Fungus Gnat Infestation After Repotting: Causes and Cures

Why Repotting Triggers Gnat Outbreaks

Moving houseplants into larger pots with moisture-retentive soil (high peat moss) creates a perfect breeding ground for fungus gnats. The excess soil stays wet too long, promoting fungal growth that larvae feed on.

Diagnose Your Situation

• Did you increase pot size by more than 2 inches in diameter?
• Does the new soil feel soggy days after watering?
• Are you watering before the root zone is truly dry?

If yes to any, you're inadvertently overwatering.

Effective Eradication Strategy

1. Correct the Growing Conditions

• Downsize pots if they're more than 3 inches larger than the original.
• Repot with a lighter mix: 1 part perlite : 2 parts potting soil (cacti: 1 part builder's sand).
• Water only when dry: Use a bamboo skewer to test moisture at root depth.
• Empty saucers within 1 hour of watering.

2. Kill Existing Larvae and Adults

• When soil is dry, water with rubbing alcohol (isopropyl 70%) – kills larvae on contact.
• Yellow sticky traps catch flying adults.
• Repeat alcohol treatment weekly for 3 weeks to catch hatching eggs.

3. Why Coffee Grounds Don't Work

A thin layer of coffee on the soil surface may deter some adults but doesn't address the root cause (wet soil) or kill larvae. It can also mold and worsen the problem.

Maintenance Routine

Going forward, adopt a "soak and dry" cycle: water thoroughly, let drain, then wait until the skewer test shows dryness at depth. This mimics natural conditions and prevents gnat recurrence without chemicals.