Fungus Gnats
Fungus gnats are the tiny black flies that drift up from potting soil and hover around your face. The adults are harmless but maddening; the real issue is their larvae, which live in the top layer of damp soil eating fungi, organic matter, and -- in heavy infestations -- tender roots. Their presence is also a reliable sign you are watering too much.
Quick Diagnosis
Small dark flies (about 1/8 inch) hovering near soil and windows, weak fliers easily distinguished from fruit flies by their attraction to soil rather than fruit. Stir the top inch of soil and look for tiny translucent larvae with black heads. Yellow sticky traps fill up quickly if gnats are present.
Causes
Understanding the root cause is the first step to fixing the problem.
- 1Consistently damp topsoil -- the required breeding habitat for gnat larvae.
- 2Overwatering, especially in low light and winter when soil dries slowly.
- 3Contaminated potting soil brought home from the store or reused between pots.
- 4Rich, peat-heavy mixes that hold surface moisture.
- 5Decaying organic matter (dead leaves, old roots) in the pot feeding larvae.
Treatment Steps
Follow these steps to treat fungus gnats in your indoor plants.
- 1
Let the top 2 inches of soil dry out completely between waterings -- this alone breaks the breeding cycle in most cases.
- 2
Place yellow sticky traps horizontally at soil level to catch adults.
- 3
Water with a Bti product (Bacillus thuringiensis israelensis, sold as Mosquito Bits) every few days for 2-3 weeks to kill larvae biologically.
- 4
Bottom-water so the soil surface stays dry while roots get moisture.
- 5
Top-dress pots with a half-inch layer of coarse sand or fine gravel, which blocks egg-laying.
- 6
For severe cases, repot with fresh sterile soil after washing the roots, and clean the pot thoroughly.
Prevention Tips
Keep your plants healthy by following these preventive measures.
- Water only when the top of the soil has dried -- gnats cannot breed in dry topsoil.
- Quarantine and inspect new plants and opened soil bags.
- Store potting soil sealed and dry.
- Remove dead leaves and debris from the soil surface promptly.
- Use pots with drainage and avoid letting water stand in saucers.