Fungus Gnats on Houseplants: Identify, Confirm, and Control Them

Tiny dark flies dying on your windowsill after bringing home a new plant? You likely have dark-winged fungus gnats. They breed in constantly moist potting soil, feeding on fungi—not your plant directly, but their larvae can damage roots.

Confirm the Diagnosis: The Potato Test

1. Cut a ¼-inch thick slice of raw potato.
2. Place it on the soil surface.
3. Check the underside after a few hours.

Tiny worms with black heads = fungus gnat larvae.

Control Strategies (From Gentle to Aggressive)

1. Cultural Control (Best Long-Term Fix)

• Let soil dry almost completely between waterings. Test with your finger 2 inches deep—water only when it feels nearly dry.
• Top-dress with ½ inch of dry sand. This deters adults from laying eggs.

2. Chemical Control (Faster Results)

Drench soil with a diluted insecticide labeled for fungus gnats (e.g., pyrethrin, neem oil, or systemic imidacloprid). Follow label directions.

Prevention

Inspect new plants before buying. Quarantine new arrivals. Avoid overwatering—it's the #1 cause of infestations.