Fungus Gnats in Potting Soil: Best Treatments

Fungus gnats are tiny dark flies that hover around houseplants, windows, and potting soil. They are annoying, but the bigger clue is usually overwatered or constantly moist potting mix. Adult fungus gnats do not bite people. The larvae live in moist organic media, feeding mostly on fungi and decaying matter, and sometimes on tender roots.

The best treatments are moisture control, yellow sticky traps, bottom watering, removing decaying debris, and using Bacillus thuringiensis var. israelensis, often called Bti, as a soil drench when needed. Spraying adult gnats rarely solves the problem because the life cycle continues in the potting soil.

If your plant has webbing and stippled leaves, read Spider Mites on Indoor Plants. If white insects fly from leaves when disturbed, see How to Get Rid of Whiteflies Naturally.

Last updated: June 5, 2026.

Quick Answer

To get rid of fungus gnats, let the top 1 to 2 inches of potting mix dry before watering, use yellow sticky traps to catch adults, remove dead leaves from the soil surface, improve drainage, and treat larvae with Bti if needed. Bottom watering can help keep the soil surface drier. Avoid constant moisture, oversized pots, and dense organic potting mixes that stay wet too long.

How to Confirm Fungus Gnats

Fungus gnats are:

small, dark, delicate flies
often seen near soil or windows
weak fliers
active when pots are disturbed
associated with wet potting mix

They are different from fruit flies, which are more associated with ripe fruit, drains, and fermenting material.

Why Fungus Gnats Appear

Common causes:

overwatering
pots without drainage
dense potting mix
decaying leaves on soil
algae or fungal growth on potting media
new plants brought home already infested
plants moved indoors from outside
constantly moist propagation trays

Fungus gnats are often a watering problem before they are an insecticide problem.

Best Treatment Plan

1. Dry the Soil Surface

Let the top 1 to 2 inches dry before watering again, if the plant can tolerate it. Larvae need moisture.

Do not dry out plants that require constant moisture beyond their tolerance. Adjust by plant species.

2. Use Yellow Sticky Traps

Sticky traps catch adults and help you monitor whether the population is shrinking. Place traps near the soil surface. Replace when full or dusty.

Traps alone will not kill larvae, but they reduce egg-laying adults.

3. Remove Organic Debris

Remove:

dead leaves
fallen flowers
mossy crust
decaying mulch
old plant tags or wood pieces

Larvae feed on fungi and decaying organic matter.

4. Bottom Water

Bottom watering lets the plant absorb water while keeping the surface drier. Set the pot in water for a short period, then remove it and let excess drain.

Do not leave pots sitting in water all day.

5. Use Bti

Bti targets certain fly larvae, including fungus gnat larvae, when used as a labeled soil drench. It does not kill adult gnats, so use it with sticky traps and watering correction.

Follow the product label. Repeat applications may be needed.

Should You Repot?

Repot if:

the mix is sour-smelling
roots are rotting
the pot has no drainage
larvae are heavy
the soil stays wet for too long
the plant is declining

Use fresh, well-draining potting mix and a clean container. Do not use garden soil for houseplants.

What Does Not Work Well

spraying adults only
covering soil with decorative moss
watering on a calendar
using sticky traps without fixing moisture
applying random insecticide not labeled for indoor plants
using dense outdoor soil in pots

Prevention

inspect new plants
quarantine new plants
use pots with drainage
water by plant need, not schedule
empty saucers
keep soil surface clean
use well-draining potting mix
avoid oversized pots for small root systems

FAQ

Are fungus gnats harmful to plants?

Adult gnats are mostly a nuisance. Larvae can damage roots in severe cases, especially seedlings or stressed plants, but overwatering and root rot are often the bigger issue.

Will cinnamon kill fungus gnats?

It is not a reliable primary treatment. Moisture control, sticky traps, and Bti are better-supported methods.

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

Expect several weeks because eggs, larvae, pupae, and adults overlap. Consistent moisture control is essential.

Can fungus gnats come from drains?

Some small flies can breed in drains. Cover soil and drains separately to identify the source if you are unsure.