Ah, fungus gnats! If you have indoor plants, chances are you’re familiar with these horrible little flying demons from hell. I have been battling fungus gnats for a few months now, and I’m happy to say, I’m winning the war at last! I’ve spent hours online research ways to beat the little buggers, have tried quite a few different approaches and now it’s time to share what I’ve learned!
OK, so what the hell even are fungus gnats? Well, they’re a teensy little fly-like critter, that looks very similar to a fruit fly. If you want to get all sciencey, they’re generally of the family Sciaridae or one of a few others - there are a few different types - and they ALL SUCK! The adults are tiny - only a few mm long, and they’re pretty rubbish at flying, so tend not to wander too far from the delicious, damp pot of soily goodness in which they were hatched.
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Fungus Gnat Life Cycle
They have a pretty rapid life cycle - the eggs are laid in the top 2 - 3 cm of the soil. When the larvae hatch out they like to wriggle around and eat stuff in your soil - which can, unfortunately, include fine roots of your beloved plants or seedlings. Once they’re stuffed and ready, they do a pupa thing, just like a caterpillar, and eventually another evil fungus gnat adult hatches out from that, ready to annoy the living crap out of you.
What’s The Harm?
Well, chances are they might well not do your plants any harm whatsoever. The only real physical danger is the larvae nibbling on delicate root systems, but with a more established plant that’s not much of a risk. The real harm is the sheer annoyance and ick factor of hundreds of tiny buzzing critters all over your plants. I’ve also found that they seem to have a taste for wine, too, and regularly find them in my wine glass. That alone is reason to declare war, in my humble opinion!
Where Do They Come From?
The come from a special section of Hell. Well, no, not really, as far as I know. We generally unknowingly bring them into our homes in contaminated soil - either a plant you’ve purchased, or potting mix or similar. They also spread incredibly quickly, let me tell you! One way to try and avoid letting them in like this is to quarantine any new plants for 2 weeks well away from any other plants, that way you can see if you have an infestation or not. Keep potting mix in well sealed containers, too.
They seem to often show up in late autumn or early winter. That's no surprise, seeing as though that's when many of us bring our plants indoors, and the cooler temperatures often mean that soil stays damp for longer.
What Do They Like?
Damp soil. The love it. They literally wallow in and eat the stuff. This can be, however, a weapon you can use. The dryer you keep the top few cm of soil in your plant pots, the less likely you are to be invaded by fungus gnats. This is also a generally good rule of thumb for houseplants in general - let most pots really dry out in between waterings. Of course, there are exceptions to this, but if you do you research on each plant you bring home, you’ll soon learn what they like.
Getting Rid of Fungus Gnats
OK, you’re convinced they’re bad now, so let’s look at ways to battle a fungus gnat invasion.
Neem Oil - This is an organic product made from the seeds of the Neem plant, and is effective in battling quite a range of houseplant pests. It’s also recommended for fungus problems, and can be sprayed on both leaves and soil. You’ll need to repeat the spraying a few times, probably, but it has reasonably positive results. I’ll be honest - on its own it didn’t totally get rid of all of them, but I definitely noticed a fairly dramatic reduction in numbers. I’d say this one is best used along with other methods. If you want to try it you can buy it HERE.
Diatomaceous Earth - This is a powder made up from the ground up fossils of diatoms (a kind of algae), and it’s totally organic and even sometimes food-grade safe, which is awesome. The powder consists of very tiny, very sharp bits, so it works by damaging the structure of the larvae, so they get all dead, which is how we like them. You basically just sprinkle this liberally on top of the soil in your pots, dig it in gently a few cm and let it do its thing. If you are using on edible plants, please do make sure to purchase one that’s food-grade. I can’t actually give you my personal review on this yet, as I can’t find any near where I live, so have ordered some from Amazon which should be arriving any day - yay! Nearly every single one of hundreds of articles I read on eradicating fungus gnats highly recommended use of diatomaceous earth, so I’m very confident in recommending it here.
Sticky Traps - oh, these are quite satisfying to use, even though they’re not enough on their own to get rid of an infestation. They’re bright yellow sticky strips (apparently the insects just love a bit of bright yellow) that you can either hang near your pots, lay small strips right down flat on the soil, or use a popsicle stick to stick little flags of sticky doom in your pots. Again, they’re non-toxic, so safe around all plants, pets etc., and aren’t the type of sticky that ruins your skin and clothes. These will, of course, only attract the adults, but every bit helps. I particularly loved using these, as there was visible progress of Tara Vs The FGs! Just change them every couple of days as needed. You can get them pretty cheaply on Amazon.
Beneficial Nematodes - I think this should be a band name, honestly, it’s just so utterly delicious to say out loud - just rolls off the tongue. Go on say it out loud, you know you want to! These guys are awesome. They’re microscopic little wormy things that you buy alive, add to your plants, and they burrow in and eat up a huge range of damaging insects. I so desperately want to try these myself, but where I live (Western Australia) we have really strict quarantine laws and I haven’t managed to find any locally yet. Therefore, I’m going by all of the hugely positive recommendations I’ve read online. This is a really natural approach to pest control, which I really like - it’s bugs eating bugs, as they were meant to do. I also think it’s fabulous that they will munch on such a huge range of pests. If you're in US, you can get them here.
Hydrogen Peroxide - This is one I read a lot about, but didn’t have terribly good luck with myself. That said, using the recommended mixture for the number of plants I have, it would also have worked out incredibly expensive, so I treated a few as a test, and left it there. It did seem to dramatically reduce the number of fungus gnats in the few pots I tested, but I think it would have needed follow up treatments to work completely. If you only have a couple of plants, I’d definitely recommend you give this a go before buying anything more expensive.
You basically just mix 1 part 3% hydrogen peroxide with 4 parts water. Let your plant soil dry out quite a bit, then soak the soil with this mixture. The larvae are, apparently killed on contact, and the mixture does absolutely no harm to your plant. In fact, I’ve read some articles that say it can benefit the roots. If you do try this out, I’d love to hear how you went. As I said, I would have needed many, many litres of peroxide to treat mine, so only did a few.
Mosquito Bits - As the name implies, these are generally used to control mosquitos, but I have it under very good authority that they are equally effective in getting rid of fungus gnats. You can just use them as a dressing on top of the soil, or dig them into the soil, or even premix with your potting mix as a preventative.
Vinegar Traps - these are quite similar to the sticky yellow traps, in that they attract the adults. Just mix a few tbsp apple cider vinegar with a few drops of dish soap and put in shallow bowls or jars near your plants. The fungus gnats are attracted to it, hop in and hopefully drown. Again, this is only effective against adults, so best used together with other methods. I’m finding it quite good, but not quite as effective as the sticky yellow flags.
Full re-pot - Yes, it’s drastic, but effective! If you can’t kill them, throw them away! Remove your plant from its current pot and really rinse off the roots so you wash away any eggs or larvae, then re-pot in fresh potting mix. Do NOT reuse your infested soil, obviously, or you’ll just have totally wasted your time. I have read that you can bake it in the oven, or put it in the sun sealed in a black plastic bag to basically cook and sterilise, but I personally wouldn’t risk it.
I’m sure there are other treatments out there, I know I’ve heard of a bacterial one, but haven’t tried it myself, but that should be enough options to get you started in eradicating the evil fungus gnat from your precious houseplants.
I would like you to remember one thing above all else, though:
The best way to combat fungus gnats is to keep your soil (especially the top) as dry as possible and do NOT over water your plants!
Constant drizzle watering isn’t the best anyway. It’s nearly always better to let your soil really dry out, then take your plant to the sink, tub or outside and give it a full soak until the water is really coming out of the bottom of the pot. Let it drain fully before putting it back where it was - do not let your plants sit in water filled trays and containers! That said, there are always exceptions, so do your homework and learn how much water your particular plant needs, and get to know its thirsty signs!
