How to Get Rid of Fungus Gnats on Houseplants

Fungus gnats are the tiny black flies that hover around houseplant soil and drift across your screen or coffee. The adults are mostly an annoyance; the real damage comes from their larvae, which live in damp topsoil and feed on fungi and tender root hairs. Killing only the flying adults never works because eggs and larvae remain in the soil, so effective control always targets the moist soil cycle.

What you need

• Yellow sticky traps
• A chopstick for moisture testing
• Bottom-watering trays
• Bacillus thuringiensis israelensis (BTI) granules or mosquito bits
• Fresh dry potting mix for topping up
• Fine sand or gravel mulch, optional

Step by step

1. Confirm the pest. Fungus gnats walk and hover weakly near soil; fruit flies circle fruit and fly faster. Trapping near soil versus near the fruit bowl tells you which you have.
2. Let the top one to two inches of soil dry between waterings in every affected pot. Larvae cannot survive in dry soil, and this single change breaks most infestations.
3. Switch to bottom watering so the soil surface stays dry while roots drink from below.
4. Press yellow sticky traps into the soil of each pot to catch egg-laying females and give you a count to track progress week by week.
5. Water once with a BTI solution: steep mosquito bits in water for half an hour and use the strained water at your next watering. BTI kills gnat larvae specifically and is harmless to plants, pets, and people.
6. Repeat the BTI watering every time you water for three weeks. This covers the full life cycle from egg to flying adult, roughly three to four weeks at room temperature.
7. Remove or scrape the top half-inch of soil from badly infested pots and replace with dry fresh mix, since that crust holds most eggs.
8. Top dress persistent problem pots with a half inch of fine sand or gravel so females cannot lay eggs in the soil.
9. Keep traps up for another two weeks after the last adult appears before declaring victory.

Common mistakes

• Spraying flying adults with insecticide and stopping there. Adults live about a week; untreated larvae simply replace them.
• Continuing the same watering schedule. Constantly damp soil guarantees reinfestation regardless of any treatment.
• Using hydrogen peroxide drenches repeatedly at full strength, which can irritate roots without reaching eggs deeper in the pot.
• Blaming compost-only soils unfairly. Any persistently wet surface works as a nursery, including saucers of standing water.

Overwatering that invites gnats usually has a root cause; if pots stay wet for many days, check whether roots have filled the container using [[repot-root-bound-plants]].

Related guides

After clearing gnats, rebuild good watering habits with the diagnosis method in [[fix-yellowing-houseplant-leaves]]. Quarantine and repot new plants correctly using [[repot-houseplants-without-shock]], and keep soil surfaces dry on humidity-loving plants with the tray approach in [[humidity-trays-for-houseplants]].