When I started collecting houseplants, I knew very little about how to care for them. In fact, I knew very few of their real names, besides the pet names I’d taken to calling them (no judgements please). As my obsession began to realise, and my collection took over my room, I began to notice something quite irritating...
Every now and then, I’d find myself swatting flies away from my face. At first it was every few days, but within weeks it became a daily pastime. I eventually noticed the flies hovering near some of my plants, and - much to my horror - that the flies were burrowing in the soil 🤢 As I do any time I find myself bewildered, I turned to the internet to uncover what the hell was going on.
Fungus gnats. They had a name.
Where do they come from?
Fungus gnats are commonly found outside, so how do they end up in your indoor plants? The most common culprit is compost. Cheap potting mixes come with the risk of gnats, who are sealed into the bag before it’s opened on repotting day. As I have said many times before, I highly recommend Soil Ninja for your soil needs. I have never found gnats in their soil.
I have also found supermarket plants to be a hotbed of gnats, often because staff aren’t trained to take care of them. I want to stress that not all supermarket plants have fungus gnats, but I would take extra care when buying them and, if possible, quarantine the plant away from others until it’s given the all clear.
Those with the luxury of a garden or balcony may choose to relocate their indoor plants outside over the warmer months. Unfortunately, pests can follow them inside when autumn rolls around, including fungus gnats. For that reason, it’s always best to thoroughly inspect your plants before bringing them back in.
Will fungus gnats damage my plants?
Fungus gnats are technically harmless to most plants. The only plants that they can meaningfully affect are seedlings or very young plants, whose roots make an appealing snack for their larvae. They are, however, in my humble opinion, the most annoying pest of any. My war with them was long, hard and extremely frustrating.
Identifying the problem
The first step in the battle against fungus gnats is to figure out if you have them. Flies are pretty common in the summer, so make sure you’re battling the right kind first.
Fungus gnats will usually be hanging out near your plants, so that’s the first thing to look for. If you disturb the soil, either by watering or tilling the surface, they’ll probably disperse, often by flying straight into your face 🙃 They’re naturally attracted to carbon dioxide, which is why your vulnerable nose holes seem so appealing when they’re hovering nearby.
In terms of appearance, they are smaller than your average fly (3-4mm long) and are brown-black. They are not particularly adept at flying, so will fly in an irritatingly, erratic way (again, usually towards your mouth as you are yawning).
The methods that don’t work
I have tried many tactics in my war. Most failed. At least I tried them so you don’t have to, right? 🤷♂️ Here are some of the methods that didn’t work:
Cinnamon
The thought behind using cinnamon is that its antifungal properties will prevent the fungus the gnat larvae eats from growing. Unfortunately, the gnats in my home were not phased at all by cinnamon. Perhaps mixing it into the potting soil when potting up would work, but by the time the gnats are present it seems to do little.
Vinegar/wine traps
You will often see people recommend a combination of wine, vinegar and dish soap to first attract the flies and then drown them (brutal, right?). After a week of using multiple little ‘traps’, I found they had caught less than a dozen flies.
Neem oil
Using a neem oil spray seemed to have very little effect on the flies. I have heard people use it as a soil drench, pouring diluted oil into the soil, however I’m not convinced this will eradicate both the adults and the larvae. Neem does have its benefits for indoor plants, however it seems as though this is one issue it can’t fix.
Diatomaceous earth
This fine powder will apparently eviscerate fungus gnats and their larvae on contact. This is all well and good if you can allow your plants to dry out, and stay dried out, for at least a couple of weeks. The problem being that when you water (even from the bottom) this powder turns into a gloopy substance and loses its gnat-shredding abilities instantly. This is both a mess and extremely inconvenient - as your hard work will likely have reduced the population without destroying it entirely.
Repotting
If you have one or two plants, repotting them may work for you. You would need to remove all the soil, including any stuck to the root ball, for it to work properly. With such a large collection I found that repotting did very little, as the gnats had infiltrated too many.
How to get rid of fungus gnats
The key to fungus gnat warfare is destroying their lifecycle. Unless you tackle both adults and their larvae, you will never curtail their population. These are the methods I recommend:
Nematodes
Nematodes are essentially your own personal army, designed to hunt down and annihilate fungus gnat larvae in the soil. Sounds fun, right? You simply water them into the soil and let them do the work for you; easy. I cannot recommend them enough and I use them regularly at the nursery whenever a fungus gnat is spotted. There are many websites where you can buy them directly, often labelled as ‘sciarid fly nematodes’ (they are the same thing).
Sundews
When you say ‘carnivorous plant’ most people think of a Venus Fly Trap. I employed a Venus Fly Trap to ‘help’ battle fungus gnats. Did it help? Not at all. In fact, just to gloat, the fungus gnats made a home in my fly trap’s soil 👎
Sundews aka drosera, on the other hand, are adept at fungus gnat warfare. Their sticky tendrils seem oh so attractive to the flies, who land on them before getting stuck and being eaten - creating a graveyard of flies. Fascinating and gross at the same time? Yes. Effective? Absolutely. I have read that butterworts can work well, too, but I haven’t used them personally.
Sticky traps
If carnivorous plants aren’t your cup of tea, then sticky traps are also effective at catching adult gnats. Set them up near your worst-affected plants and you’ll soon have collected a battlefield’s worth of flies. They are far less interesting than sundews, in my opinion, but they do the job just as well and require less attention.
Fungus gnat prevention
You will find that many articles tell you to simply ‘stop overwatering’, and while there is some truth in this, it’s not black and white. It is true that the majority of plants should dry out at least an inch or two between waterings, but it is also true that some species of plant will hate you if you let them dry out too much e.g. ferns or Calathea.
Where possible, ensure you are letting your plants dry out so that the top soil doesn’t stay wet for too long. A high-quality, well-draining soil with perlite, pumice or bark will help a lot with this. On that topic, ensure leftover potting soil is sealed after use in an airtight container. You should thoroughly check over any soil or plants you are bringing into your home, so that gnats can be identified and dealt with before they spread.
In conclusion
Nematodes are an absolute game-changer, and are what finally won the war for me. Used in combination with sundews, I eliminated the gnats entirely. It took a few weeks and a couple of applications of nematodes, but I can safely say that swatting flies away is just a distant memory 😌
Do you have any other tried and tested methods for tackling fungus gnats? Let me know!
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