Small dark flies that rise from the soil when you water, or hover around the pot, are fungus gnats. The adults are mostly a nuisance. Their larvae live in wet soil and can chew fine roots, which matters for seedlings and weak plants.
Why you have them
Fungus gnats breed in soil that stays damp, especially soil rich in compost or peat. The fix is mostly about letting the soil dry.
What to do
- Let the top 2 inches (5 cm) of soil dry out between waterings. Larvae can't live in dry soil.
- Empty the saucer, and don't leave standing water.
- Catch adults with yellow sticky traps placed near the soil.
- Cover the soil with a thin layer of dry sand or grit, which makes laying eggs harder.
- For a bad infestation, use a soil treatment sold for fungus gnat larvae (many contain Bti, a bacterium that targets them), following the label exactly.
- If the soil smells sour and the plant is drooping, check for root rot too.
When to scan
Adult gnats are hard to tell from other small flies. If you are not sure, a photo of the soil surface and the plant will help. If the plant also droops or yellows, scan the whole plant too.