Fungus Gnats: Why They Appear and How to Get Rid of Them
By The Budometry TeamPublished Updated
Adult gnats are annoying but harmless. Their larvae, which eat fine root hairs in the top layer of wet soil, are the real problem — and they only exist because the surface stays wet.
Part of our pillar guide: Plant Training, Pests and Troubleshooting
Fix the cause first
Gnats lay eggs in soil that is wet at the surface. Let the top inch dry fully between waterings and most of the population collapses on its own.
Three tools that work together
Yellow sticky traps catch adults and show you whether the population is falling. A quarter inch of sand or a layer of diatomaceous earth on the soil surface blocks egg laying. Bacillus thuringiensis israelensis (BTi), sold as mosquito bits, kills larvae when steeped in your water.
Give it two weeks
You are breaking a life cycle, not killing a colony. Expect adults to keep appearing for 10-14 days after the larvae are gone.
Frequently asked questions
- Can fungus gnats kill a plant?
- Rarely a mature plant, but they can kill seedlings and clones whose entire root system sits in the top layer.
The full guide on this topic
Plant Training, Pests and Troubleshooting
Topping, LST and SCROG explained, plus how to identify and stop spider mites, fungus gnats and powdery mildew, and how to read leaf symptoms correctly.