How to Get Rid of Fungus Gnats in Houseplants, Least Toxic First

To get rid of fungus gnats, let the top inch of potting mix dry out before each watering, trap adults on yellow sticky cards, and treat the larvae with Bti (Bacillus thuringiensis subsp. israelensis) if numbers stay high. Plan on three to four weeks, because the life stages overlap. The adults don’t damage plants, and in homes the larvae rarely do much harm to established ones, so there is no need for harsh sprays.

University of Wisconsin-Madison Extension calls changing the growing conditions the single most important step. Everything else supports that one change.

Make sure they’re fungus gnats

Adults are dark, delicate flies, about 1/16 to 1/8 inch long, that run across the soil or flutter weakly near pots and windows. Larvae are whitish, with shiny black heads, and grow to about 1/4 inch. They live in the top inch or so of moist mix.

To check for larvae, press a few slices of raw potato onto the soil and look underneath after three or four days. That is a monitoring trick, not a treatment. Adults flying around a fruit bowl are more likely fruit flies, which is a different problem.

Why they build up in winter

Larvae feed on fungi and decaying organic matter in damp soil, so peat-based mix that stays wet is ideal. In winter, low light slows plants down and pots stay wet longer, yet many of us keep watering at the summer rate. Plants brought indoors in fall can also carry eggs or larvae with them. At about 75°F, UC IPM puts one generation at around 17 days, and a female can lay up to 200 eggs, so numbers climb quickly.

Steps, least toxic first

1. Let the surface dry. Wait until the top inch is dry before watering. If you’re not sure how much your plant can take, see our guide to overwatered vs underwatered plants. Plants that need constant moisture need gentler treatment, so favor the steps below.
2. Water from below. Stand the pot in a shallow tray, let it take up water, then drain it. This keeps the surface drier. Empty saucers each time.
3. Set out yellow sticky traps. Cut them into small squares and mount them on skewers just above the soil. They catch adults and show you whether numbers are falling. Replace them every few days, and keep them away from pets and children.
4. Add a dry top layer. Spread 1/2 to 1 inch of coarse sand or fine gravel over the mix. Wisconsin notes this helps keep females from laying eggs in damp soil.
5. Clean up. Remove dead leaves and fallen flowers from the pots, since decaying matter is food for larvae.
6. Treat the larvae with Bti. It is a bacterium sold as granules, liquid or mosquito bits, and is toxic only to the larvae of certain flies. Wet the soil well so it reaches them, following the label. It doesn’t last, so repeat about every five to seven days for several rounds.
7. Try beneficial nematodes for a stubborn problem. UC IPM names Steinernema feltiae as the most effective species. They’re living animals, so apply them right away, and keep the mix moist and between about 60 and 90°F.
8. Use a labeled spray only as a last resort. Pyrethrin products labeled for houseplants can knock back adults for a while. UC IPM suggests moving plants outdoors to treat and waiting about a day before bringing them back, which only works in mild weather. Never fog indoors, and never mix products.
9. Discard a badly infested plant if nothing else helps, especially if it’s cheap to replace.

Options at a glance

What to expect

You’ll likely see adults for a while, even as the problem improves. Wisconsin expects about three to four weeks of changed watering and top-dressing to control a fall infestation. Bti alone, applied once, won’t do it. If trap catches are falling after two weeks, you’re on the right track.

What doesn’t work

• Vinegar traps. Wisconsin notes these don’t work for monitoring fungus gnats. They’re meant for fruit flies.
• Bt for caterpillars. The common garden spray is a different strain and won’t touch gnat larvae. Check that the label says israelensis.
• Spraying the air. UC IPM advises against fogging or spraying flying adults.

When to worry about plant damage

In heavy numbers, larvae can feed on root hairs or tunnel into roots. Seedlings, cuttings and young plants are the most vulnerable, and collapsed seedlings can look similar to damping-off. On mature houseplants, wilting is much more likely to come from water problems or root decay than from gnats, so if yours is wilting in wet mix, follow our root rot rescue.

How to keep them away

• Water by feel, not by the calendar, especially from November to February.
• Use pasteurized potting mix, and keep opened bags closed and away from infested plants. Damp bags can harbor gnats.
• Avoid adding unfinished compost or manure to pots.
• Keep new plants apart for a few weeks with a sticky trap beside them.

Bottom line

Fungus gnats are a sign of wet mix. Dry out the top inch, water from below, use yellow traps, and add Bti if needed. Keep it up for three to four weeks, since the pest has several stages in the soil at any one time. Stronger products are rarely worth it indoors.

FAQ

Are fungus gnats harmful to my plants?

Adults aren’t. Larvae mostly eat decaying matter and soil fungi, and serious root damage is rare in homes, though seedlings and young plants are more at risk.

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

Plan on three to four weeks. A full generation takes a few weeks, so it takes that long for the adults to die off once new ones stop emerging.

Does hydrogen peroxide kill fungus gnats?

It’s a popular home remedy, but it isn’t part of the Wisconsin or UC IPM control programs, so we can’t recommend a rate. Drying the mix and using Bti are the proven routes.

Will repotting get rid of fungus gnats?

It can help a badly infested plant if you remove most of the old mix and use fresh pasteurized mix. It won’t last if you go on keeping the surface wet. Winter is also a poor time to repot unless you have to.