Get Rid of Fungus Gnats

Fungus gnats are the most common houseplant pest there is - those tiny black flies that drift up in a little cloud whenever you move a pot. The good news: they’re harmless to people, they don’t bite, and adult gnats do little damage to plants. The annoying news: they’re persistent, and getting rid of them for good means understanding what they actually are.

What Fungus Gnats Actually Are

A fungus gnat infestation has two stages, and you must treat both:

Adults - the tiny black flies you see. They live only about a week, can’t really fly well, and are mostly just annoying. Each female lays up to ~200 eggs in damp soil.
Larvae - tiny translucent worms living in the top few centimetres of soil. These are the real population. They feed on fungus, organic matter, and decaying roots in damp soil, and in large numbers they can nibble healthy roots.

Here’s the key insight: fungus gnats are a symptom, not the disease. They breed only in constantly damp soil. If you kill the adults but leave the soil wet, a new generation hatches within days. To win, you must fix the moisture and break the breeding cycle.

Why You Have Fungus Gnats

Overwatering - by far the most common cause. Soil that’s always damp on top is a perfect nursery.
A new plant brought them home - they came in its soil.
A bag of contaminated potting mix.
Soil rich in undecomposed organic matter that stays moist.

If you have gnats, you almost certainly have a watering habit that’s a little too generous - fixing that is half the battle.

The Step-by-Step Plan to Get Rid of Them

Do all of these together. One method alone rarely works.

Step 1: Let the soil dry out

This is the most important step. Let the top 3-5 cm of soil dry out completely before you water again - and keep it that way. Fungus gnat larvae and eggs can’t survive in dry soil. For plants that tolerate it, letting the soil go quite dry between waterings starves the larvae directly.

Step 2: Switch to bottom watering

Water the plant from below - stand the pot in a tray of water for 20 minutes so it soaks up moisture from the bottom. This keeps the top layer of soil dry, which is exactly where gnats breed. It’s one of the most effective long-term fixes.

Step 3: Top the soil with a dry barrier

Cover the soil surface with a 1-2 cm layer of something dry and inhospitable: coarse sand, fine gravel, or perlite. Adults can’t reach the soil to lay eggs, and newly hatched larvae can’t get out. A simple, very effective physical barrier.

Step 4: Trap the adults with yellow sticky traps

Place yellow sticky traps flat on or just above the soil. Gnats are drawn to yellow and stick fast. This steadily cuts the adult population and lets you monitor whether you’re winning (fewer caught over time = success).

Step 5: Kill the larvae in the soil

To attack the larvae directly, use one of these:

BTI (Bacillus thuringiensis israelensis) - a natural bacterium, sold as mosquito bits/dunks or gnat-specific products. Soak it in your watering water; it’s harmless to plants, pets, and people but lethal to gnat larvae. The most effective larvae treatment.
Hydrogen peroxide drench - mix one part 3% hydrogen peroxide with three or four parts water and water the soil with it. It fizzes, kills larvae on contact, then breaks down into water and oxygen. Safe for most plants.

Step 6: Repeat and be patient

The gnat life cycle runs a few weeks. Keep all the above going for at least 3-4 weeks, even once you see fewer gnats - you’re outlasting every egg already in the soil. Stopping early lets the population rebound.

When Nothing Works: The Repot Option

If an infestation is severe and stubborn, the nuclear option is to repot the plant. Remove as much of the old infested soil from the roots as you reasonably can, and repot into fresh, dry potting mix in a clean pot (our repotting size calculator helps you pick the right pot). Combined with the steps above, this resets the problem.

How to Prevent Fungus Gnats

Don’t overwater. Let soil dry properly between waterings - the #1 prevention.
Bottom water plants prone to gnats, keeping the surface dry.
Quarantine new plants for two weeks and check their soil before they join your collection.
Top vulnerable pots with sand or grit as a permanent barrier.
Use fresh, good-quality potting mix and store opened bags sealed and dry.
Don’t leave water standing in saucers.

Frequently Asked Questions

Are fungus gnats harmful?

To people, no - they don’t bite and carry no disease; they’re just annoying. To plants, the adults do little harm, but large numbers of larvae can damage roots, especially of seedlings and small plants.

Why do I keep getting fungus gnats?

Because the soil stays too damp. Fungus gnats breed only in constantly moist soil. If you kill the adults but keep overwatering, a new generation hatches within days. Fix the watering.

What kills fungus gnat larvae in soil?

BTI (mosquito bits soaked in your watering water) is the most effective. A diluted hydrogen peroxide drench also kills larvae on contact. Letting the soil dry out starves them.

Do yellow sticky traps get rid of fungus gnats?

Sticky traps catch and reduce the adults, but they don’t touch the larvae breeding in the soil. They’re one part of the plan - use them alongside drying out the soil and treating the larvae.

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

Plan for at least 3-4 weeks of consistent treatment to outlast the full life cycle. Stopping as soon as you see fewer gnats lets the remaining eggs rebuild the population.

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