Tiny flies over the soil: fungus gnats
Small dark flies that rise from the potting mix when you water or disturb the pot are usually fungus gnats. The flying adults are mostly a nuisance, but their larvae live in the mix and feed on fungi and fine roots, so a large population can weaken seedlings and stressed plants.
The key point is that spraying the flying adults achieves almost nothing: most of the population is in the mix as eggs and larvae. Control has to target the mix and its moisture, not the air above it.
General beginner information only — not a substitute for advice from your local agricultural extension service. For edible plants, use only a product that is labelled for that particular edible crop, for indoor use, and follow any harvest interval on its label; try non-chemical steps first, and never use a product that is not labelled for indoor use on that plant.
Checklist to diagnose
- Do tiny dark flies rise from the mix when you water or tap the pot?
- Is the top of the mix constantly damp, with no dry interval between waterings?
- Are seedlings or stressed plants weakening, or is the problem only the flies themselves?
- Is the pot draining freely, or sitting in water?
- Are there dead leaves or debris on the mix surface for larvae to feed on?
Rescue steps
- Let the top of the mix dry out between waterings — extension guidance commonly describes allowing the top couple of inches to dry — and always empty the saucer.
- Remove dead leaves and debris from the mix surface, and keep the pot draining freely.
- Set out yellow sticky traps at soil level to catch adults and to show whether numbers are falling.
- If drying and traps are not enough, use a product labelled for fungus gnat larvae in houseplant mix, such as one containing Bacillus thuringiensis israelensis, following its label exactly; it targets larvae, not flying adults.
- Keep the drier watering pattern after the flies are gone: returning to constantly damp mix brings them back.
When to give up
- Fungus gnats alone rarely kill an established plant. Give up on the plant only if its roots are already rotted or it has no healthy growth left — see the root rot page — not because of the flies themselves.
Plants on this site
This page gives general indoor advice: it applies to indoor-grown plants generally and is not a claim about any particular plant on this site.