Fungus Gnats on Houseplants: Causes, Cures, and Prevention

Eliminating Fungus Gnats (Not "Nats")

The tiny black flies are fungus gnats. They breed in constantly moist soil. The reader in Spokane, WA tried soapy water without success.

Root Cause: Overwatering

Gnats thrive in wet, organic-rich soil. The fix is cultural:

• Water by weight: Water thoroughly, drain excess, then wait until pot feels light.
• Dry top inch: Let soil surface dry completely between waterings.
• Check every 5–10 days indoors.

Immediate Controls

• Yellow sticky traps: Catch adults; place near plants.
• Potato slices: Lay ¼-inch raw potato slices on soil; larvae congregate underneath. Discard daily.
• Horticultural charcoal + perlite: Repot with 1 part charcoal + 1 part perlite mixed into fresh soil for better drainage.
• Safer Soap (professional formulation): Apply as soil drench per label.
• Turf lime: Light dusting on soil surface raises pH, deterring gnats (they prefer acidic soil).

Prevention

• Use sterile, bark-free, peat-based soilless mixes with plenty of perlite.
• Never let pots sit in drainage water >30 minutes.
• Buy from reputable nurseries; inspect soil before bringing home.

Drying out the soil is the only permanent fix. Pesticides are temporary. Visit a local garden center for product recommendations suited to your area.