How to get rid of fungus gnats in houseplants

By Sophie Sparkes · October 2026 · 5 min read

If small black flies drift up every time you water a houseplant, they are almost certainly fungus gnats, also called sciarid flies or compost flies. They are mostly a nuisance rather than a danger, but they breed fast, and they are a sign that something about your watering needs to change.

Short answer: fungus gnats breed in damp compost, so the fix is to let the top few centimetres of compost dry out between waterings. That stops the larvae surviving. Add yellow sticky traps to catch the adults, and for a stubborn problem water in a biological control (beneficial nematodes) sold for sciarid flies. Most infestations clear in three to four weeks.

How to tell it is fungus gnats

• Tiny, dark, slim flies, about 2-3mm long, with long legs and grey wings.
• Weak, zig-zag flight, usually close to the plant or the soil surface, and they rise up when you water or move the pot.
• They hang around plants, not the fruit bowl. Fruit flies are rounder, tan coloured and gather around food and the bin.
• Larvae in the compost: if you look closely at the top layer, you may see tiny translucent worms with shiny black heads.

Why they appear (and why autumn is peak season)

The adults lay eggs in moist compost that is rich in organic matter. The larvae feed on fungi and decaying material in the compost, and occasionally on fine roots. In autumn, houseplants grow more slowly and need less water, but most of us keep watering on the summer schedule. The compost stays damp for longer, and gnats move in. Plants brought indoors for winter, and fresh bags of compost, can also bring them in.

Adult gnats do not bite and do not harm healthy, established plants. Seedlings and cuttings are more vulnerable, because their roots are so small.

How to get rid of fungus gnats

1. Let the compost dry out (this is the big one)

Larvae need moist compost to survive. Let the top 2-5cm dry out completely before you water again. Push a finger into the compost: if it feels damp, wait. This alone clears most infestations, because each generation fails to complete its life cycle. Our guide to telling when a plant needs water covers the finger test in detail.

2. Water from below

Stand the pot in a bowl of water for 20-30 minutes, then let it drain. The roots get a drink while the surface, where eggs are laid, stays drier. Never leave a pot standing in water afterwards.

3. Catch the adults with yellow sticky traps

Gnats are drawn to bright yellow. Push a sticky trap into the compost or hang it just above the plant. Traps will not solve the problem on their own, because the larvae are in the compost, but they reduce the number of adults laying new eggs and show you whether the numbers are falling.

4. Use a biological control for bad infestations

Beneficial nematodes (Steinernema feltiae) are microscopic worms that hunt sciarid larvae in the compost. They are sold in the UK as a powder you mix with water and pour on. They are safe for people, pets and plants, and work best in moist compost at normal room temperature, so apply them, then return to drying the surface between waterings.

5. Top dress, or repot as a last resort

A 1cm layer of horticultural grit or fine gravel on top of the compost makes it harder for adults to lay eggs. If a plant is badly infested and the compost stays soggy, repot it into fresh compost in a pot with good drainage.

How to stop them coming back

• Water less in autumn and winter. Most houseplants need noticeably less water from October to March.
• Make sure every pot drains. Decorative pots without holes trap water at the bottom.
• Empty saucers 15 minutes after watering.
• Seal open bags of compost, and keep them outside or in a shed rather than indoors.
• Check new plants before putting them next to your others.

Fungus gnat questions

Are fungus gnats harmful to plants?

Not usually to established plants. The larvae mostly feed on fungi and decaying matter in the compost. Large numbers can nibble fine roots, which matters for seedlings, cuttings and young plants. A heavy infestation is more useful as a warning: it usually means the compost is staying too wet, which is a bigger risk to the plant than the gnats are.

How long does it take to get rid of fungus gnats?

Usually three to four weeks. That is roughly one full life cycle at room temperature, so you need to keep the compost surface dry for long enough that no new generation can develop. Adults you see in the first week or two were already on their way; numbers on the sticky traps should fall steadily after that.

Why do I have fungus gnats in winter?

Because houseplants use far less water in the darker months, so compost watered on a summer schedule stays damp for days. Central heating keeps the room warm enough for gnats to keep breeding. Cutting back on watering is the fix.

Are fungus gnats the same as fruit flies?

No. Fungus gnats are slim and dark with long legs, and they gather around houseplants and compost. Fruit flies are rounder and tan coloured, often with red eyes, and gather around fruit, food waste and the kitchen bin. The fixes are completely different, so it is worth checking which one you have.

Do fungus gnats bite?

No. Fungus gnats do not bite people or pets and do not carry disease. They are a nuisance, and a sign that the compost is staying too wet, but they are not a health risk.