It’s highly likely that fungus gnats will make their home in your plant collection at some point. Anyone who’s ever encountered them will conclude that they are supremely annoying; therefore, they must be destroyed. These tiny black flies will often be seen lazily circling your house plants. You may also find their tiny white larvae in your potting mix. Usually they are content to live outdoors but will inevitably make their way indoors in search of greener pastures.

How to kill fungus gnats

Are fungus gnats killing my plants?

Yes! Fungus gnats feed on the roots of your indoor plants, as well as other types of organic matter found in potting mix. In most cases the damage they’re causing in the short term is fairly minor and the main issue is that they are very annoying. The presence of fungus gnats is also an indication that there might be issues with over-watering, drainage, or airflow.

How do I get rid of fungus gnats?

To easily get rid of gnat flies we want to reduce soil moisture, reduce humidity, and increase air flow. These environmental changes disrupt the gnat fly life cycle and remove possible habitat. I highly recommend starting with these options as they are free and easy. Once you’ve figured them out, they'll also help you to avoid future incursions. Continue reading below to find out how it’s done.

Let the soil get as dry as possible

Gnat flies lay their eggs in the top inch of soil and sometimes around the pot’s drainage holes. They need the soil to be moist for their larvae to survive and generally have a much easier time reproducing in boggy conditions.

The easiest thing we can do is to disrupt their habitat by watering less frequently, with the goal of allowing as much time as possible before re-wetting the soil. This might mean waiting two to four weeks in between waterings depending on the size of your plant. A few types of ferns or palms may not be able to tolerate this treatment, but generally everything else will be fine.

You’ve probably heard people mention they only water their Peace Lily when it wilts. This is the vibe we are going for. If you’re unsure of what your plant can tolerate, check out a guide online or on Youtube, or ask at your local nursery. Disregard the watering advice that came on the plant’s tag.

Air Flow

Gnat flies are very weak flyers. They love living inside where they can lazily cruise around with very little wind disturbance. If you can increase air movement they are going to have a much harder time flying around and reproducing. If you can’t open a window, try temporarily keeping any problem plants in a sheltered outdoor position. 

If neither of these options work for you, you can try using an oscillating electric fan. Use the lowest setting and set it up a few meters away from your plants so it’s gently moving the air around them.

More air flow will help to dry out your soil faster too!

Remove cover pots and pour out standing water

Make sure all trays and sources of standing water are totally empty and dry. This includes drains, sinks, and flower vases.

If you've kept your plant in its plastic pot and are using a cache (cover) pot, make sure to take it out of this decorative pot so that you can maximize air circulation.

Advanced Option

If the techniques above still haven’t yielded results after a few weeks you may need the assistance of a few products. These options are relatively cheap, and are great for anyone who is still struggling. You MUST use these products in conjunction with the free methods.

Sticky Traps

Sticky traps are a great way to kill mature gnat flies and help reduce larger infestation. They are cheap and each one lasts a long time. The bright yellow colour attracts pests and stops them from being able to lay more eggs. Keeping one tucked out of sight can be a great way to monitor for possible gnat populations and prevent infestation.

If you’d rather DIY it, try making your own vinegar traps. Into a cup of warm water mix a few tablespoons of apple cider vinegar, 1 teaspoon of sugar, and a few drops of dish soap. The gnats will be attracted to the mixture and will not be able to float on the surface due to the soap reducing the surface tension. If you can partially cover the top of the trap it’ll help prevent escape (you can use something like tape or plastic wrap).

Fungus Gnat Sticky Trap

Thank you anonymous customer for this truly inspiring photo!

Neem oil

The neem tree has a rich history of use throughout India as a medicinal plant. All types of indoor and outdoor gardening can also make use of the fungicidal, anti-bacterial, and insecticidal properties of neem oil. For the purpose of getting rid of fungus gnats we recommend using a neem oil concentrate.

The first method of deploying neem oil is to incorporate it into your watering regime. Simply follow the dilution guidelines on the back of the bottle. My favourite brand says to dilute 10ml of neem oil into 5L of water (roughly 0.3oz neem oil to a gallon of water). Water all your plants with the neem/water mixture. This will be absorbed and distributed throughout the whole plant. The next time the gnat fly larvae take a bite of your plant’s roots they will be ingesting the neem oil. Repeat this process three more times to catch any survivors. 

Do not increase the frequency of watering. Only apply the neem/water mixture when you would normally give your plants a drink to avoid root rot.

The second method of application is to use a spray bottle to apply the neem and water. Spray down the plant’s foliage with the solution, where it can be absorbed and will be distributed throughout the rest of the plant. The surface of the soil can be lightly sprayed down too. This will help reapply the neem without completely soaking the soil again. 

There are also dried neem granule products that can be sprinkled over soil to create a similar effect.

Neem oil is a very safe substance and is not toxic to people or animals, although it can repel some birds. If you’re concerned about using neem around your birds, the other methods in this article are still effective.

Improve soil drainage

We can avoid creating a habitat for gnat flies by enhancing our current soil or by exchanging it for something with the qualities we need. To help resist gnat flies we want a potting mix that will allow more airflow through the soil, reduce the water holding capacity, and to increase the speed that the soil dries out. 

There are a lot of options for substrates we can mix into our existing soil. A product like pumice, or perlite will work great. I personally prefer to mix in a chunky orchid bark as I find that it looks better. Orchid bark is very similar to regular potting mix but is just made up of larger pieces of mulched pine bark. It looks just like the bark chip used in playgrounds except that it's the same color as potting mix. It’s also much more environmentally sustainable than the other options.

There are several pre-mix potting mixes that work great for most indoor plants. A cacti succulent mix is a great option as this offers some bonus drainage. Even better would be an aroid mix (if you can find it.)

My preferred method is to equally mix one bag of premium potting mix and one bag of orchid bark. I’ll usually put a thin layer of the orchid bark on the top of the soil as it looks really great!

Adding rocks to the bottom of your pot provides no added drainage. None!

Mosquito Bits - Bacillus thuringiensis israelensis

The strain of bacteria, Bacillus thuringiensis israelensis aka ‘BTI’, lives naturally in the soil and kills all kinds of insect larvae including gnats! A concentrated product is used by pond owners to kill off mosquito larvae but savvy gardeners have been using it too.

A few years ago I could only find it at a particular koi fish supply store. Now it’s a bit more available in Australia and can be found on eBay or Amazon. It comes in the form of little pellets that are soaked in water to activate the bacteria. The water is then tipped over the soil, killing the larvae in 24 hours.

BTI is totally safe and easy to use. This is a great option if you are looking for a quick and easy solution. The brand I prefer sells 8oz (225g) packets, which will last you for years due to how concentrated it is.

Final Thoughts

Encountering gnat flies is somewhat inevitable. Keeping some of the product mentioned on hand is a great idea, but generally changing your watering practices is the best option for long-lasting success. There are a few other methods out there but I’ll leave them for a follow up article in the future.

Greener House Nursery is an Indoor Plant Nursery located at 95 Sydney Road, Brunswick, in Melbourne, Australia. We have the specialised knowledge to make sure you get the right house plants, pots, and accessories for your home. Weather you're looking for low-light, low-maintenance, pet friendly, hanging, or air purifying plants, we've got you covered.