How to Get Rid of Fungus Gnats in Houseplants

To get rid of fungus gnats, you have to target the larvae in the soil, not just the adults flying around your face. Let the top inch or two of potting mix dry out between waterings, catch adults on yellow sticky traps, and drench the soil with water treated with BTi (the bacterium in Mosquito Bits and Mosquito Dunks) to kill the larvae. Keep it up for three to four weeks, because one round almost never clears an infestation.

Why you have fungus gnats

Fungus gnats are small, dark, weak-flying insects that hover around pots. They're a sign of moisture, not a dirty home. The adults lay their eggs in damp potting mix that's rich in organic matter, and the larvae live in the top few inches of soil, feeding on fungi, decaying material and, when numbers get high, fine plant roots.

That means the real cause is almost always soil that stays wet for too long. Overwatering, pots without drainage holes, saucers full of standing water, peat-heavy mixes that hold moisture for days, and lower light in fall and winter (when plants drink less) all set the stage. A new bag of potting mix or a newly bought plant can also bring them in. If you're not sure whether your watering is the problem, our guide to overwatering vs underwatering walks through the signs.

The adults themselves don't harm plants. They're a nuisance. The larvae are the part that matters, and in a heavy infestation they can damage roots enough to stunt seedlings, cuttings and small plants.

The fungus gnat life cycle

Knowing the timeline explains why the fix takes weeks. According to University of California IPM, at around 75°F eggs hatch in about three days, larvae feed for roughly ten days, and pupae take about four more, so a full generation can run in about 17 days. Colorado State University notes a single female can lay up to 200 eggs. Generations overlap, so at any moment your soil holds eggs, larvae and pupae all at once.

The practical upshot: anything that only kills adults will leave a fresh wave hatching a week later. You need to interrupt the soil stages for at least two full cycles, which is why the plan below runs for about a month.

Step 1: Let the top of the soil dry out

This is the single most effective change, and it costs nothing. Eggs and young larvae need moist soil near the surface to survive. Water thoroughly when the plant needs it, then wait until the top inch or two of mix is dry before watering again. Stick a finger in to check rather than watering on a fixed calendar.

Empty saucers and cachepots 15 to 30 minutes after watering so the pot isn't sitting in a puddle. If a plant is in a pot with no drainage hole, move it into one that has one. Plants that need consistently moist soil, like ferns, can't be dried out as hard, so lean more on the other steps for those.

Step 2: Use yellow sticky traps

Yellow sticky cards attract adult gnats. Cut them into small squares and push them on a stake just above the soil in each affected pot, or lay them flat on the surface. They won't solve the problem on their own, but they reduce the number of adults laying eggs and give you a simple way to monitor progress. When the cards stop collecting new gnats, you're close to done.

Replace them once they're covered, and keep them away from pets and curious kids.

Step 3: Treat the soil with BTi

BTi (Bacillus thuringiensis subspecies israelensis) is a naturally occurring bacterium that kills the larvae of mosquitoes, black flies and fungus gnats. It's sold for mosquito control as Mosquito Bits (granules) and Mosquito Dunks (floating donuts), and it's widely used on houseplants because it targets fly larvae and is considered low risk to people and pets.

The common home method:

• Soak a tablespoon or so of Mosquito Bits, or a piece of a Dunk, in a gallon of water for at least 30 minutes, or overnight.
• Use that water for your next few regular waterings, so it soaks through the root zone where larvae feed.
• Some people also sprinkle a few granules directly on the soil surface.

BTi breaks down fairly quickly in soil, so repeat applications are normally needed. Use the treated water each time you water for about three to four weeks. Always follow the label on the product you buy.

Beneficial nematodes (Steinernema feltiae) are another larvae-targeting option that extension services recommend, applied as a soil drench. They're live organisms, so use them soon after purchase.

Step 4: Bottom water

Watering from below keeps the top layer of soil drier, which is exactly where gnats lay eggs. Set the pot in a bowl or sink with a few inches of water for 15 to 30 minutes, let it wick up, then drain. You can still pour BTi water into the tray. Every so often, water from the top to flush out fertilizer salts.

Step 5: Repot if it's bad

If a pot is crawling with larvae, the mix is old and soggy, or the plant shows signs of root rot, repotting can reset things fast. Remove the plant, shake off as much old mix as you can, trim any mushy roots, and repot into fresh, well-draining mix in a pot with a drainage hole. Adding perlite or bark makes the mix dry faster. Store open bags of potting mix sealed, since gnats will happily breed in them.

A thin top dressing of coarse sand or fine gravel over the soil surface can also make it harder for adults to reach moist soil to lay eggs. It works best combined with the steps above.

What not to bother with

A lot of popular fixes either only catch adults or don't have good evidence behind them.

• Vinegar or wine traps. They catch some adults, but sticky traps do that better, and neither touches the larvae.
• Cinnamon on the soil. Often recommended online, but there's little evidence it controls larvae.
• Potato slices. These are a monitoring tool. Larvae gather under them so you can see if they're present, but they don't reduce numbers in any meaningful way.
• Household bug sprays. Spraying adults in the air won't fix a soil problem and isn't something you want drifting around your living room.

Diluted hydrogen peroxide drenches are another common suggestion, but they can harm roots if mixed too strong, and extension guidance focuses on drying the soil, BTi and nematodes instead.

Fungus gnats or something else?

If the flies are hanging around your fruit bowl or kitchen drain rather than your plants, they may be fruit flies or drain flies. And if your plant is declining in ways that gnats wouldn't explain, it's worth a closer look. Planted can identify the plant from a photo, help diagnose problems like pests or watering issues, and give you a care schedule matched to your home, which makes it easier to stop overwatering in the first place.

Common questions

How long does it take to get rid of fungus gnats?

Plan on about three to four weeks of consistent effort. With a generation taking roughly 17 days at room temperature, you need to keep larvae from maturing through at least two cycles. You'll usually see far fewer adults within the first week or two, but keep going until the sticky traps stay clean for a couple of weeks.

Are fungus gnats harmful to my plants?

The adults are harmless to plants. The larvae mostly eat fungi and organic matter, but large populations can feed on fine roots and damage seedlings, cuttings and small or stressed plants. In a typical home, the bigger issue is the wet soil that's attracting them, which can also lead to root rot.

Can I use Mosquito Bits on all houseplants?

BTi is widely used on houseplants and targets fly larvae specifically, so it's a common choice for most collections. Read the product label before use, and keep granules stored away from pets and children. Using treated water at your normal watering frequency avoids adding extra moisture to the soil.

Why do fungus gnats keep coming back?

Usually because the soil is still staying wet, or because treatment stopped before the last generation of larvae matured. New plants and unsealed bags of potting mix are other common sources. Let soil dry between waterings, finish a full month of BTi, and check new plants before putting them next to the rest.