You've just brought a beautiful new plant home, or your houseplants are looking great. Until you see small black flies fluttering around the pots. Chances are they're fungus gnats. They're harmless to people, but can seriously damage your plants if you don't act.
What are fungus gnats?
Fungus gnats are small, dark flying insects that resemble a tiny mosquito. They grow to about 2 to 4 millimetres and move slowly and clumsily through the air. As adults they live only a few days and barely eat. The problem lies in the larvae, which live in the top layer of moist potting soil.
How do they get into your plants?
The main cause is overwatering. Fungus gnats lay their eggs in moist soil, just below the surface. The wetter the potting soil, the more attractive it is as a breeding site. Commercially available potting soil sometimes already contains eggs or larvae, meaning a new plant can be the source without you realising it.
What damage do they cause?
The larvae feed on organic matter in the soil, but in the case of a heavy infestation they also attack the fine root hairs. This weakens the plant, causing poor growth, yellow leaves or in the worst case death. Young cuttings and seedlings are most vulnerable.
How do you solve the problem?
The most effective approach starts with watering. Let the top two centimetres of potting soil dry out before watering again. Fungus gnat larvae do not survive in dry conditions. Yellow sticky traps on sticks near the pot help catch adult gnats and break the reproductive cycle. For persistent cases, replace the top layer of soil with sand or perlite.
Prevention is better than cure
Only water plants when the soil really needs it. Use pots with good drainage so water doesn't collect. Always check new plants before placing them with your others.