How to Get Rid of Fungus Gnats

Fungus gnats are small (about 1/8 inch) dark-winged flies in the families Sciaridae and Mycetophilidae. The adults you see flitting up when you water are mostly harmless, but they signal a thriving population of larvae in your potting mix. Those translucent, black-headed larvae feed on fungus, decaying organic matter, and the fine feeder roots of your plants, which can stunt growth in seedlings and young plants.

The single biggest driver of a fungus gnat outbreak is chronically moist soil. Gnats lay eggs in the top 1 to 2 inches of damp, organic-rich mix, and the eggs hatch in about three days. Because the full life cycle runs roughly three to four weeks, lasting control means breaking the breeding cycle at both the larval and adult stages while letting the soil dry out between waterings.

Signs to look for

• Tiny black or gray flies rising from the soil when you water or disturb the pot
• Adults drifting weakly around windows, lamps, and the plant canopy
• Shiny, clear larvae with black heads in the top inch of moist soil
• Seedlings or young plants wilting or growing slowly despite adequate water
• A persistent population that never quite disappears, with new adults appearing every few days

What causes it

How to fix it

1. 1
   Let the top 2 inches of soil dry out completely

   Stop watering until the top 1 to 2 inches are bone dry, ideally for several days. Larvae and eggs cannot survive in dry mix, so this single step kills most of the next generation.

2. 2
   Place yellow sticky traps at the soil surface

   Set horizontal or low vertical yellow sticky cards right at pot level to capture egg-laying adults. This both reduces the breeding population and lets you track whether numbers are falling.

3. 3
   Treat the soil with Bti (Bacillus thuringiensis israelensis)

   Mix a Bti product such as mosquito bits into your water at the labeled rate and drench the soil at each watering. This bacterium specifically kills gnat larvae and is harmless to plants and pets.

4. 4
   Apply a hydrogen peroxide drench if needed

   For a faster knockdown, drench the soil with a mix of 1 part 3% hydrogen peroxide to 4 parts water. It fizzes on contact with larvae and kills them, then breaks down into water and oxygen.

5. 5
   Top-dress with a dry, inhospitable layer

   Cover the soil with a 1/2-inch layer of coarse sand, fine gravel, or horticultural grit. The dry surface blocks adults from reaching the moist mix to lay eggs.

6. 6
   Repot badly infested plants into fresh, fast-draining mix

   If the population persists after three weeks, remove the plant, rinse loose soil from the roots, and repot into fresh mix amended with perlite. Discard the old, contaminated soil.

How to prevent it

• Water only when the top 1 to 2 inches of soil are dry, and empty saucers promptly
• Add perlite, pumice, or coarse sand to your mix to speed up surface drying
• Always use pots with drainage holes and avoid letting plants sit in standing water
• Inspect and quarantine new plants for two weeks before placing them near your collection
• Store opened bags of potting mix sealed and dry to keep them from breeding gnats