Learn what causes gnats & how to get rid of them!
If you have any houseplants, you have probably dealt with fungus gnats. Fungus gnats are teeny tiny black flying insects that are attracted to wet potting soil.
My plants are 100% gnat free. Through trial and error, I have developed a personal toolkit of things I use to keep the gnats at bay. And I’m sharing all of the details with you today! So let’s dive in with some basics.
Key takeaways
- Fungus gnats are attracted to wet potting soil.
- Allowing the top few inches of soil to dry out, aerating the soil, and bottom watering are effective strategies to create a less hospitable environment.
- Sticky traps, machines like a Katchy, vinegar, sand or gravel, hydrogen peroxide mix, soap and water, neem oil, cinnamon, beneficial nematodes, mosquito bits, and systemic pesticides are other options.
- Persistence is key; experiment and adjust treatment as needed.
- Keep infested plants away from healthy ones to prevent gnat spread.
Lifecycle & causes
Fungus gnats have a four-stage lifecycle; they start as eggs, then develop into larva, then pupa, and then adult (source: UC IPM). The gnats lay their eggs in damp soil, usually the top layer. And they lay 200 eggs at a time!
These eggs then hatch and turn into larvae…which feed on the fungi produced from the damp soil. Once you see them buzzing around, they are at the adult stage. They don’t fly very long distances and always seem to aim for your face. The whole lifecycle is about 2.5 weeks.
Given their proclivity for nesting into wet soil, the chief cause of fungus gnat infestations in houseplants is overwatering. The reality is that most common houseplants don’t need nearly as much water as we think they do.
Getting rid of gnats
Fungus gnats don’t generally hurt your plants, but they are often signs of problems in your care routine. So it’s best to address them early and get rid of them. Here are some methods you can try out.
1. Establish an appropriate watering routine
Establishing an appropriate watering routine will help to kill off any eggs or larva in the soil. It will also prevent fungus gnats from moving in in the future. Always check to make sure the top few inches of soil is dry before watering (for more common houseplants). You can get a moisture meter, but I don’t fuss with one. I just use my finger.
I honestly find that backing off of watering my plants a bit can knock out a fungus gnat problem. Many people overwater their plants, and just letting the top few inches of soil dry out before watering again can make a HUGE difference!
2. Use sticky fly traps
I included my favorite yellow sticky stakes fly traps (affiliate link) in the houseplant lovers gift guide I made last year. I’ve since started buying some budget yellow fly paper, cutting it down in strips, and adding it to the stakes. Or just hanging it up.
These work great to control adult populations. The adult gnat populations are attracted to the yellow color on the fly traps. They get stuck in the super sticky yellow fly paper and die. Sorry, gnats.
And once you control adult populations, there isn’t anything to lay eggs. The second I see a fungus gnat, I throw some sticky paper out. If you can catch them soon, you might prevent a bigger problem.
3. Invest in a Katchy machine
I bought a Katchy (affiliate link) many, many years ago after I saw one of my planty friends online post about hers. I had a bad fungus gnat problem at the time and couldn’t get it under control. So I caved and bought it…and it’s kind of amazing!
The bonus with the Katchy is that it actually looks really nice…much better than a bunch of bright yellow fly paper hanging around your house. You can also quickly switch out the sticky fly traps to refresh it.
It has a UV light on top of it, and you turn it on at night so the flies are attracted to the light. The fan in the machine then sucks the flies down onto the fly paper.
4. Try vinegar
Vinegar is a simple and straightforward way to help control adult gnat populations. This is also a great option to use in addition to sticky traps. You can simply fill a small shallow bowl with vinegar and add a few drops of dish soap.
Refresh every few days as it fills up with dead gnats. They love this stuff. But if you don’t want to have bowls of stuff sitting around, this might not be a good choice.
5. Use sand or gravel to dress the soil
Since fungus gnats lay eggs in the top few inches of the soil, dressing the top of the soil with sand or gravel can help to prevent them from moving in. I have used decorative sand or pebbles to do this, but it isn’t my favored method. I know some love this method, though—especially for plants like cacti and succulents that prefer a grittier soil to begin with!
6. Add a hydrogen peroxide mix
In addition to vinegar, hydrogen peroxide is another thing you probably have in your house already that can help to control fungus gnats. Use roughly 1 part hydrogen peroxide mixed with 5 parts water to water dry soil will help kill everything off. This is harmless to almost all common houseplants—just don’t use straight hydrogen peroxide!
7. Try a soap and water mixture
I talked a bit about this in my article on how to debug plants before bringing them in for the winter. I typically use an insecticide spray and a soapy water mixture to kill off anything trying to hitch a ride indoors on the leaves or in the soil.
I don’t overthink this. I simply put a few drops of dish soap (a mild kind without a degreaser) into a watering can and fill it up with lukewarm water. It will bubble a bit. Then I water the plant.
If your plant is already damp, you can simply spray the top layer of soil to prevent further overwatering. If the plant is ready for watering, just use the soapy water.
8. Sprinkle cinnamon
I have tried this in the past, but I don’t think it did much for the infestation I was battling at the time. A lot of people recommend cinnamon and say it works well for them, though. You can sprinkle cinnamon, which is a natural fungicide, on top of the soil.
I’d recommend doing this when the soil is dry, working it into the top inch or so of soil, and then watering the plant. This seems to fall into the “can’t hurt, might help” category, so it doesn’t hurt to try it since you might already have cinnamon at home!
9. Add beneficial nematodes
Beneficial nematodes are another great option. They are microscopic worms that you can’t even see. However, once in the soil, they will eat a ton of different types of bugs (source: UMD Extension).
Including fungus gnats. (And thrips, if you ever have those.) They are not harmful to you or the plant at all. The downside is that they do not affect pests on leaves or the stems. Just the soil.
10. Try neem oil spray
Neem oil is another method you can use to control fungus gnats. You can buy a neem oil spray that is already diluted, or you can buy a neem oil concentrate and dilute it yourself.
I find buying a concentrate goes farther—I have had mine for a few years, and I just reuse an old spray bottle. However, not everyone enjoys the pungent smell of neem oil. If that’s you and you’ve still gotten this far without finding a solution…read on!
11. Use mosquito bits
While mosquito bits (affiliate link) are marketed as kill mosquito larvae, guess what? It works for fungus gnats, too! Mosquito Bits are a microbial larvicide. They use a bacterial active ingredient that kills off bugs in the larval stage of development. We use these in our outdoor pond and rain barrels, too.
For use on fungus gnat larvae, you can make a “tea” by soaking mosquito bits in water. Then water the plant thoroughly with the tea. Repeat until there are no more signs of gnats. I’d recommend combining with sticky tape, too.
12. Try a systemic pesticide
You can also use a systemic pesticide to kill off anything in the soil. But this is the nuclear option. I’m not anti-pesticide, but I try to avoid them whenever possible. However, on indoor plants where there isn’t an ecosystem with predators, I do resort to them sometimes.
Systemics are pesticides that you can work into the top few inches of the soil, and when you water the plant, the insecticide kills everything off. Unlike some other soil-based treatments, this will also kill things sucking from the stems and leaves, too.
You probably don’t need this level of insecticide for fungus gnats. But if you’ve tried everything else and do need to resort to them, they’ll work. I used insecticide granules to kill off some thrips I had in some of my houseplants, including my large monstera deliciosa. It worked! And it also killed off all of the fungus gnats I had at the time—added bonus!
Preventing fungus gnats
Once you get rid of your gnats, you don’t want them to come back. Below are some of the things I work into my regular care routine to make sure I don’t create an environment that fungus gnats will find enticing.
1. Let the top few inches of soil dry
Letting the top few inches of soil dry out helps to keep things healthy and to prevent the moist environment necessary for fungus gnat breeding. Watering in planters without drainage holes can also lead to soil retaining too much moisture, so it’s best to avoid that, too.
2. Aerate the soil
I like to occasionally aerate the soil in my potted plants to help promote air flow. This not only helps with moisture issues in the top few inches of the soil—it also helps prevent the soil from caking together too much and shrinking from the sides of the pots.
I like to use a fork or a chopstick for this. You don’t have to wait until you’ve overwatered a plant to do this, either. It can help soil and plant health all the time as long as you don’t disturb the roots.
3. Bottom water plants
Bottom watering your plants is one way to help prevent fungus gnats. Some plants like to keep their soil moist, so it’s not a great idea to let them dry out too much between waterings.
Bottom watering is when you set a planter with a drainage hole into water and let the plant soak the water up through the hole. This helps to prevent fungus gnats because the top layer of soil is never really wet.
The bottom-most layer of soil gets moist, and the roots pull up the moisture. I bottom water some of the plants I have because it keeps water from pooling on leaves—for example, my hoya carnosa compacta.
4. Set up a barrier for the drainage hole
I have read that gnats can also sneak into plants through the bottom drainage holes and lay eggs there. I typically put a piece of a coffee filter in the bottom of my planters when repotting just to prevent dirt from spilling out, so that might be helpful.
This is one of those “can’t hurt, might help” tools, though. Just doing this probably won’t knock out a fungus gnat problem, but it can be one tool in the arsenal to remember.
5. Isolate infested plants
If you get a plant that already has gnats, keep it away from your other plants. Fungus gnats can spread quickly. Always quarantine new plants that you buy. If you notice any issues, treat them before introducing the newbies to your existing plants.
In conclusion…
I hope this article helps you understand what causes fungus gnats in houseplants and how you can get rid of them. Not everything works for everyone, so it might take a bit of trial and error.
Don’t give up, though! Start small and see what works for you. Kick up the treatment only if necessary. I have tried many different methods and have been gnat-free for years. You can do it, too!
Here is a link to the Dropbox folder with the guide!
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Brittany is a seasoned DIY home and garden expert, running a creative brand since 2014 that inspires others with approachable plant care guides, woodworking tutorials, and decor projects. She is a certified project manager and has completed extensive coursework in the art and science of growing your own plants. Her work has been recognized by major publications, and she routinely collaborates with fellow DIY industry leaders—but her favorite thing to do is inspire you! Learn more about her here.
So, which was it that got rid of the fungus gnats?? The Katchy or the insecticide granuals?
It’s hard to say. The Katchy helps control populations that aren’t too out of control, and I used it last month when I noticed that I had some moving in to my plants. But treating the thrips with the insecticide knocked out a bad infestation.