Fungus Gnats on Houseplants
Fungus gnats are small dark flies (~2 mm) whose larvae live in the top 2–3 cm of moist potting media, feeding on organic matter and fine roots. The adults are mostly a nuisance; the larvae can damage seedlings and stressed plants. They indicate consistently overwet soil.
How to identify fungus gnats
Adults: small dark flies that flit around the pot and the air just above it. Larvae: tiny translucent worms with shiny black heads, found in the top layer of wet potting mix.
Damage to look for
Slowed growth in seedlings and young plants, mushroom-like off smell from the soil, occasional yellowing of lower leaves on small plants. Mature plants tolerate gnats with little damage.
Life cycle (why they spread so fast)
Egg to adult in ~3 weeks indoors. Each female lays ~200 eggs. Larvae stay in the top 2–3 cm of media, which is why surface-drying breaks the cycle.
How to get rid of fungus gnats
- Let the top 2–3 cm of potting media dry completely between waterings — this kills larvae and stops egg laying.
- Apply Bti (Bacillus thuringiensis israelensis) as a soil drench (mosquito bits work) every 7 days for 3 cycles — kills larvae, harmless to plants and humans.
- Use yellow sticky cards near the pot to trap adults and monitor population.
- For severe cases, repot in fresh, well-draining mix and dispose of the old soil.
How to prevent fungus gnats
- Avoid bagged potting mix sitting open in a humid room — it can come pre-infested.
- Use a well-draining mix with chunky perlite or bark so the surface dries fast.
- Bottom-water plants prone to gnats so the surface stays dry.
Per-species treatment guides
Treatment varies slightly by plant. Find your species below for a dedicated guide.