Fungus Gnats In Seed Starting: What They Are & How To Get Rid Of Them

If you start seeds indoors, you will deal with fungus gnats. Let me show you what they are, why they show up, and how you can stop them. Welcome back to the Hobby Homestead. If you're new here, my name is Amy and I am gardening and homesteading in the suburbs in Ontario, Canada in Zone 6A. And one of the things that happens to me almost every year when I'm starting seeds inside is an infestation of fungus gnats. And I'm here to tell you that there is a way to combat them. Actually, a lot of ways that all need to be used together. So I'm a researcher by profession as well as by personality. I have a PhD. I've always been obsessed with researching things, getting into the nitty-gritty, and that has paid off as a gardener because whenever I have a question, something I don't understand, something that's new to me, I just dive deep into the research and also do a lot of trial and error learning. And then I get to the bottom of things. And while my usual YouTube style is just to take you along for the ride, show you my experience as I'm doing things, today's video is a little bit different in that I was so fed up with fungus gnats I did a deep dive into the research on fungus gnats. And therefore this video is going to be a very informed video, much more of a sit down and tell you what I have read kind of a video. But also I'm going to share with you all of the tactics that I use, which ones have been successful, which ones have been less successful, and which ones I think you should be using together in concert to have the best chance of actually getting rid of fungus gnats in your plants. First I'm going to tell you why they are a problem for seed starting indoors because you're not going to care about how to fix them if you don't understand why they're a problem. It's actually their larva that causes the issue for our delicate little seedlings. While the adult fungus gnats that fly around can be a little bit annoying, they might flutter in your face if you're sitting near your plants, they will lay their eggs in the soil and then the eggs hatch into larvae and the larvae eat the plant roots. Now usually if we're talking something like a house plant or outdoor plants that are established, established plants don't have a problem putting up with a little bit of nibbling by fungus gnat larvae, and fungus gnat larvae tend to go for the most delicate little roots and also just fungi and molds and things inside your seed starting soil. So while established plants may be totally fine, fungus gnats are a real problem when you're starting seeds and you have delicate little baby plants and they don't have huge root systems yet. If you have a seed that has only just germinated and it has a tiny little root and fungus gnat larvae are in there eating it, that's enough to kill your seedling and if not kill it then at least set it back, stress it out so that you don't have healthy enough seedlings to do well outside. Another issue with fungus gnats is that they can actually carry a disease and transfer it to your plant and this is a disease that causes damping off. If you have heard of damping off or if you've experienced damping off you will know how frustrating this is. Once the seedling has got damping off it will just be gone in like 24 hours. You'll have healthy looking seedlings and then the next day you check on them and they're wilted down to nothing and fungus gnats can carry the disease that causes this issue. I've even had fungus gnats be a problem for my indoor mushroom growing. I mean it makes sense they like to eat fungi decaying material and my mushroom grow kits are always kept very moist. That's the way to grow mushrooms. You want to either spray them with water all day long or I keep mine in a little tent with a humidifier so it's very moist and fungus gnats love that kind of environment and I in recent years have had multiple harvests ruined because the mushrooms were full of fungus gnat larvae and they'll even eat the growing mycelium which can mean you don't even get a harvest out of a mushroom grow bag. So they are annoying and they cause real havoc when you are growing food indoors. Now let's talk about what fungus gnats are. Fungus gnats belong to the family Diptera and they can actually be a few different species. They're about three millimeters long that is one-tenth of an inch and they do look like fruit flies from a distance especially if you're not familiar. They are actually related. Fruit flies are also from the order of Diptera. Fruit flies tend to be a little chunkier and they have shorter legs and they have these bright red eyes whereas fungus gnats have more slender mosquito-like bodies with longer legs. They are gray black in color and the other difference is where you will find them. Primarily fruit flies will be around your fruit bowl or around your compost heap or around other like sweet fermenting sugary things in your kitchen. Fungus gnats will primarily be on soil so if you have house plants or if you have seeds that you're starting indoors you'll find the fungus gnats fluttering very close to the soil level. Fungus gnats apparently don't like to fly very much or they're not strong flyers so while they will fly around if disturbed for the most part you'll find them right there on the soil or right around the plants. Fruit flies fly around a lot more than that. Now both of these insects fruit flies and fungus gnats have also been confused for drain flies just because both of these types of flies can also be found around your drains. They like decaying matter, they like fermenting things and so if you have yucky drains they can go down in there too and then you'll find either of those insects coming out of your drains. But the best way to know if you have fungus gnats is to notice if they are on your plants and around your soil. That's the telltale sign. Also they're tiny little bodies very delicate with long legs. Now they don't live very long which might sound like a good thing but really what it means is that they have a really fast turnaround in their life cycle. So the adults live about four days they've laid their eggs in that time up to 200 eggs each. A terrifying thought when I think about how many fungus gnats I have fluttering around right now. So the eggs will hatch within another four days and then they become the larva and this very destructive larval phase can last 10 to 14 days before they then pupate and then become adult flies again. Some sources I have found have said that fungus gnats really prefer growing mediums that are high in peat or that use peat moss. I haven't used peat moss in many years and I still have a ton of fungus gnats every year that come back to haunt me. So I am not sure that's necessarily true. I think sure they may love peat but they'll eat any any growing material that is soil based or has organic matter in it. Now let's talk about how to control them. First I'm going to tell you how the professionals do it because this is like the way to eradicate fungus gnats but it isn't a way that home gardeners really should try or really could try because these products are not readily available unless you're growing on a commercial scale. But I did find that there is a pesticide or an insecticide that will kill the larva right away and its effects last one to two months. So one application and you're good basically for at least half of your seed starting season. The problem is this pesticide or insecticide is very toxic to aquatic invertebrates and much of what we spray in horticultural or agricultural applications makes its way into our waterways as runoff. So I definitely don't want to promote that insecticide here I'm not even going to name it also because I don't remember what it's called but there is something out there that can really wipe them out. So if you're a commercial grower you may need that. There's also a species of Bacillus thuringiensis Bt that works against fungus gnat larva. Bt is a bacterium that lives in the soil there are a lot of different species of it and if you are a gardener you may have heard of it as something that you would use against the cabbage looper. Bt is something you would spray on the foliage of your brassicas your kales and your cabbages and that kind of thing and then when those cabbage loopers the green caterpillars eat it they shrivel up and die. The bacterium I think creates a toxin that kills them. So this is an otherwise harmless and safe insecticide. It's a naturally occurring bacterium. Its use is a little debated when you're talking about really gardening in harmony with nature because usually a species that would attack one type of caterpillar would actually kill all types of caterpillars. So if it gets sprayed in the wrong places it could end up killing beneficial insects. But it is useful in applications where you have just a ton of brassicas all protected in one space and you have an issue with those those little cabbage white butterflies fluttering around laying their eggs and then their caterpillars will decimate brassica crops if their population gets out of hand. So that is when Bt is commonly used in horticultural practices or agriculture but there is another species that has been found to kill fungus gnat larvae but apparently it's not easy to get on the open market. This is again something that's really only available to commercial growers at least here in Canada. Now let's talk about how the home gardener or the home seed starter can manage fungus gnats. I'm going to break this down into several categories or several tactics and then we'll wrap up at the end with my recommendations on how you use these tactics. So the first one is for control of the adult flies and that is oh no stuck it to my leeks oh no and that is these yellow sticky traps you can see how super gross that is covered in adult fungus gnats and also insecticidal soap which is something that you can spray on your plants and if it comes in contact with the adult fungus gnats then this will coat them and cause them to die. These are not solutions in and of themselves in my experience. I find the yellow sticky traps very very useful as an indicator of a problem so I recommend using them actually even if you don't think you have a fungus gnat problem just stick one or two around your little seedlings and wait and see and if things get stuck in them that look like tiny little fruit flies then you have fungus gnats and then you can move on to next steps to deal with them. The insecticidal soap I find a little bit tricky to use because if you don't get your ratios just right I buy it in a concentrated form and then I dilute it myself and then I've had instances where I didn't dilute it enough and it burned the leaves on my seedlings and I actually lost some seedlings that way and so it did not help me. It does work if you get the concentration just right or if you buy some at the store that's already mixed you can make it without buying a concentrate as well with just like dish soap and an oil or something like that but you also have to watch if you're spraying an oil on your leaves that the lights are not too bright where you've sprayed the oil or you can end up with leaf burn from that so it's just a little bit riskier unless you're going to use something that's like pre-mixed or you know what you're doing when you're mixing the concentration and so these are ways to kill the adults but as I've said I mean an adult can lay 200 eggs in their lifetime and even if you catch five fungus gnats on one of those sticky traps you may have another 20 that have gone uncaptured and they've laid 200 eggs each and so before you know it you've got a ton of larvae in your soil that you wouldn't even know about with a sticky trap and that can't be captured with a sticky trap and usually insecticidal soap doesn't get to them either because they are under the surface layer of the soil. So what we want to do is prevent the issue in the first place and my favorite way to do this is to sort of sterilize your seed starting soil with boiling water. I have done this in a lot of ways in the past I've tried the oven method but after some more research I learned that putting soil in an oven can cause it to release some toxic substances it's not good for us to breathe in also it really smells so I did that once and didn't like it. Boiling water works really really well so if you buy a seed starting mix or a potting soil whatever you use even if you're mixing your own with bulk ingredients you might want to consider this you just put all of your seed starting mix your soil into a big tub or a bucket with a lid and you're going to hydrate that anyway right normally you would pour water over it to hydrate it this time you pour boiling water pour as much as you can before it becomes too saturated give it a good stir and then get the lid on and let it continue to steam in there and that will kill a lot of if not all of the larvae that might already exist in that soil because let's face it the places where you buy your soil from are not immune to fungus gnats and a lot of these materials will have the larvae in them already so it's not your fault they just came in the stuff that you bought that's what I tell myself every year so that does work now this year I didn't do that because I had spent a little more money on a fancy compost because it was a living soil you know it's it's real compost it's made locally it's passively composted so there's tons of beneficial microbes in there and I thought why not just leave it au natural and see if it can balance itself well no it came with lots of fungus gnat larvae of course and I did not boil it and they went nuts immediately so boiling water over your potting mix is a great thing to do now you do have to make sure that you do it to all of your soil at the same time so the way I start seeds usually since I start them in January and then I start some more in early February and then mid-February and then early March mid-March late March I start at a bunch of different times in the year I'll usually just mix up a batch of my seed starting mix and then sterilize that and use that but then I leave my bulk ingredients or my bag of the rest of my soil sitting somewhere else while the larvae are still in there and they're going to hatch and they're going to fly around and reproduce and so boiling that one batch is not sufficient if you have other sources around your home for them to reproduce so you got to do it all which can be kind of hard but that's the only way really to get ahead of them by sterilizing your soil another tactic with the soil itself after you've planted your seeds is to let the surface layer of the soil dry out between waterings now I love bottom watering for this where you just water into the bottom of a tray and let it soak up from from the bottom of the pot and then the surface layer of the soil tends to stay drier fungus gnats are attracted to everything growing in that wet soil on the surface layer so they come down there and lay their eggs in that perfect perfect material now I do find it hard to keep the surface layer dry when I'm sowing seeds when you first sow things you know I have a humidity dome on because I want to keep all that soil around the seed moist at all times or humid and so it really only works later on in the season when I'm not sowing fresh seeds anymore but I have more established seedlings another thing you can do is sprinkle something like sand or fine vermiculite over the surface of all of your pots or soil blocks because then it's just not as hospitable an environment for them to lay their eggs on that surface layer but also for me that's something I find hard to do consistently and when I pot up I mix all that stuff into the soil and so it's something that it can work as one tool in the toolbox but it's just one tool keep that in mind that's not going to be sufficient on its own I did mention what some commercial growers will do in terms of insecticides or using bt but there is something that home gardeners have access to that can work as a tool in the toolbox and that is beneficial nematodes also the bacteria in mosquito dunks so these are things that you can apply to your soil either when you're mixing the soil together getting ready to pot up or to sow seeds or when you already have seedlings and pots you can apply it to that soil as well I have tried a new product this year that's called nema nights this was not inexpensive by any stretch of the mind but it was worth it for me because I had a fungus knot issue in my house plants before I even was ready to start seeds and I knew it was going to be a bad year for fungus knots and my seedlings if I didn't get a handle on those in my house plants it worked really really well for about a month I had like I had swarms flying up around one of my house plants and after applying this it's I didn't see them for a good two weeks but the way you use this is you're supposed to put a little serving of these little nematodes into each plant pot and then you water every day for three days and then in two weeks you do the same thing again and I think you have to do it every two weeks because they did end up coming back they were gone for that first you know after several days I didn't see any more and then I saw a couple after that two weeks not very many though and then when I reapplied I didn't see any more for another two weeks but lo and behold they're back so it does require frequent reapplication it's one of my tools in my toolbox that I'm going to continue using at least for this year because I have a whole thing of it and it really does stretch it goes a long way it's just annoying how you have to apply it it takes daily maintenance for a while so when I have little pots with my seedlings in them if I have like 24 pots on a tray I have to put some of these in every pot and then top water like pour water on top of the soil for these every day for three days and that goes against my whole bottom watering promotion here to keep the top layer soil dry and then I have to do it again two weeks later and then two weeks later and then two weeks later and so it's a lot of work it works but it's a lot of work and you really have to stay on top of it which is what's happened here I did not stay on top of it also you have to be consistent and every source of soil needs to be treated and I went ahead and started more seeds and forgot about these and so I had sources of soil laying around that didn't have any nematodes and then the fungus gnats will reproduce and move on to the next pot so consistency and continued maintenance are important with something like this now my friend Sarah from the Sunny Hill Homestead has always recommended mosquito dunks and I tried it for the first time last year and it worked for me but also it's something you have to be consistent with just like with these mosquito dunks are a heck of a lot cheaper than these fancy nematodes that are specifically for fungus gnat larvae you can purchase them in like any hardware store box store garden center here here in Canada you can get them at Canadian Tire and Home Hardware and like those popular places for gardening stuff at this time in the year and you just break off a chunk of one of these discs put it in a watering can let it sit for 24 hours and then use that to water your plants I have been doing that now for two weeks and I still am overrun with fungus gnats so I don't know if I'm not being consistent enough or if I have old mosquito dunks but it's a losing battle for me right now there are two other strategies tools in the toolbox that I have read about but haven't used so if you have used these please comment down below if it worked for you one is to water your soil with diluted hydrogen peroxide that one just scares me a lot because again I'm not the greatest at measuring and I've messed up my insecticidal soap dilution before so if I mess up hydrogen peroxide I'm just worried that concentrated hydrogen peroxide will be worse for my seedlings than the fungus gnat larva so I've been hesitant but if you've tried that please let me know also some people will say that cinnamon is a deterrent to the fungus gnats that they don't want to lay their eggs where there's cinnamon so you could try sprinkling some of that on your soil I haven't tried it it just seems like one of those old wives tales that might not be grounded in evidence I guess I could try to do a little trial but I just haven't tried that yet and then finally diatomaceous earth applied to your soil mix can apparently wipe out the fungus gnat larva as well or at least help to reduce them diatomaceous earth is basically just ground up crustaceans it's a fine fine fine powder you don't want to breathe it in but it is safe technically to have around your food crops just don't breathe it in it's abrasive and so it will abrade I don't know the word for that it will scratch up the surfaces of exoskeletons and I assume very delicate things like transparent slimy little larva and then it causes them to dehydrate and die so those are the actually three things that I have seen recommended and I just haven't tried myself so it's kind of funny I have this huge list of ways to combat fungus gnats and here I am recommending to you how to combat fungus gnats how to get rid of them and yet there are fungus gnats swarming behind me again every year that's because my purpose here is to inform you that it is never going to be one solution problem you are always going to need to use multiple tools in your fungus gnat toolbox and be consistent with them so here's what I'm going to change for next year based on my 20 years of dealing with fungus gnats not every year but a lot of years they come back and haunt me this is what I'm changing based on what I've learned I'm going to start by pre-treating all of my house plants before it's time to start seeds for the season I did that this year but I wasn't consistent it is so hard for me to be consistent with these things but I promised myself that next year I will start four weeks before my seed starting pre-treating all my house plants with a nematode or with mosquito dunks and I will keep that routine up to make sure I have eliminated them from the existing plants and soil in my home because that's where they come from if I'm sterilizing all my potting mix that's where they're coming from and then I also will be pre-mixing all of my seed starting soil into one giant bin not a little bit at a time I'll use a big tote bin that should be enough for this season and I will pre-mix it all and I am going to hit that sucker with boiling water to sterilize it and then I'm going to add nematodes I'm going to make sure that that soil has been very well pre-treated and I will make sure not to leave bags of soil sitting out like right there there's a bowl of soil that's just sitting out because I didn't need it the last time I was sowing seeds and it is full of fungus gnats so I've just left this big like welcome mat I've left a buffet sitting up there for the fungus gnats while I'm diligently treating each little pot on my shelves that's where they're reproducing right now so I'm going to be more consistent and more diligent next year I will also continue to use yellow sticky traps as an indicator it will let me know how many I have roughly if I catch a ton I know I have a greater ton and if I catch one or two I know I can get on top of the problem right away and I will continue watering with mosquito dunk infused water because I know my friend Sarah has a lot of success with it and I am sure my problem has always just been lack of consistency so my plan because my watering can is quite small and I put little chunks of mosquito dunk in and then fill it I need more than one watering can to fill to water all these plants so just like I plan to start using a jar a large bin for my soil so it's all treated at one time I plan to fill a bucket with water add a mosquito dunk let it sit and that's what I will fill my watering can with and just keep that keep that bucket treated with the mosquito dunks if you've had issues with fungus gnats in the past and you have found ways to combat them please write them in the comments below it's really great to lean on community in times like these when we feel very overwhelmed and frustrated with how things are going in our seed starting endeavors and there will always be things like that even if you are a very seasoned seed starter things can always go wrong especially if you lack the ability to be very consistent and reliable with your practices like I am sometimes guilty of but the bottom line here is that these are proven tools in your seed starting toolbox and the trick is to use them consistently and together I told you what I will be doing for the rest of this year and next year and that is what I recommend for others as well to combine those tools particularly sterilizing your soil if you are worried about it containing the larvae and making sure that you don't already have a problem in your houseplants that you may not have noticed because those houseplants are established enough to withstand the nibbling of the larvae so get up close with your houseplants use a sticky trap see if you have any and then eradicate that problem first before you go seed starting I always recommend bottom watering as a gold standard but I recognize it's not always possible and so just lean toward bottom watering when you can to let the surface level of the soil dry out it is scary to let soil look dry when you're starting seeds but it's actually better than keeping it fully saturated all the time and then finally if those tactics together are not enough there are some predators of fungus gnat larvae that you can try like a nematode or mosquito dunks that you apply by watering in use those things together use them strategically in the right order as you prepare for the season ahead of seed starting and you should be able to eradicate or at least keep at bay the problem of fungus gnats in your seed starting mix thanks for watching and I hope that you'll come and grow with us