roots to chew on. You shouldn’t expect the gnats to disappear by themselves. Action is needed to save your plant.
How do I get rid of fungus gnats in potting soil?
Here are some methods you can use to get rid of bugs in your potting soil.
Use a combination of them and see what works best for you. Most of them are natural techniques to keep the gnats away from your plants.
But I also included a section for commercial pesticides, just in case.
Does cinnamon kill fungus gnats?
Cinnamon has long been a DIY home remedy to kill fungus gnats. The trick is to use Ceylon cinnamon and sprinkle it all over the soil in your house plants to quickly kill the larvae.
The larvae must eat the cinnamon in order for this approach to be effective. If the gnat only touches the cinnamon, nothing will happen. Thus, you’ll want to cover the entire top layer of the soil in Ceylon cinnamon.
The more coverage you have, the better the chance of the worm eating the cinnamon.
But don’t overdo it. You need to find a balance between providing enough cinnamon to kill the gnat but not turn your soil’s metrics (pH, kH, NKA, etc.) to another level.
After the gnats are no more, scoop out the entire top level of the soil to get rid of the cinnamon. Don’t leave it there. Cinnamon wills both larval and adult fungus gnats and also helps repel them from coming to your plant.
This works for both indoor and outdoor plants that have fungus gnats. But if you just have a new bag of soil that’s infested, this doesn’t really work. This is a nice natural way to get rid of gnats.
Dryer sheets have been said to repel fungus gnats, but the main problem is that it’s hard to get the scent all over the soil. If you have a bag of soil that you just opened and it’s swarming with gnats, place 2-3 scented dryer sheets into the bag and leave the bag open.
The gnats hate the smell and will fly out of the bag.
As for the larvae, they can’t fly yet, you’ll have to deal with them separately using the other methods on this page.
Assuming you’ve already planted the soil, you can use sticky traps around the plant that has fungus gnats.
These are just traps that you hang on a nearby branch, stem, or leaf and the adult gnats will fly into them and get stuck. You can use them on both outdoor plants and houseplants. You can buy them for cheap at most hardware stores. Use as directed on the label.
These are a different type of trap, but basically they’re just stakes you stick into the soil that has gnats. The gnat flies into the stake and gets stuck.
You can pick them up at hardware stores and use them as directed by the label. These are best used for houseplants that have fungus gnats rather than a new bag of soil.
There are fly killing devices that can help control free-flying gnats.
This is especially useful for indoor houseplants or contained places like a small greenhouse. They can also help kill other pests that fly. This is useful for exploiting their weak flying ability.
You can reduce the amount of water you use to help prevent fungus gnats.
They’re attracted to moist roots saturated with water.
So reducing the amount of water you use can help reduce the number of nymphs and make your plant less appealing to them.
You should also reduce the amount of fertilizer you use.
Excess fertilizer means excess nutrients. This could lead to fungus developing on the topsoil which can attract more fungus gnats.
Add a sand layer
Adding a sand layer to the topsoil can prevent fungus gnats.
Check with your plant first to see if it can handle a top layer of sand. If so, add a 0.5” sediment bed of sand right on the topsoil. This will prevent the nymphs from entering the soil and protecting your plant’s root systems.
Attract natural gnat eaters
There are many natural predators that eat fungus gnats.
The most popular and effective one is a bacterium microbe known as bacillus thuringiensis, AKA Bt.
This microbe is deposited directly into the soil where gnats are present and will disturb their lifecycle. Depending on where you buy it from, the process is different.
But generally, you’ll want to apply Bt on a cloudy day. You’ll mix the Bt solution with water and then spray the solution or use a watering can.
Some Bt products may have you using an injector or syringe requiring a dilution rate. They come in a sealed container to preserve their efficacy are usually found online or in specialty nurseries. Use as directed.
You should avoid using Bt when possible as it’s an extreme measure in my opinion. But if you just can’t kill the fungus gnats, then you can consider using nematodes as a last resort.
How to get rid of gnats in houseplants using houseplants hydrogen peroxide
Use some peroxide as a spray to kill fungus gnats.
You can mix the solution with half water and half peroxide into a spray bottle. Spray it on adult gnats and any visible worms.
This will kill them instantly.
Remove the dead pests to prevent attracting other bugs to the plant. Check to see if the plant reacts to the peroxide before applying to the whole plant.
You can do this by testing it on a single leaf and waiting 2 days.
You can use regular pure white vinegar to kill fungus gnats. Mix two tablespoons of vinegar with a half cup of warm water. Add six drops of dish soap.
Use a spray bottle and spritz the soil surface around your plant. This will help deter and prevent any further fungus gnats from surfacing.
You can also use it on adult gnats as a repellent. Be sure that the acidity of the solution doesn’t harm your plant before you go all out. You should test the mix on a small leaf first and check for damage after 24 hours.
Nothing? Then go ahead and apply the solution. Repeat until the fungus gnats are gone. You can use this for both new soil and current soil that’s been infested with these bugs.
Apple cider vinegar
Just like regular vinegar, you can do the same technique with ACV. Use the same steps above.
ACV can be acidic too sensitive plants, so test on a small leaf first before using on the entire plant.
Dish soap is one of the most popular all-around DIY techniques because it’s cheap, easy to make at home, and very effective.
You can make your own dish soap mixture by using a dish detergent (8 drops) and 1 cup of water. This will create a mixture that bubbles somewhat after you spray it on the soil. The dish soap drowns the gnats and kills them over time.
Be sure to test iron a small leaf first before applying it to the whole plant. If there’s a reaction, dilute the mixture or use fewer drops of soap.
You can use any soap, but if you’re planting organics, you can use an organic or natural dish detergent. And be sure to wash and assess your harvest before you eat.
How does Dawn dish soap get rid of gnats?
Dawn dish soap can be used to make a DIY solution of soapy water by drowning them. The surface tension of soap bubbles is too thick for them to pop. Thus, fungus gnats drown.
Will soapy water kill fungus gnats?
Soapy water will drown and kill fungus gnats provided that you make the proper solution.
The ratio can be tricky, but once you find the right one, you can use the soapy water to kill the pests upon contact.
Diatomaceous earth, or DE, can be an effective way to get rid of fungus gnats.
This is a natural powder that’s commonly used for pools or food. You can buy food-grade DE at most department stores. Use the non-toxic type that’s 100% pure food-grade DE. when you apply it, wear a protective mask and gloves. Mix the DE into the soil completely.
This works for both existing soils that have become infested with gnats or new bags of soil that already had gnats. Just pour the diatomaceous earth into the soil and mix completely.
There are no exact portions to measure- just use enough to where it’s visible when mixed, but only when you look carefully. Yes, that’s not precise.
But you don’t want to swarm the whole soil base with DE. less is more. As long as there’s visible DE for every square inch of soil, you should be okay.
You can apply this to existing plants that have gnats in the soil by removing the soil, mixing it with DE, then replacing the soil. For new bags, pour all the soil into another container and mix.
Alternatively, for existing plants, you can just add some DE to the top layer only if you’re feeling lazy. Not as effective as saturating the whole plant.
Attract predators (Hypoaspis)
Other than the Bt nematode, there are a few other pests that regularly eat fungus gnats.
You can special order them online or buy them at specific nurseries. The trick is to order them and then contain both the fungus gnats and the natural predators in a secure area.
This means using a small greenhouse or another container to put both the soil and the predators together.
Over time, they’ll eat the fungus gnats, both the adults and larvae until the soil is sterile from gnats.
Of course, depending on the number of gnats you have, the volume of soil, and how many predators you order, this can take time.
- You don’t have to do anything once you set it up
- Relatively easy and cheap
- Gets rid of the fungus gnats to nearly 100%
- Takes a long time
- You may have to sustain the predators
The most common predators used for gnat elimination are Hypoaspis predatory soil mites.
Each Hypoaspis eats about 3 prey per day on average, so depending on how many you order, the time it takes will vary.
Use as directed by the breeder. Most fungus giant predators work best when the larvae population is minimal.
What temperature kills gnats?
Gnats are difficult to kill by temperature alone.
They can withstand both freezing and extreme heat. Some people will attempt to freeze their potting soil to get rid of the gnats. This works, but it requires very low temperatures for a sustained
period of time.
You can try freezing small amounts of soil by placing it into a zipper bag and freezing it for 3 days. This may take multiple attempts before you kill all the gnats.
On the other hand, you can also use heat to kill gnats.
Pour the soil into a container and seal it with plastic food wrap. Place the container outdoors on a hot day in direct sunlight and the container will heat up. Let it sit out for the entire day.
Wait until the next day during sunrise to unseal the container and check for gnats. Be careful as it may be hot. The point of leaving the container out overnight is to suffocate the gnats using the trapped heat.
Both of these measures don’t work as well as others, so using temperature for gnat control is a poor methodology. Don’t rely on it to do anything unless you have extreme lows or highs.
Gnats live comfortably between 75-80 degrees. You’ll want to use temperatures below 20F or above 100F to kill them.
Note that they can stand freezing temperatures because they have proteins that prevent this. Thus, they can both overwinter and tolerate extremely cold temperatures.
T drops for gnats
T drops (also sometimes called “tea” drops) are Tanlin drops.
They’re a synthetic pesticide advertised to kill fungus gnats and a whole host of other common pests.
Tanlin needs to be used as directed by the product label and usually requires dilution with water. This mixture is usually 100% safe for plants and harmless for animals, humans, and plants.
Tanlin kills gnats at the plant’s root system and will kill the gnats that are feeding on them. Combined with Nilnat, T drops can be very effective to kill maggots found in the soil.
Some people online have reported that it may stunt plant growth temporarily, but plants do recover. T drops can be used on both outdoor and indoor plants for the majority of applications. You can find the product online or at nurseries and dilute accordingly.
Here are some store-bought solutions to kill fungus gnats.
If you want a spray that actually works effectively against fungus gnats, look for something that has pyrethrin as an ingredient.
Pyrethrin is highly toxic to gnats and will kill them quickly.
You should follow the directions on the product label at all times.
Avoid using pyrethrin sprays when possible.
Always use natural or organic methodology to get rid of fungus gnats when possible. But if you need to use pyrethrin, try to use something with it in combination.
This could be anything that won’t interact with the spray, such as a sticky trap, stake, or some other neutral product.
Don’t use DE, vinegar, essential oils, or any other kind of material with pyrethrin. Pyrethrin is the fastest way to kill gnats reliably.
How do I get rid of fungus gnats in my house?
If you’re set on using temperature to kill fungus gnats, baking the soil is the only surefire way to kill all the eggs and worms.
The pro to baking is that it kills everything and basically sterilizes your potting mix.
The con is that it’s a hassle to do, not practical for everyone, and also makes your oven smell for weeks.
If you have an outdoor kiln or furnace, this may work better:
- Start off by laying down the soil spritzing it with water. This will moisten it and prep it for baking. This is optional but I highly recommend that you do it to make it easier.
- Pour all the soil slowly into an ovenproof container. Use a clay pot, baking pan, or some other container. Make sure it’s overproof and has enough volume to contain all of the soil you want to cook.
- Filter out rocks, debris, sticks, and stones. Make sure there’s nothing but pure soil. Mulch it with your hands to ensure proper air circulation.
- Break up any noticeable clumps or throw out hardened soil.
- Cover the container with aluminum foil and poke a few holes in it.
- Put the container into your oven and bake at 200F for 30 minutes.
- Continue to monitor the temperature to make sure it approaches 200F every 10 minutes. If you notice an odor appear out of nowhere, open the oven door to air it out.
- Be sure to turn on your oven fan and open your windows and patio doors. It’ll stink. Lower the temperature to remove the scent. There should be a slight odor, but not a strong one. If you smell a strong manure odor, release the heat and turn the temps down.
- After 30 minutes are done, let it cool to room temperature. Keep the oven door shut. Let it cook.
- Remove the pan from the oven after it’s cooled to ambient temperatures.
- Check for pests. If you did everything right, you should see no more fungus gnats. You may have to remove any pests that you see in the dirt. Repeat the baking if necessary.
- Clean up.
How do I keep gnats out of my soil?
There is no single way to do this. You should use a combination of all of the above methods covered on this page.
Use a few of the following techniques to keep fungus gnats out of your soil:
- Set up yellow sticky traps
- Use sticky stakes
- Spray vinegar on the soil surface
- Sprinkle diatomaceous earth on the soil
- Apply Bt
- Bake the soil
- Try dryer sheets
- Use T drops
Don’t use all of them because that’d be overkill. Use as few methods as possible until you see that the gnats have disappeared. Then you know that it’s working.
Remember that preventing the fungus gnats from infesting your soil proves to be better than trying to get rid of them.
This applies to old soil that was suddenly overtaken with gnats. New soil that you just opened can be baked, mixed with DE, or use Bt.
Will repotting plants get rid of fungus gnats?
Repotting your plants that already have fungus gnats won’t do much and can actually help the pests infest new plants. If the gnats are clinging to the repotted plant, you may inadvertently transport them from one planting site to another.
Unless you soak the plant in a solution that kills all fungus gnats, there will likely be some clinging onto the leaves, roots, or stem that you miss.
Repotting only works if you can 100% make sure that there are NO nymphs on the plant before you replant.
Otherwise, you need to treat the plant in the same soil. You can always try replacing the soil with new soil if this is the case.
How to prevent gnats in houseplants
Here are some tips to prevent fungus gnats from getting all over your soil in the future.
Always quarantine new soil
When you buy new soil from the store, you should quarantine it by placing it in an isolated container.
Check the soil every few days for two weeks. You’re looking for bugs, eggs, and other pests.
You’ll have to stir the soil to make sure you don’t miss any maggots, worms, or nymphs hiding deep in the soil.
This should be something you do with anything that you plan to add to your yard- just like quarantining plants.
Don’t buy discounted soil
As mentioned earlier, don’t buy discounted soil.
These are usually damaged, opened, or used. You don’t know where the soil has been, how it was stored, or if it already has eggs brewing a future pest problem.
The few bucks you save definitely isn’t worth the hassle you’ll have to deal with. New bugs in the yard? No way.
Check the packaging
Always check the packaging for the soil you buy. Check for small holes, tears, rips, and taped-up soil bags.
This is commonly found in cheap department stores- they may tape up damaged bags of dirty and mark them down for resale.
This is just asking for a pest problem and should be avoided. Some bags of soil contain small holes for ventilation- this is normal.
Mix in diatomaceous earth
For a new bag of soil, you can mix in DE as a precaution even if you don’t suspect there to be pests.
DE is safe for most plants and can help prevent and kill gnats in the future. It’s easier to mix in the stuff BEFORE you plant rather than trying to do it AFTER there’s a gnat problem.
Check your plants on a schedule
You should always be checking the plants in your yard regularly.
Every time you check, take note of any damage, changes, or other anomalies you notice.
Then act upon it. Check it out. Research it. Neglect will just forward the pest problem.
Use gnat prevention
Even if you don’t currently have a fungus gnat problem, you can always take measures to prevent them in the future.
There are gnat seasons where the gnats are rampant.
If you live in an area where they’re native, you can start by doing the following the repel fungus gnats:
- Layer your soil with DE- both indoor and outdoor plants.
- Set up sticky stakes or sticky traps to catch any adult gnats. If you see some caught, you know that there are gnats present.
- Regularly check your plant for gnat damage.
- Continue to check your soil for signs of gnats.
- Quarantine new soil and plants.
- Use vinegar or dish soap on any adult gnats.
- Check the leaves, stems, and other parts of the plant for damage.
- Prune the plant on a schedule.
Here are some references you may find useful:
- Fungus gnat – Wikipedia
- Fungus Gnats as Houseplant and Indoor Pests – CO State
- Fungus gnats – Missouri Botanical Garden
Did you get rid of the fungus gnats?
You have everything you need to know on this single page.
Now go forth and get to work!
You can now manage, control, and exterminate fungus gnats and save your plants from these pests.
If you have questions, drop me a comment. Or if you own this page helpful, please tell a friend!
This took a lot of time to put together, so you telling others about it definitely helps and is the most you can do for me =]!
Thanks for reading.
Currently an active researcher in the pest control industry for the past 8 years- with a focus on using natural and organic methods to eliminate pest problems.
I share handy DIY pest techniques I come across here to help out others (and possibly save them from a mental breakdown).
Fight nature with nature.