Most gardeners, myself included, can’t be without plants in our lives. We fill the sills and shelves of any well-enough-lit location with green, growing things. And who’s to argue? Indoor plants are ideal sources of happiness for places where winter holds sway for months of the year. Indoor plants connect us to nature every month of the year. But they come with their set of problems too. One of those is the annoying fungus gnat.

You may think that you have some fruit in your kitchen that has gone off and attracted fruit flies. If, when you check, as I did, and find no produce that has gone off, it’s time to look for another possibility. Indoor houseplants commonly attract fungus gnats. Once you have an infestation, it’s annoying, embarrassing, and maddening to deal with. Well, we’re here to help.

What Are Fungus Gnats?

Fungus gnats are small flies that commonly infest houses. Unlike other indoor flying pests like the common housefly, fruit flies, or drain flies, fungus gnats inhabit moist soil of potted plants. They are small, thin, even angular in appearance with translucent, veined wings, unlike fruit flies which are more plump in appearance and drain flies which are fuzzier.

Life Cycle

It is important to understand the fungus gnats’ life cycle because it allows you to attack them when they are at their most vulnerable. 

Fungus Gnats have a life cycle of only four weeks, but during that time, they cause all kinds of trouble and make total nuisances of themselves. They start out as eggs laid in clusters of 30 to 200 that sit in the first one to two inches of soil. The eggs hatch and the fungus gnats emerge as larva which live in the soil for two weeks. Fungus gnats usually lay their eggs where fungus or mold grows. White mold is a significant contributor to a fungus gnat population. 

If you don’t deal with fungus gnats in their larva stage, they enter a pupal stage where they form little chrysalises near the surface of the soil. After 3 to 7 days, they emerge as adults with wings, ready to be a nuisance in your life.

Adults live for only a week before they die, but during that time, they’re laying more eggs in your plants’ soil to start the cycle of destruction all over again.

Are Fungus Gnats Dangerous?

Fungus gnats aren’t dangerous to humans or pets. They don’t bite or sting and they carry no diseases. You can’t get sick from fungus gnats, but they can be an indicator for mold which can harm your health.

However, they are a real problem for plants, especially young seedlings (like those seeds you started indoors to get a head start on the gardening season). When you think of gnats, you generally think of the adult flying insects, but that’s not when fungus gnats do the most damage.

When larval fungus gnats hatch, they feed on fungus in the soil and, horrifyingly, also on your plants’ roots! Plants with an infestation of fungus gnat larva show slowed or stunted growth. The plants start a cycle to repair their roots before they grow any new leaves. Damaged roots can’t take in as many nutrients, so the leaves on affected plants may start to yellow and die. This leaf die-off then starves the plant of much needed chlorophyll they use to repair the root damage the fungus gnats continue to cause. It’s a vicious cycle that can result in the death of your plants. As such, it requires a firm and harsh solution.

Why Do Houseplants Attract Fungus Gnats?

Adults are mainly attracted by moisture. When you water your plants, it attracts the adult gnats as an area that can incubate their young and grow mold for their meals once they hatch.

Gnats are also attracted by extractions of CO₂. Combined with the moisture of your eyes, nose, and mouth, the CO₂ you exhale means that they love to fly in your face whenever you’re breathing. When I dealt with my fungus gnat infestation, I was mortified whenever I had guests over for a visit and gnats repeatedly “attacked” my guests. 

Fungus gnats use their finely tuned sense of moisture and CO₂ to locate their food since mushrooms and other fungi produce CO₂ as they break down organic matter in the soil.

How To Get Rid of Fungus Gnats

Because fungus gnats spend most of their lives as larva in the soil, and that’s where they do the most damage to your plants. It’s also where they are most contained. Flying adults are difficult to eradicate because they move around. Still, I recommend a two-pronged attack of both the larva AND the adults at once.

Fungus gnat larva have soft bodies, so the methods of killing them are similar to the methods to kill aphids and spider mites: Drowning and Desiccation. To attack the fungus gnat larva, you’ll want to soak the soil with your chosen cure.

Death by Drowning

Drowning may seem a silly remedy for a creature that’s attracted by excess moisture, but there are ways to make it work. Everything on this planet needs some form of oxygen to live. Fungus gnats, like many small insects, breathe through their bodies. That means that if you coat them in a sticky substance, they’ll suffocate.

Neem Oil

Neem oil is a natural oil from the seeds of the neem tree, an evergreen found in the Indian subcontinent. It smells a little like oranges to me and so can be quite pleasant compared to other options on the markets.

Mix one-part neem oil to seven-parts water in a spray bottle and spray down the top of your soil. If your plants are indoors, take them outdoors before spraying the soil. This will result in a lot less icky cleanup for you on your windows and walls, trust me. Be careful not to get any of this on the leaves! If you do, take a damp cloth and wipe the leaves down again. Neem oil won’t hurt your plant if it’s not left on it for too long, but over time it can clog the pores of your plant’s leaves and suffocate it just like the fungus gnat larva.

This works well enough, but it doesn’t get the larvae that exist a little further under the soil.  Fungus gnat larva exist in the top two inches of soil, after all. Plants can also absorb the neem oil into their roots. While it won’t harm the plant in small quantities, it essentially removes the active ingredient you’re trying to use to kill the larva in the soil. Neem oil works, it’s just not the ideal solution. (I really didn’t intend that science pun, honest.)

Death by Desiccation

Desiccation methods work by sapping the moisture directly from the fungus gnat larva that are living in your soil. Desiccation will also dry out any eggs that haven’t yet hatched and any chrysalises where the larva are becoming adults.

Hydrogen Peroxide

My favorite method, and the one that I recommend, is to mix one-part hydrogen peroxide with three-parts water in your watering can and water your plants with it.  Be sure you know what percentage of hydrogen peroxide you’re using so you don’t burn your plants.  Most hydrogen peroxides purchased at drug stores are a diluted 3%, which works great, but you should still dilute it further. If your hydrogen peroxide is more concentrated, use less of it when combining it with water. You’ll have to make several applications to make sure you get all the larva. I recommend watering with hydrogen peroxide solution every other day until your infestation is eradicated.

Hydrogen peroxide will do three things to help you out:

1. It will kill the larva upon contact.
2. It will kill the fungus and mold that the larva are munching on
3. It will bolster your plants’ roots.

How does hydrogen peroxide accomplish these magical things? Think of it this way: Hydrogen peroxide is written out in scientific notation as H₂O₂. When Hydrogen peroxide comes into contact with catalase, an enzyme in most living cells, the catalase breaks apart the H₂O₂. The oxygen atoms steal the electrons from the catalase, damaging the living cells and breaking down the H₂O₂ into simply H₂O and O, Water and Oxygen. What two things are good for plants? Water and Oxygen! Your plant will thank you for the literal breath of fresh air.

Adult Fungus Gnat Eradication

So, now you’ve killed the larva of your fungus gnat infestation! Because fungus gnats spend most of their lives as larva, focusing on killing the larva will eventually kill your entire infestation. But why not go completely genocidal on these annoying creatures that have invaded your home? Attack the adults too, so that they can’t lay their eggs in the soil in the first place!

It’s harder to catch fungus gnat adults because they’re everywhere. But keep in mind they are attracted to moisture and CO₂ and they are ultimately looking for a place to lay their eggs. Where? In your potting soil, of course! So, even though they fly all over your house, the best place you can catch them is near your plants. 

Place fly traps in and around your plant pots to trap the fungus gnat whenever they come into contact with the sticky coating. The fungus gnat will stick there and won’t be strong enough to free itself. Eventually, it will die of starvation. 

My favorite fly traps are so cute and cheerful that they look great … at least when they aren’t coated with a thousand dead and dying fungus gnats. But at least then there’s the satisfaction that you are winning the war.

Attacking fungus gnats at two points of their life cycle is the most effective way to have an immediate significant reduction in your infestation. It will still take a week or two, but if you’re diligent in the application of a hydrogen peroxide or neem oil solution and change out the fly traps regularly, you will succeed at eradicating every fungus gnat from your house.

How to Prevent Fungus Gnats

You’ve heard the old saying: “An ounce of prevention is worth a pound of cure.” It’s far easier to prevent a fungus gnat infestation than to get rid of one. How can you stop these pesky creatures from coming back or getting a foothold in the first place?  Happily, you have several options:

Don’t Overwater Your Plants

Moisture is the main reason fungus gnats are interested in your plants to begin with. Remove temptation from their little lives entirely by not overwatering your plants. Water that sits on the surface of the soil is prime for growing mold and attracting gnats. Don’t water blindly. Check your plants before giving them a drink. If the top inch of your plant’s soil is still moist, the plant doesn’t need water that day. Check it tomorrow.

Spread Sand on the Topsoil of your Potted Plants.

If you live near a beach or have sandy soil in your yard, you’re in luck! An inch or so layer of sand on the top layer of your houseplants’ soil will deter fungus gnats. This has a couple of uses: (1) It keeps fungus away because water doesn’t gather on the surface of your potting soil. (2) Fungus gnats don’t see sand as an ideal place to lay their eggs. Sandy beachfront property is not a pretty home to a fungus gnat.

Don’t have a ready supply of sand near your home? Fear not. You can buy sand specifically made for plant pots at many major hardware stores, floral shops, and hobby depots. Online, you can search for “Horticultural Sand,” and find a myriad of attractive options for under $20 USD.

Regularly Sprinkle Cinnamon on the Topsoil of your Plants.

Cinnamon is a wonderful, natural anti-fungal agent. It kills fungus immediately. It also deters unsightly mushrooms from growing and is a natural remedy for dampening off in young seedlings. Where mold can’t grow, fungus gnats won’t be as interested. Plus, the powdered texture of ground cinnamon acts a little like sand, further dissuading fungus gnats from moving in.

Open Any Purchased Potting Soil Outside.

The soil in the places I’ve lived is mostly made up of clay. This means we’ve bought a LOT of potting soil for the gardening that we do. The problem with potting soil, or any other kind of bagged soil that you get from stores or online, is that you don’t really know what you’re getting. It could be full of rocks, old mulch that’s broken down, weeds, and even insect eggs.

Companies that sell potting soil are supposed to regularly run their soil under a hot oven to sterilize it. This sterilization process kills off harmful substances living in the soil like the eggs of pests and fungus spores. However, if the company has a quality control incident – or simply doesn’t care to do the job properly, you’re the one who pays the price. Sadly, I learned about this the hard way.

One year, after purchasing soil from a brand I’d trusted for years, I cheerfully brought it home and set up shop on my floor to pot some plants. Imagine my horror when I cut open the bag of potting soil, and a cloud of tiny, black gnats launched into the air from out of the bag of soil. Revolted by the incident, I immediately closed the bag and took it outside. The damage, however, was done. The little gnats dispersed throughout the house and found new homes in all my indoor plants. My war on fungus gnats had begun.

Opening any purchased soil outside completely avoids the entire situation I ended up in.

Sterilize Your Own Soil.

If you buy soil, you’re essentially paying the companies you buy from to do this step for you.  But face it, the person who will care the most about you and your plants is you. Don’t trust the companies to get it right, ensure it gets done right by doing it yourself.

To sterilize your own soil, fill a standard baking dish with a couple inches of soil and cover it with aluminum foil, shiny side down. Place the pan in your oven for two hours at 200 degrees Fahrenheit (94 degrees Celsius). Check it every 30 minutes or so with a standard meat thermometer. When the temperature reaches 180 degrees Fahrenheit (82 degrees Celsius) on the outside edges of the pan, pull the pan out and let it cool. This will kill anything harmful in the soil.

Fungus Gnat Free!

There you have it!  Fungus gnat and fungus free in just two weeks!  Give it a try and let us know if it worked for you!