How I Finally Got Rid of Fungus Gnats (And What Didn’t Work)

Fungus gnats are one of the most common problems for indoor gardeners—and one of the toughest to solve. When these tiny flying insects strike, you want them gone fast—but it took me several failed attempts before I solved the problem for good.

There are a ton of fungus gnat treatments out there, and I tried most of them in my quest to get rid of these pesky bugs in my 60-odd houseplants. Here’s what I learned about how to kill fungus gnats—and how to prevent them from affecting your plants in the first place.

What Are Fungus Gnats?

Outdoors, fungus gnats feed on fungi and organic matter in the soil as it breaks down. Indoors, they’re an unwelcome pest that lays eggs in your houseplants’ soil.

Once fungus gnats are introduced, it’s easy for them to spread. You’ll see them buzzing around, which can be a nuisance.

They don’t bite or pose a health risk to people or pets, but they can harm your plants. Fungus gnat larvae can chow down on the roots of your plants. An infestation can cause wilting, slow growth, or even plant death.

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Where Do Fungus Gnats Come From?

Fungus gnats typically enter an indoor environment in one of two ways: via growing medium like compost or potting soil or in a new houseplant.

In my case, the source was either a cheap philodendron purchased from the grocery store or bagged compost from the company that picks up my kitchen scraps every week. The plant probably would have been infested at the commercial greenhouse where it was grown—it’s not uncommon for greenhouses and outdoor nurseries to manage a baseline level of fungus gnats.

But since I noticed the issue shortly after a big repotting session with that compost, and since it affected so many plants so quickly, I suspect that was the culprit.

What Didn’t Work

I researched my options, which ranged from watering adjustments to household items like dryer sheets to commercial organic pesticides. Here are some of the common treatments that didn’t solve my fungus gnat problem.

Bottom Watering

Fungus gnats seem to prefer moist soil, so by bottom watering, you’re keeping that top layer dry while giving your plants the hydration they need. You can also allow plants to dry out fully between waterings, which works well for drought-tolerant varieties like succulents and snake plants but poorly for others like ferns and philodendrons. The fungus gnats persisted.

Hydrogen Peroxide

I had read that watering your plants with hydrogen peroxide—straight or diluted—could eliminate fungus gnat larvae, but that it could also stunt growth by killing soil microbiology. I tried this on my problem philodendron, which seemed fine, if still infested, but I was too nervous to use it on my entire collection.

Sand on the Soil Surface

I came across a suggestion that applying a shallow layer of coarse sand to the surface of your houseplants could get rid of fungus gnats. By turning that top layer of soil into a fast-draining desert with no organic matter, the bugs would lose their habitat and die off.

The sand didn’t help, but it did make a mess every time I watered, and I didn’t like the way my plants looked with sand instead of regular soil around their stems.

Beneficial Nematodes

I was optimistic about beneficial nematodes, microscopic organisms that you stir into the water you apply to plants. They enter the bodies of fungus gnat larvae in the soil and secrete a bacteria that kills them.

I applied these to all of my plants according to package directions, but the infestation continued.

What Finally Worked

After trying and failing to get rid of fungus gnats, I finally succeeded with this combination of strategies.

Sticky Traps

On their own, these bright yellow pieces of plastic coated in adhesive won’t cure a fungus gnat infestation, but they’re an important tool in monitoring and reducing the adult population. Use them to gauge fungus gnat levels and see if other treatments are making progress.

Bti (Bacillus thuringiensis israelensis)

Bti—short for Bacillus thuringiensis israelensis—is a naturally occurring bacteria in soil that kills pests like fungus gnats and mosquitoes. It’s non-toxic to people, birds, mammals, fish, and beneficial insects.

It’s also approved for use in organic farming, but it is an EPA-registered pesticide, so it’s important to follow all package directions when using products containing Bti.

You can purchase Bti as a powder that you mix into water and then apply to your plants’ soil, which is what I did—three applications over the course of a few months, and my fungus gnat problem was history.

Some people prefer to mix mosquito bits, little pellets containing Bti, into the top layer of their houseplants’ soil, but those bits must be replaced periodically after they lose their effectiveness.

Fungus Gnat Prevention

Now that my plants and I have survived a fungus gnat infestation, I never want to experience another one. Instead of amending with compost, I use a potting mix containing both a biofungicide and beneficial mycorrhizae to support soil health while reducing the risk of fungus gnats.

I also inspect new plants carefully and quarantine them away from my main collection for a few weeks after I bring them home.

I use sticky traps to gauge whether they’re cleared for entry. But now that I know what kills fungus gnats and stops them for good, I worry a lot less that I’ll have another infestation on my hands.