Fungus Gnats: The Full Treatment Guide
What Fungus Gnats Actually Are
The tiny flies hovering around your plant aren't the problem — their larvae are. Adult fungus gnats live only a few days and don't harm plants directly. But each female lays up to 200 eggs in moist soil, and the larvae that hatch feed on organic matter and, critically, on plant roots. A bad infestation causes root damage that mimics overwatering: yellowing, wilting, and slow decline on an otherwise well-cared-for plant.
The Lifecycle (Why Sticky Traps Alone Don't Work)
Fungus gnats complete their lifecycle in about 17 days at room temperature:
- Eggs hatch in 3 days in moist soil
- Larvae feed for 12–14 days (this is the damaging stage)
- Pupae develop in soil for 3–5 days
- Adults emerge and live 7–10 days, laying up to 200 eggs each
Yellow sticky traps catch adults and reduce the egg-laying population, but they don't touch the larvae already in the soil. You need to address both the surface and the root zone to actually break the cycle.
What Works
Step 1: Let the soil dry. Fungus gnat larvae need moisture to survive. Allowing the top 2–3 inches of soil to dry completely between waterings kills larvae and makes the soil inhospitable for new eggs. This alone significantly reduces populations.
Step 2: Hydrogen peroxide drench. Mix 1 part 3% hydrogen peroxide with 4 parts water and water your plant normally. The solution kills larvae on contact as it fizzes through the soil, then breaks down into water and oxygen — harmless to roots and beneficial soil organisms. Repeat with every watering for 3–4 weeks.
Step 3: Yellow sticky traps. Place near the soil surface to monitor and catch adults. Watch the numbers — falling trap counts over 2–3 weeks confirm the treatment is working.
Step 4: Top dressing. A layer of coarse sand, fine gravel, or diatomaceous earth on the soil surface prevents adults from laying eggs in the top layer.
Treatment Timeline
Treat every watering for 3–4 weeks minimum. The 17-day lifecycle means you need at least two full cycles of treatment to break it. Don't stop when the flying adults disappear — that's just the adults dying off. Larvae in the soil are still developing.
How You Know It's Working
Sticky trap counts drop week by week. By week 3, traps should be nearly empty. Soil stays drier longer without plant decline. Roots look white and healthy at the next repot.