How do I get rid of the little black flies around my houseplants?

Fungus gnats are more a message than a pest. The adults are a nuisance and do no harm at all, and the larvae feed on fungi and decaying matter in wet compost, damaging roots only in large numbers or on very small seedlings. What they reliably indicate is compost that never dries at the surface, which is a problem in its own right.

Step by step

1. Confirm what they are.Small dark flies, 2 to 4mm, with long legs, that run over the compost surface and fly weakly in short hops when disturbed — usually towards your face and a screen. They are not fruit flies, which are rounder, browner, faster and are around fruit rather than plants.
2. Let the top of the compost dry between waterings.The single most effective measure. The eggs and young larvae live in the top two or three centimetres, and they cannot survive that layer drying. Water thoroughly, then wait until the surface is genuinely dry to a depth of a couple of centimetres before watering again.
3. Water from below.Stand the pot in a tray of water for twenty minutes and then remove it. The rootball wets and the surface stays dry, which is exactly the opposite of what the gnats need. Do not leave pots standing in water afterwards.
4. Top-dress every pot.A centimetre of horticultural grit, fine gravel, sand or perlite over the compost surface. It dries very fast, so the adults will not lay in it and emerging adults struggle to get out. This is the measure people skip and it does a great deal of the work.
5. Put out yellow sticky traps.Adults are strongly attracted to yellow. Traps do not solve the problem on their own, because they only catch adults, but they cut the breeding population, they tell you whether numbers are falling, and they remove the nuisance while the other measures take effect.
6. Remove dead leaves and debris from the compost surface.The larvae feed on decaying organic matter. Fallen leaves lying on the compost are both food and shelter, and clearing them is free.
7. Check your compost.A bag left open outside, or one stored damp, often arrives already colonised. Store compost sealed and dry, and consider repotting a badly affected plant into fresh compost — which also lets you check the roots while you are there.
8. Use a biological control if they persist.Nematodes specific to fungus gnat larvae, or predatory mites, watered into the compost are effective and safe on edibles and around pets. They need moist compost to work, which is a slight tension with the drying-out strategy, so use them alongside traps and top-dressing rather than instead of drying out.
9. Treat every pot in the room.Adults fly between pots and lay wherever the compost is damp. Treating one plant while the others stay wet simply relocates the problem.
10. Do not use household fly spray on plants.It is not intended for plants, it damages foliage and it does nothing to the larvae in the compost, which is where the population is.

Tips

• Peat-free composts hold moisture differently and can look dry on top while wet below, which encourages both overwatering and gnats. Judge by weight or by pushing a finger in properly.
• Seedlings and cuttings are the one case where fungus gnat larvae genuinely do damage, because the root systems are tiny. Use fresh compost and cover trays.
• A carnivorous plant such as a sundew on the windowsill catches a satisfying number of adults and is not a serious control measure.
• Numbers naturally rise in winter, because houseplants are watered on summer habits while using far less. Halving the winter watering solves most cases on its own.

Common mistakes

• Relying on sticky traps alone — They only catch adults. The larvae in the compost keep producing more, and the traps look like they are failing.
• Watering on a schedule — It is the direct cause. Check the compost each time instead — a plant needing water weekly in July may need it monthly in December.
• Letting the whole rootball dry out to kill them — It stresses the plant, and peat-free compost is very hard to re-wet once it has dried right through. Dry the top layer, not the plant.
• Assuming they are eating the plant — They are not. If a plant is declining as well, look for the real cause — root rot, root pests or pot-bound roots — rather than blaming the gnats.

Questions people ask