So you've got fungus gnats 🪰

Tips and tricks to get rid of these pesky little flies

Ever see small black flies walking on the rim of your plant container? See them flying or crawling around the plant or the soil? Or, have small black flies hovering in front of your face, crawl across your book or drown themselves in your tea?

If so, you’ve got fungus gnats.

But, rest assured: while they are incredibly annoying and seem to multiply at the rate of fruit flies, most plant resources suggest that fungus gnats are harmless.

Winter is the ideal season for fungus gnats and having over 120 houseplants in my home also means that they recur pretty much every winter.

In this post, I’ll share how I deal with them, and the many remedies that my favourite plant resources suggest. Special thanks to

Tom Pendergast

for suggesting a post about fungus gnats!

What fungus gnats are

Fungus gnats are small black flies that are actually pretty bad at flying. They prefer to walk or hover. They are attracted to the moist soil of your houseplants—one of the key tips that people often give is therefore to let your plants dry out properly before watering them again. While this is an option for some houseplants, moisture-loving houseplants require moist soil at all times, so drying them out is not an option. Fungus gnats lay their eggs in the top layer of moist soil (1-2 inches, or 2 to 5 centimetres, generally).

Most of the plant resources that I consulted suggest that while annoying, fungus gnats don’t do actual damage to plants. My favourite Dutch houseplant author though, writes that “the larvae that hatch from the eggs may gnaw at the roots of your plants and damage them”.

As always, if your plant is struggling, it’s a good idea to explore what is wrong.

Thread: houseplant first aid 🏥

Personally, though I’ve had my fair share of experience with fungus gnats, I’ve never seen my plants being affected by them in any way (i.e., no brown leaves, mysterious wilting, fungus, etc.).

How to get rid of fungus gnats

If you don’t have that many fungus gnats and/or you are not easily annoyed, there’s no reason you can’t just leave things as they are. Usually they’ll disappear as the weather warms up and the circumstances indoors are less inviting for fungus gnats.

If you do want to get rid of them, here are some options.

Yellow sticky insect traps

Stick one of these in your plant’s soil, and once the gnats walk across them they’ll get stuck and die. It looks gruesome, and part of me definitely feels bad to cause suffering of this kind. But these traps are effective in catching and killing the fungus gnats. They’re not effective, though, in making sure that the gnats haven’t already laid eggs. In the long run, then, especially if you have many plants and/or gnats, it doesn’t really fix the problem.

An insect trap with dead fungus gnats in my study

Keep your plant tidy

Dead or rotting leaves on the plant’s soil keep it moist, which is great for many garden plants, but not great if you’re trying to prevent moisture-loving gnats to set up shop in your house. When you’re doing your watering round, pick the dead leaves out of the pot.

Cinnamon and sand

Iris van Vliet, my favourite Dutch houseplant author, suggests sprinkling a layer of cinnamon on the plant’s soil, which keeps the gnats from laying eggs. Another remedy is replacing the first inch/2 centimetres of potting soil by sand, which dries more quickly than potting soil.

Neem oil

In Plant Society, Jason Chongue recommends removing the top 2-5 centimetres (1-2 inches) of soil from the pot, letting the remaining soil dry out and treating that with neem oil pesticide. He suggests doing multiple treatments, five days apart. I’ve not tried neem oil pesticide myself, but have heard great things about it.

Jason Chongue’s neem oil pesticide recipe:

4 cups of water

1 tsp dishwashing detergent

2 tsp neem oil

spray bottle or mister

Mix and apply to the plant and soil, continue until all traces of pests are removed.

Sticky cards and apple-cider vinegar

Lauren Camilleri and Sophia Kaplan of Plantopedia also recommend sticky cards (or insect traps) against fungus gnats. They also suggest a mixture of apple cider vinegar and dishwashing liquid (250ml/1 cup of vinegar and a few drops of dishwashing liquid) in a shallow dish to attract the gnats.

I use dishwashing liquid and water in a spray bottle in the garden to blast lice off young plants, and have used vinegar to kill fruit flies, so I can well imagine this working to attract the gnats as well. It doesn’t, of course, get rid of the eggs that the gnats may have laid before drowning in the liquid/dying stuck to the sticky card.

Are you enjoying this post and do you want to support my work? Buying me a ko-fi is the fastest way to my heart (as it allows me to buy more plants and books about plants). 💚

My remedies against fungus gnats

Carnivorous plants

Initially, my partner and I figured we’d go very natural in getting rid of the fungus gnats. We bought a handful of carnivorous plants, dotted them around them house and waited for them to devour the fungus gnats. While a great idea, it had a couple of drawbacks.

Like the sticky cards, this doesn’t get rid of the eggs in the soil. Most importantly, though, I found it pretty much impossible to keep the carnivorous plants alive for much of the year: they need so much light, that they only survived grouped together on a south-facing windowsill in summer. And ironically, once they started to die, fungus gnats happily started hanging out in the pots of these plants… Also: the number of fungus gnats was just too large for the carnivorous plants to deal with.

Nematodes

A few years go, an acquaintance suggested nematodes as a remedy against fungus gnats. At first I was more than a little grossed out at the idea of buying bugs and setting them free in my house, so to speak, but this has been the most successful solution so far.

I’ll admit, administrating the nematodes is a bit of a faff and requires a bit of planning—ideally, all plants need to be watered with the mixture, which means that I have to ensure that they are all dried out enough in order not to drown them.

Nematodes are tiny little worms—the ones most frequently used against fungus gnats are Steinernema feltiae—that eat the gnats’ eggs. Once the food supply runs out, the nematodes die.

I buy my nematodes from this Dutch online store, though a quick Google search reveals several UK and US suppliers as well.

Administrating the nematodes

The nematodes can’t be kept at home for a long time—in the fridge, the ones I buy last seven days. So before ordering, I made sure that I was taking a little longer between watering my plants so that they’d actually need water.

A handful of nematodes: they seem to come in a kind of sawdust like powder.

Depending on the specifications of the supplier, you dissolve the nematodes in water: the ones I got need to be dissolved in 20 litres of warm water (I took half for 2 times 10 litres). After much vigorous stirring, I watered my plants the way I’d regularly do.

Nematodes dissolved in water.

The nematodes do their thing, eat the eggs and the problem is solved. More or less at least: the procedure needs to be repeated twice in the case of the ones I bought, and the success rate for me and my plants personally has never been 100%. Still, the majority of fungus gnats disappear. I also combine the treatment with sticky cards to capture the adult gnats—just how full the cards get also gives me a good sense of how many gnats are still around.

Do you have fungus gnats hanging out in your plants’ soil? And do you do something about it, and if so, what? Do leave a comment and join the conversation.🌱

What I’ve been up to

Two weeks ago, I shared my tips for houseplant first aid, and invited you to ask your questions.

Over at Female Owned, I’ve added paid subscriptions (very exciting!) and wrote about 16 months off social media and how my business was affected.

Thank you for reading A Houseplant Journal! I love this hobby project, and have a whole list of topics that I want to explore in the Spring. Got an idea for a future post? Leave a comment and tell me :)