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Ah, fungus gnats, the bane of all houseplant owners, (and my boyfriend, who’s plagued by them, and is pretty uninterested in the plants).
Why do they inevitably turn up and bug you whilst you’re trying to drink a casual glass of wine and watch Gardener’s World?
Fungus gnats are attracted by rotting organic matter and love the moist* soil inhabited by your houseplants. They’re also rather partial to light sources, and carbon dioxide (who isn’t?).
But don’t worry, I googled it and there’s stuff we can do. But first, let’s get back to basics…
What are fungus gnats?
Well, for a start, they’re annoying little twats.
I’d give you the scientific name, but there are actually six or seven different kinds, with loads of different variants – but they’re all small, black, and feed on fungi in soil.
The one in the picture above is Diptera Mycetophilidae, but since there are about 3000 species in that genera, it's not really narrowing things down.
I do recommend that you watch them traipsing around on the soil of your houseplant because considering they’re literally called flies, they’re crap at flying.
Do fungus gnats damage plants?
Not to any real degree.
The actual adult flies don’t – they’re just annoying as hell.
However, larvae can.
As I mentioned before, the primary diet of fungus gnats is *drumroll* er, fungus. But if the larva is especially hungry, or there's a lot and they've exhausted their supply of fungus, then they'll turn to the next best food source - the roots of your plants.
This is so unlikely to be an issue. If the flies were at levels they could significantly damage your plants, they’d be so thick in the air you couldn’t live there.
Will fungus gnats harm me (or my pets/kids)
It really depends on what you mean by harm.
Physically, no. Fungus gnats don’t/can’t bite, but by Christ, they can be annoying.
They like to get right up in your face or fall in your wine or get between your mouth and the next bite of your sandwich (they don’t tell you what to do about flies flying in your mouth in vegan school).
Their desire for CO2 makes our mouths and noses (and fizzy wine) absolutely irresistible, so try not to take it too personally – you’re not attracting flies due to an imperfect showering regimen, you’re just breathing.
Gnats won’t hurt your pets or kids though (depending on how you consider your plants, I suppose), though if your dog is bothered by gnats you can make citronella sprays to repel them.
How do I prevent fungus gnats?
Well, from what I’ve read I assume that to stop them from swarming around your face you need to stop breathing.
Good luck with that!
Bear in mind that fungus gnats are extremely common.
Their life cycle is also such that you could easily bring them into your home on a new plant and have no idea – it takes 17 for them to mature from egg to flying fly, so you could be bringing in eggs and larvae without realising.
If finding an alternative to breathing is a bit extreme for you then you could first review your watering routine.
Overwatering is one of the biggest issues when it comes to houseplants, and can cause a plethora of problems, fungus gnats being one of them. Make sure that the top of the soil is completely dry before you water.
If you’re a serial over-waterer, get yourself a cheap moisture probe. If you just can’t stop, I have a post on how to stop overwatering your plants here.
Try bottom watering
I have a whole article on bottom watering here, but the idea is that you sit your plant pot in a dish of water, and allow the capillary action of the soil to wick water up through the plant.
Then remove the plant before the top has a chance to get wet.
This won’t harm the plant – there’s plenty of water in the soil – but it makes the soil WAY less hospitable for the fungus gnats.
Remove all decaying matter from the top of the soil.
For some reason, I used to think that if I left the dead leaves on the surface of the soil it would act as a kind of fertiliser; break down and feed the plant.
Maybe it does, but it also attracts fungus, and therefore gnats, like a boss.
Put sand on top of the soil.
Just a thin (maybe a centimetre or so) layer will really help with keeping the surface of your soil dry.
This is on my to-do list – so far I’ve only tried it on my cactuses.
Diatomaceous earth works too. It’s fossilised microalgae which can absorb four times its own weight in water and instantly dehydrates gnats and larvae in contact.
It also provides a layer on top of your plants which would require the larvae to move through the equivalent of broken glass. Nice.
It only works when it’s dry though, so works best in conjunction with bottom watering.
Try not to bring them into the house
Easier said than done, but check for flying adults, or adults crawling around on the soil. If you really wanted to, you could bring a magnifying glass and check for the teeny white larva.
How do I get rid of fungus gnats?
- Stop. Overwatering. Your. Plants.
- If you’ve potted your indoor plant in compost repot it using an indoor potting mix. I make my own potting mix because it’s super free draining, and is peat-free.
- Put a layer of steel wool on the surface of the soil.
- Buy some beneficial nematodes. They’ll eat the larva. You can buy them online. Check out Betsy Begonia’s video on them here.
- Yellow sticky traps
- Neem oil
- Put a slice of potato on top of your plants. Chuck it every few days – apparently, it attracts the larva so when you chuck the potato, you’re chucking the larva too. I haven’t tested this, but let me know if you have!
- Systemic granules, and other pesticides like Mosquito bits.
Alternative methods of getting rid of fungus gnats?
You can get all kinds of traps and sticky things to get rid of fungus gnats but a lot of people don’t like them.
If I were to use a trap I'd need one that killed the gnat instantly - the sight of them flapping around on the traps is heartbreaking.
A more natural way to reduce fungus gnat numbers is to get carnivorous plants that eat them, like this sundew:
I got a carnivorous plant, specifically, a Sundew plant.
He’s effective. Like, super effective.
One thing to note about Sundews is that they will not tolerate tap water, so I water mine with water that I harvest from the sky using a highly technical rainwater collection system comprising three buckets left outside.
They’re actually pretty picky, but I’ve found that keeping damp moss in the cache pot does a good job of keeping him moist but not waterlogged.
- Stop overwatering your plants.
- I mean it.
- It’ll kill ’em and bring in fungus gnats
- Maybe try bottom watering?
- Clean up decaying leaves
- Put sand or diatomaceous earth in your pots
- Buy sundews
*Apologies for those of you that don’t like it, but I’m going to be saying moist A LOT in this article.