Gardening & Homesteading

Controlling Fungus Gnats and Damping Off Organically

Controlling Fungus Gnats and Damping Off Organically

Late winter and early spring is when most gardeners start seedlings indoors and get the gardening season underway. Growing your own garden plants from seed is highly rewarding and can allow you to enjoy unique or heirloom species that are not found in garden centers.

However, fungus gnats and damping-off can stymie even the most seasoned gardeners and seriously affect the success of growing seedlings. But before you spend money on expensive chemicals, the solution to these problems is much safer and cheaper than you would expect.

Fungus Gnats

Fungus gnats and fruit flies are DIFFERENT

Fungus gnats (Sciaridae spp.) are the most common houseplant pest, and are distinguished from Fruit flies (Drosophila spp.) because of their darker color. While fruit flies hang out primarily near fruit, rotten food and funky fridges, you’ll find fungus gnats in wet plant soil, in sewer areas, and in household drains.

They’re also attracted to CO2 (carbon dioxide) which is why they fly up right in your face.

Fungus gnat adults are about 1/8-inch long, spindly looking flies with long legs and long, thin antennae. They resemble tiny mosquitoes more than they do common flies. These tiny, black insects seem innocuous enough as they hop all over your plants.

But, while the adults rarely cause any problems, the young larvae will feed on the fine root hairs of your plants, causing them to lose vigor and develop yellow leaves. They can also tunnel into the base of cuttings or plant stems, which can be devastating for succulents, leeks, or onion seedlings in particular.

Fungus gnats search out moist soil in which to lay their eggs. The eggs are about 1/100 inch in length and laid in clusters. Larvae are translucent gray to white worms, about 1/4 inch long, with shiny black heads. These insects can infest a crop from wet, algae-covered areas in the garden, from contaminated potting soil or by flying short distances from plant to plant.

The easiest way to prevent fungus gnats is to water your plants properly. Overwatering, which causes your potting mix to remain moist for extended periods of time, attracts fungus gnats which seek out a steady supply of fungi, algae, and decaying plant matter for their larvae to eat.

If you allow the soil to dry out between watering, you can disrupt the availability of food for the fungus gnat, and make your soil less attractive to them.


Darn those tiny lumberjacks!

Not only will controlling your watering help prevent fungus gnats, it will prevent the second most common problem for indoor gardeners: Damping off.

There is nothing more heartbreaking than saying goodnight to a tray full of robust, little seedlings and waking up to find them all knocked down and laying dead on the soil, as if felled by tiny lumberjacks in the night.

This condition is caused by several fungi such as Phtophtora and Pythium. These fungi are often carried by fungus gnats, and live at the soil line, where air meets the moist soil surface.

When your potting soil is kept continuously moist by overwatering, and your seedling roots are weakened by fungus gnat larvae, the damping-off fungi can easily infect your seedlings.

The telltale symptom is a constricted stem, just at or below the soil surface. Once seedlings are infected, they tend to fall over dead, and must be replanted. There is no cure for damping off; it can only be prevented.

Potting mixes containing compost or peat moss seem to be particularly affected by fungus gnats and damping off. If the problem is reoccurring for you, think about switching from a peat-based mix to one that only contains perlite or vermiculite.

Because fungus gnats have a quick life cycle, it is important to reduce their numbers by using methods that control them both as adults and as larvae. Whether you choose physical controls and/or biological controls, there are a variety of affordable, safe choices for ridding your indoor garden of pesky fungus gnats and their plant-killing larvae.

Physical Controls

Sand – Controls larvae
Adults lay their eggs in the top 1/4 inch of moist soil. If you dress the top of your soil with a 1/4–1/2 inch of sand, it will drain quickly and often confuse the adults into thinking the soil is dry. You can use colorful decorator sand and have fun with this!

Vinegar – Control adults
A good trap for both fungus gnats, and especially fruit flies, is to put out baby food jars filled halfway with apple cider vinegar or cheap beer with a couple drops of dish soap added to break the surface tension. Once you’ve filled the jars, screw on the lids, and poke several holes into them large enough for fungus gnats to enter.

Place these jars in areas where you are having problems with either fungus gnats or fruit flies, and they will dive into the vinegar and drown. Strain and reuse the vinegar until you have gained control of them.

Potato slices – Controls larvae
Slice raw potatoes into 1-inch by 1-inch by 1/4-inch pieces. Place the slices next to each other on the surface of your potting media to attract fungus gnat larvae. Leave the potato slices in place for at least 4 hours before looking under them. (Be prepared to be grossed out a bit.)

Once you have seen just how bad the problem is, replace the potato slices every day or two to catch and dispose of as many larvae as you can, and consider adding additional control measures.

Note the HORIZONTAL orientation of the trap

Sticky Traps – Controls adults very effectively
Make your own sticky trap by smearing Vaseline or Tanglefoot on a 4″x6″ piece of bright yellow cardstock, and place the card horizontally just above the surface of your potting media, where it will catch the adults as they leap from the soil. Set another trap vertically to catch incoming gnats, whiteflies, thrips, and more.

I often lay these traps on the edges of pots, or make little holders out of old, bent forks to hold them horizontally or vertically, as needed. Where to find pre-made yellow sticky traps and holders online.

Food Grade Diatomaceous Earth – Controls larvae
Food grade diatomaceous earth is another effective treatment for fungus gnats. Diatomaceous earth (DE) is mineralized fossil dust that is both natural and non-toxic to the environment. Make sure you get food grade diatomaceous earth—not pool grade, which is not pure enough for use around food gardens and pets.

Always wear a simple dust mask when working with DE: Inhaling any kind of dust is never a good idea.

DE contains microscopic shards of silica that physically shred any insect that walks through them, therefore it will not work in hydroponic gardens. But if you mix some into the top layer of infested soil—or better yet, into your potting mix before planting—it will kill any gnat larvae (and adults) that come in contact with it, as if they were crawling through crushed glass.

DE works the same way to kill fleas, bedbugs, slugs and other insects too, so don’t use DE to control fungus gnats in your worm bins. (Poor worms!) Some people eat DE therapeutically to kill intestinal parasites. Where to find food grade diatomaceous earth online.

Biological Controls

Hydrogen PeroxideControls larvae
Mix one part 3% hydrogen peroxide with four parts water. Allow the top layer of your soil to dry, and then water your plants with this solution as you normally would. The soil will fizz for a few minutes after application; this is normal.

The fungus gnat larvae will die on contact with the hydrogen peroxide. After a few minutes the fizzing stops and the peroxide breaks down into harmless oxygen and water molecules. Repeat as needed.

Chamomile Tea Controls damping off
Weak chamomile tea (after it has cooled) is another natural fungicide that is effective in stopping damping off, though it does not treat fungus gnats at all. Simply brew a quart of strong tea, let it cool, and add it to your 1-gallon watering can. Add more water to the can until full, and use whenever you water.

Cinnamon Controls larvae and damping off
Cinnamon powder is a natural fungicide that has been shown to be particularly effective against damping-off. It helps control fungus gnats by destroying the fungus that the larvae feed on. Only true Ceylon cinnamon, or Cinnamomum verum, will work.

Simply sprinkle enough cinnamon to form a visible layer across the top of your potting media, and repeat every few weeks, if needed. DO NOT use to control fungus gnats in worm bins as cinnamon will kill your worms. Where to find true cinnamon powder online.

BT – Bacillus thuringiensis var. israelensisControls larvae very effectively
Bacillus thuringiensis (Bt) is a naturally-occuring bacteria that kills many types of worms, caterpillars, larvae and insects. There are specific strains of Bt called “israelensis” or “H-14” that specifically kill fungus gnat larvae, and are sold under various brand names like Gnatrol or Knock-Out Gnats. (Other varieties of Bt will not work for fungus gnats.)

Used extensively in organic greenhouses, Bt-i can work to stop fungus gnats where nothing else will, though it is a little pricey. Bt-i is safe for use in worm bins, and can help control mosquito larvae too. Where to find Gnatrol online.

Beneficial NematodesControls larvae
There is a type of nematode, Steinernema feltiae, that can be used to drench the soil each time you water. These tiny worm-like creatures will enter the larvae of soil pests like the fungus gnat and release a bacterium which is lethal to it.

Nematodes are expensive, and are best used on a large infestation of many plants, because they are hard to control in small quantities. Nematodes kill a variety of soil-borne pests, and are safe for use in worm bins, too. Where to find beneficial nematodes online.

About the author

Dawn Gifford

Dawn is the creator of Small Footprint Family, and the author of the critically acclaimed Sustainability Starts at Home - How to Save Money While Saving the Planet. After a 20-year career in green building and environmental sustainability, chronic illness forced her to shift her expertise and passion from the public sphere to home and hearth. Get the whole story behind SFF here.


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  • Hi, thank you for all of these ideas. Right now I’m trying the trap paper, potato slices, and an apple cider vinegar trap on each side of the pot. I’m a first-time grower so I had no clue that plants could still get infested with pests indoors! Please let me know if the hydrogen peroxide remedy is safe for young seedlings.

  • I have a small gnat problem in which I use a vacuum cleaner to suck up the adults. I just tap the bins and agitate the top soil to get the buggers flying and bam. I do this 5 or 6 times a day and any other time I have to water or whatever. Seems to work. I am also using D. earth as well.

  • Thank you so much. So fur the best informative put togather article on the web i could find. You awesome. Thank you for sharing intensive research. It takes time!

  • I saw mosquito dunks in Home Depot / Lowes. Are they same as gantrol or knock-out-gnats? The packages says BTI which sounds like the same strain of bacteria. Has anybody used them?

  • My seng tui money tree has had stunted growth because of the blighters! I’m using cinnamon and the drying tenchnique, so hopefully it’ll work

  • I’m so glad I found this! I introduced a plant to my indoor garden recently I think was infected with fungus gnats, and now the gnats and the fungus they carry are destroying my herbs 🙁 I’ve already tried the diatomaceous earth, have a fan blowing on my plants, and sticky traps for daaaaays but they are becoming more numerous by the minute. Just tried the hydrogen peroxide trick, and will be investing in those nematodes and such. Losing my mind seeing my cute little baby tomato plants and herbs get ravaged by these mofos and the fungus they spread!

  • I do in house pest control and we are have Fungus Gnats in one part of the facility. It about 4 stories above the ground, and it is a very wet and damp area. There are sugar and food particles everywhere. I catch the FG in insect light traps with glue boards in them. I can’t seem to find the source, I’ve spent hours looking for where they are coming from and the light traps aren’t controlling them either. Any suggestions? It is a food facility so we are very limited in what we can try. Thanks for any help you might have.

    • They really could be coming from any damp or moist areas. They especially like to live in drains. Anything you can do to mitigate the moisture in the area will help your other efforts, as will closing up the drains with stoppers until you need to use them. Good luck!

  • Does anyone know if DE (diatomaceous earth) still works once it gets wet? I would guess it does, otherwise how could one water their plants? Anyone know?

    • Several sources state that DE is less effective when wet, because it tends to clump together. To get around that, try using a watering bulb inserted near the root of the plant instead of freely watering the surface. Or, try using a “plant spa” style pot, that is meant to be watered by pouring water into a tray under the pot, then the water soaks into the lower layers of soil where the plant roots can find it. (Note that such pots only work for deep rooting plants and do not work if you fill with layers of rock under the soil.)

  • I am specifically looking for a solution to fungus gnat infestation in my indoor container vegetable/herb garden. I purchased knock out gnats and now I have to throw away all my plants because I wasn’t suppose to use it on edible plants.

    • There are many solutions in the post that should help. If your food is a fruiting crop, like tomatoes, you shouldn’t need to throw it out. You just don’t want to eat lettuce, for example, that you have poured Bt onto. Always apply any solutions to the soil, and not the foliage of your plants.

  • There seems to be conflicting advice out there on whether Ceylon Cinnamon or Cassia Cinnamon is the better fungicide. This website says that only Ceylon will work. But other sites, including this scientific study, appear to suggest that coumarin is the more effective fungicide and Cassia has more coumarin in it than Ceylon, suggesting that Cassia is the better fungus inhibitor and killer. Would love to know more if you have more literature on this and or can point us in the right direction.

    • Thanks for sharing a study that shows both can work. I think this information will matter most if you have a commercial scale operation that depends on regular control. For the home gardener who just wants to save a few houseplants or seedlings, I recommend whatever is in the cabinet or easily available at the store. In practical terms alone, that will work best and fastest. If you can get Ceylon, I find that that works best in my home.

  • Just a quick correction, DE (Diatomaceous Earth) only impacts insects with an exoskeleton. Worms don’t have exoskeletons so it actually will NOT harm them.

    • Food-grade DE will not harm earthworms, but pool-grade DE can be harmful to them. DE can also kill slugs and snails, and food-grade DE, when ingested, can kill intestinal worms and parasites.

  • Thanks everyone! I have a beautiful indoor garden here in Korea– about the only way to get chives, basil, cilantro. I also have some tomatoes and strawberries and flowers. I lost some young spinach and greens and young basil to the little buggers, but after almost a week of drying out my soil and a day of cinnamon and peroxide washes, the bugs are gone and the plants seem OK. I hate to kill anything, but these things just don’t deserve mercy. This site (and others) have been so helpful! It’s not possible to get the recommended pesticides here (organic pesticides worked OK on the adults– but I’d used them for weeks and the larvae continued). Peroxide and cinnamon are slightly more expensive here– but well worth it as my garden is organic and I love it.

    • I’ve had two of my pepper seedlings dampen off .. And my herbs have seen brighter days. Will straight peroxide harm my tomato seedlings? At this point..I just wanna get rid of these buggers. My last remaining pepper seedling has barely breached the surface..hasn’t even stood up yet. Do I spray the peroxide all over? Or lightly pour it around the outside? Newbie here…thanks.

      • To stop damping off, you’ll want to use cool chamomile tea, which will not harm your plants. Straight peroxide might be strong for tiny seedlings, so you might want to plant a lot of them so you have enough to cover some losses, or try something a little less harsh on tiny plants, like diatomaceous earth.

  • I used cinnamon for some of the potted plants and they seem to work, but I sprinkled some on my seed pods in my aero garden and some of the already sprouted seeds didn’t like it too well. My mint immediately wilted, my Curley parsley wilted. And so did some of my basils. The chives didn’t come up yet at all so I don’t know how it effects that. The tomatoes seem to be doing just fine.

  • I had a really bad fungus gnat problem in one of my bigger pots. I tried an organic fungicide (Garden Safe), but it didn’t help at all. Used the yellow sticky traps, and they worked to get the adults, but there must have been so many eggs that I couldn’t keep up. I finally put a layer of diatomaceous earth on all the soil and that solved the problem immediately. I’m leaving it there for at least another three weeks to make sure I got all the stages of the gnats. I will have to water carefully and reapply in the meantime, but it will be worth it. I definitely recommend trying the DE if you need a quick solution!

  • This is one of the most thoroughly detailed, informative pieces I’ve ever read on fungus gnats! Very nice page design, as well.

    Thanks for the great information! Definitely bookmarking your site.

  • Would hydrogen peroxide be harmful to succulent roots? They’re just so sensitive compared to other plants I’ve taken care of. Others here have mentioned treating with straight undiluted 3% hydrogen peroxide; have any of you tried the undiluted stuff on succulents?

    • Hydrogen peroxide burnt the leaves of my Peace Lily.
      I have a Gnat problem now with my Happy Plant and want to avoid burning the leaves so tried the cinnamon remedy suggested and hasn’t helped. Going to my local gardening shop to ask for a Gnat control mix.

  • Your article on gnats is great! Thanks so much! Would DE made from Safer brand be adequate to apply as a top dress, or is there another brand that works better? Also, Gnatix (brand name) is crushed glass that I’ve been told works amazingly well. Ever try that?

  • Wow! I can not thank you enough for this info!! These little pests seem to LOVE my succulents and i don’t use any pesticides on my plants. This is the first useful info i have come across in my search for a cure, THANKS A MILLION!
    *also thanks for the idea (your 4yr olds personal corner)i have a 3yr old helper who LOVES to garden with his mommy, only he thinks my succulents and cacti area ALWAYS thirsty. “She was thirsty mama, she needed drink,Eli take care of her mama”somehow I’m always too late to stop it 😉

  • I’ve been trying some of these. The potatoes only made the little flies copulate… They seemed to have found it cheekier than the soil itself.
    I have now made my own sticky traps -vaseline over yellow post-its- and although it was fun making those little handsome bastards, little flies don’t seem lured by it… Think the potatoes might have filled that void inside their funky little being.
    Could mine be a slightly different type of gnats? Maybe a “you won’t fool me” type?

    • You’ll want to remove the potatoes frequently, if not daily, to get them to work. Also make sure your sticky traps are BRIGHT yellow. I will often shake the pot or stir up the soil a bit to get them to jump. They get stuck on the traps much more quickly that way. If you have a huge infestation, you’ll want to use nematodes as the most effective method.

  • I have a bunch of potted plants and all are infected. I tried the diluted version of peroxide 3% and water and it had no effect. Out of desperation I poured straight peroxide right onto one of my plants and it seems to have done the trick. Seems like a zillion gnats came flying out of the plant and flew off. The soil bubbled up then settled back down. The plant doesn’t seem any the worse for it either. It is over a week now and I haven’t seen a gnat on that plant since, nor do I see any larva in the soil. Now I will do all the rest of my plants too. Thanks for the tip.

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