Fungus gnats are the small black flies that scatter up out of the pot every time you water or brush past a plant. They’re annoying more than dangerous, but a bad infestation can spread across an entire plant collection fast if the conditions that caused it don’t change.

What fungus gnats are

Fungus gnats look like tiny mosquitoes — thin black bodies, long legs, weak fliers that hover near the soil surface or gather at windows. People often mistake them for fruit flies, but fruit flies are drawn to ripening produce in the kitchen, while fungus gnats are drawn to soil. If they’re congregating around your plants and not your fruit bowl, they’re gnats.

The adults you see flying around don’t actually damage the plant. The problem is what’s happening below the surface: females lay eggs in moist soil, and the larvae that hatch are what feed there, eating fungus and decaying organic matter in the top layer of soil. In large numbers, larvae will also nibble on fine roots, which can set back a seedling or a freshly rooted cutting. On an established houseplant with a developed root system, though, they’re rarely more than a nuisance.

Why they showed up

Wet topsoil is the cause almost every time. Fungus gnats need consistently damp soil to lay eggs and for larvae to survive, so a plant that’s watered before the top layer dries out — or one in a mix that holds onto moisture too long — is exactly the breeding ground they’re looking for. This is also why gnats tend to show up in clusters: one overwatered plant can seed an infestation that spreads to neighboring pots.

New plants are the other common source. Eggs or larvae can hitch a ride in bagged potting soil, in a nursery pot that’s been sitting wet, or in soil brought in from outside. A gnat problem that appears shortly after bringing a new plant home usually traces back to that plant, not anything you did.

How to get rid of them

Let the topsoil dry out more between waterings. This is the single most effective fix, because it breaks the life cycle at its source — eggs and larvae can’t survive once the top inch or two of soil dries out. For a mild infestation, sticking to a proper dry-back schedule alone often clears things up within a couple of weeks.

Top-dress the soil. A 1/4 to 1/2 inch layer of coarse sand, diatomaceous earth, or fine gravel spread over the surface dries out quickly and physically blocks adult gnats from reaching the moist soil underneath to lay eggs. It’s a cheap, low-effort step that pairs well with everything else here.

Set out yellow sticky traps. Placed flat on the soil surface or on a stake just above it, these catch adult gnats before they can lay more eggs. They also double as a gauge — fewer gnats stuck to the trap each week means the infestation is actually easing, not just less visible.

Drench the soil with diluted hydrogen peroxide for a heavier infestation. Mix roughly 1 part 3% hydrogen peroxide to 4 parts water and pour it through the soil until it runs out the drainage holes. It will fizz on contact — that’s the peroxide breaking down and killing larvae in the soil — and it doesn’t harm most houseplants once it’s diluted to that ratio.

Water in Bti for a more sustained fix. Bti (Bacillus thuringiensis israelensis) is a natural bacterial larvicide sold in pellet form as “mosquito bits.” Soak the pellets in water, then use that water to water the plant as usual. It targets fungus gnat larvae specifically without harming the plant, pets, or beneficial insects, and repeated applications keep new larvae from establishing as eggs continue to hatch over the following weeks.

How to prevent them coming back

The long-term fix is the same as the cause: stop letting the soil stay wet. Water based on how dry the soil actually is rather than a fixed schedule — see watering houseplants the right way for how to judge that by feel instead of guessing. Make sure every pot has a drainage hole so water isn’t pooling at the bottom and keeping the lower soil perpetually damp, and if a particular plant’s mix seems to stay soggy for days after watering, it may be time to repot into something that drains faster — see best soil mix for houseplants for what to look for.

Finally, quarantine new plants for a week or two before placing them near the rest of your collection, and check the soil surface for gnats before you do. Most infestations start with one plant that came in already carrying eggs — catching that early saves you from treating your entire shelf later.