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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.

Damping-Off

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 – Bacteria thuringiensis var. israelensisControls larvae very effectively
Bacteria 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.

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Thank you for your support!

MEDICAL DISCLOSURE: Your health is between you and your health care practitioner. Nothing in this blog is intended for the treatment or prevention of disease, nor as a substitute for medical treatment, nor as an alternative to medical advice. Use of recommendations is at the choice and risk of the reader.





45 Comments

  1. 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.

  2. 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.

  3. The good news for those of us finding out just now that we’ve been lied to for years about our cinnamon is that for the purposes of killing fungus and therefore fungus gnat larvae, cassia (cinnamomum cassia) is just as antifungal and antiviral as is ceylon (cinnamomum verum). The cinnamon you have on your shelf, even though it’s probably not true cinnamon, will be just as effective. Cassia is still a hot spice and contains hot oils, and it’s not the same species of plant but it is from the same genus.

  4. Thanks for all the help. I have some house plants, including herbs (I’m in an apartment) and the fungus gnats are back. So I was thinking that I should try the hydrogen peroxide solution first. If that does work, then I’ll get some DE. How does this sound? Will either hurt my herbs (dill, rosemary, basil and lavender)? How many treatments with the peroxide solution should I do before determining if it’s done the trick?

    • 2-3 times should do the trick, unless you have a massive infestation. It shouldn’t hurt your herbs, but if you are worried, use a more dilute solution.

  5. I have a drainage ditch field next door to my house where noseeums gnats and other insects are very prevalent . I have tried almost everything but nothing seems to work. Is there any way to break this cycle around my deck area so this kids and family can enjoy our outdoors? We also have a very large garden which doesn’t seems to mind All the flies gnats and noseeums….

    • Unless you can get rid of any standing water, moist soil and organic matter in the ditch, there won’t be much you can do. That ditch is a little ecosystem. You can install citronella or essential oil torches around your deck to get at least some relief.

  6. Hello and thanks for the post. I have a nasty infestation of fungus gnats. I don’t water much (twice a week in Northern Cali) but my plants are all potted. PB started after I muched with cocoa shells and worsened with added dry blood to the soil.
    I’ve tried nematodes and Stratiolaelaps scimitus (=Hypoaspis miles)
    SKU: 1154002-G (twice and the second time they sent me 4 times the dose by mistake… used it all!). So far no success. Can I use chamomille and/or cinammon or will I kill the nematodes and beneficial mites ? How about Bacteria thuringiensis var. israelensis ?
    Should I not much this year ? What do you think about blood ? My plants love it (until the gnats hurt them!)

    • Anything that keeps the soil from drying out can encourage fungus gnats. However Bt can help and other beneficials can help.

  7. Thank You! Stuff I didn’t know yet!

  8. Thank you SO much! I lost so many seedlings last year due to dampening off – I had no idea what was wrong with them and it was so discouraging. Now I know!

  9. Does hydrogen peroxide kill earthworms too? I like to keep earthworms in all my plants.

  10. I have a question, I am growing wheatgrass, and I can’t tell if what I have are fruit flies and/or gnats, I am not a usual grower, anyhow, what can I spray on my wheatgrass if anything that will repel the gnats/fruit flies, and what is the best thing I can add to the soil now, and before I add seed to the soil?

    Also the grass looks good, and is almost fully grown, can I still eat the greass even though those things have been on them? Sorry for all the questions, I haven’t found the answers I have been looking for so maybe you can help. Great artical btw, I am bookmarking this page!

    Thanks
    Adam

    • The only ways to be rid of fungus gnats are using the methods in the post, and there are several ideas for you to try before your next planting. There really is nothing you can spray though, and it would ruin the wheatgrass anyway. The gnats won’t hurt the quality of your grass if it has already grown in well, so don’t worry about juicing it. :)

  11. God Bless you for sharing this info. THis has been the bane of every attempt ive been making at growing indoor herbs, seedlings etc. So helpful to find all these remedies in one place onece again ty.

  12. great stuff,but how much do i use in each 5gal plant pot if mix it in with the soil?
    thank you….

    • How much of what? Cinnamon, nematodes, diatomaceous earth? Generally, you need only apply cinnamon or DE to the surface, where the gnats live and breed. How much will depend on your pot width and infestation level. I would surely cover the top of the soil.

  13. This is some great information. Thanks for sharing!

    Please join us again Thursday at:
    The HomeAcre Hop

    ~Ann

  14. Great info Dawn! Thanks

  15. Thank you for the best info so far!
    I am starting an urban container garden this year, and I have 6 large pots and a balcony to work with. I have seedlings starting indoors. I bought 6 bags of Organic Miracle Grow veggie soil, and soon I will pot and set everything out on the balcony.
    I did some research on the fabulous new soil that I purchased, (the organic miracle grow) and it seems that almost 90% of the people who bought this soil reported a severe gnat infestation from larvae that is mixed in the soil.
    If I do have gnats in the soil once I open it, what would be my options? This is a situation where I wouldn’t have them on the surface alone, but mixed in the entire thing. What would work best?
    Thank You

    • Getting some Bt is probably your best bet if the soil is pre-infested, as well as using proper drainage and watering techniques. If the soil isn’t pre-infested, treat using any of the remedies in the post! Good luck!

    • Hi Alina; I am one of the 90% who have pre-infested bags of Miracle Grow sils (not so nice a surprise last weekend when I opened the bags!). Were you able to kill them all off in the bag? If so, what worked for you. I’m at a standstill nright now – I think my gnats like dry conditions as well as wet!!! :(. Thanks.

  16. I have been battleing those darn fungus gnats for a week now! I have tried everything, I am on the potato slices and the cinn. Now with the cinn. is it okay to use organic cinn. sticks? I would think so. I dont know what to do I did stop my oregano from diein, but my pumpkin and squash seedlings are now there wilted. I have been puttin a fan on them during the day to help circulate the air. I also have a grow light on. The sticky traps that I do just keep feeding the flies rather then trap them.

  17. Thank you for this! This has been the best I’ve found in dealing with fungus gnats and fungi. I have a question re cinnamon though. You mentioned that only Ceylon Cinnamon works, but don’t both Ceylon and Cassia Cinnamon contain cinnamaldehyde which kills fungi? I’m concerned because I bought two bags of ground cinnamon, pretty sure they’re cassia.

    • Cinnamaldehyde is present in both types of cinnamon and it kills fungi in your soil, however cassia tends to attract ants. Anecdotally, many gardeners report that Ceylon cinnamon seems to work better too. But don’t despair; give your cinnamon a try and see how it works!

  18. The best article I found online for how to naturally deal with soil gnats. Thank you thank you thank you! I’ve tried three different methods and am positive that the little buggers ought to stop flying into my nose soon. :)

    • Glad to have helped! You need to catch them at each stage of their lifecycle and keep at it, since they reproduce so quickly. A combination of tactics works best! Good luck!

  19. what kind of plant?

    • Fungus gnats affect all plants because they are soil-dwelling organisms.

  20. I have a small potted highbrush blueberry plant and the soil is full of these worms and every now and then the flies are in the soil and on top of it. Also I believe they are in a potted Mint plant as well, but in the mint plant I have also found something similar to the worms but about 3 times as big around but the same length. Not maggots, so what could they be? Ugh, what a pain all these disgusting little creatures are. Thank you for this page & information collected, I hope to try out a few of these ideas & hope it will do the trick. Thank you thank you.

    Gregor
    Medford, Oregon

    • It’s really hard to say without seeing them, but it sounds like fungus gnats, especially if the larvae are only about 1/8-inch long or less. A good Bt soak (mentioned in the article) could help with both plants however.

  21. Thank you so much for your detailed information, which explains fungus gnat control better than any other site I’ve found. I have a question about using Bt in my 2-tray stacked worm bins. Many worm bin sites recommend Bt, but they don’t explain its use in enough detail. I used Gnatrol (3 t. per gallon) and soaked both levels very thoroughly, then let the excess solution drain off. I have read that Bt should be applied a second time 4 days after the first application, but this will no doubt make my worm bin bedding very soggy (even with draining). Do you if this is the way worm bins are typically treated with Bt? I don’t want to risk drowning the worms. Having multi-level bins is tricky because the gnats have so many entry points. Thank you.

    • Fungus gnats are composting organisms, so it is very natural to find them in a compost bin of any type. Their presence there is more of a nuisance than anything harmful. Since fungus gnats prefer to lay their eggs in the top half inch of moist soil/compost, you should completely cover the compost at the top of your trays with a few sheets of moist newspaper to create a barrier to the adults who want to lay eggs in the compost. Consider soaking the newspaper in BT before placing it. That way you don’t make your bins too wet for the worms, but you still get the BT on the surface of the soil. Be sure to also use yellow sticky traps simultaneously to catch adults at the top. Good luck! Getting fungus gnats out of worm bins can be very tough since it is a natural habitat for them.

  22. Hi Dawn,
    I love love love your blog and I have a question for you. Will diatomaceous earth get rid of millipedes in a house plant or is there another natural solution that you know of? I am about to dump a beautiful dracaena because of them.

    Thank you so much!
    -Jennifer

    • Thanks so much! DE should work well to help eliminate millipedes. They prefer cool, moist areas with decaying plant matter in them, so eliminating those areas from around your home will help too. Millipedes cause no harm, but they are a nuisance, and they smell when they squish or die, so if you can remove the plant to a warm dry area outside for a few days, and pull any mulch or other decaying plant matter away from your house, they may just go away without much hassle.

  23. Wow! This is great information. Thank you so much for sharing at Rural Thursdays!

  24. I am so impressed, this has to be the most comprehensive article I’ve read on any one plant pest- a wealth of information and an excellent resource. Thank you so much for sharing this with us on Seasonal Celebration. have a great weekend! Rebecca @ Natural Mothers Network x

    • Thanks a lot! That’s a high complement to pay a research nerd like me! You made my day!

  25. Wow, this is a lot of helpful information! Thanks!

  26. Fantastic article! Thanks a lot, will definately be taking your advice just what we need for our little seedlings!
    Sarah

  27. Fascinating! I’ve got two little garden helpers this year and am afraid we’ve overwatered several times because it’s just “so fun!” I’d already cut back, but thanks for the additional push to do so!

    • I keep a designated area of the garden just for my 4-year old’s “experiments.” She is free to water there as much as she would like. It has become really more of a place to make mud pies than grow anything, but she’s happy, and the garden isn’t overwatered!

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