Fungus Gnats (Sciarid Flies): How to Get Rid of Them

Tiny black flies lifting off the compost every time you water? They are almost certainly fungus gnats, also called sciarid flies. They're one of the most common pests in grow rooms, propagators and houseplant pots, and the good news is that they are easy to get on top of once you know where they breed.

What fungus gnats look like

Adults: small, dark grey to black flies, a few millimetres long, with long legs and long antennae. They are weak fliers, so you see them running over the surface of the compost or flying a short way when disturbed.
Larvae: thin, see-through white grubs with a shiny black head, living in the top few centimetres of damp compost, coco or soil.

Not sure? Hang a yellow sticky trap just above the pots for a few days. Fungus gnats are one of the pests the traps catch, and a trap is the easiest way to see how many you have.

Are fungus gnats harmful?

The adults are a nuisance but do no harm, and they don't bite. The larvae are the problem. They feed on fungi and rotting organic matter in the growing medium, but also on fine roots. A big population can set back seedlings, cuttings and young plants, and the damage they do to roots can let root diseases in. Big, established plants usually cope with a few.

Why you have them

Fungus gnats need a damp surface to lay their eggs in. The usual causes are:

watering too often, so the top of the pot never dries out;
water sitting in saucers or trays under the pots;
compost rich in organic matter, or bags of compost left open where flies can get at them;
warm, still air in a tent, propagator or greenhouse.

How to get rid of fungus gnats

You need to break the breeding cycle, so tackle the adults and the larvae at the same time and keep going for a few weeks: in a warm room the whole life cycle takes only a few weeks, so one treatment on its own rarely clears them.

Let the top dry out. Water less often and let the surface of the compost dry between waterings. Watering from below and emptying saucers helps. Moving air from a clip fan dries the surface faster too.
Catch the adults. Sticky fly traps near the surface catch the flying adults before they lay more eggs. On their own they won't solve the problem, but they tell you whether what you're doing is working.
Treat the larvae in the growing medium. CX Tanlin is a biological control made for fungus gnats (and root aphids and thrips). It is diluted and applied to the growing medium or added to your watering. Biological controls such as beneficial nematodes and predatory mites, which live in the compost and eat the larvae, are also sold by specialist suppliers.
Keep it up. Carry on for at least three or four weeks, until the traps stay clear.

Stopping them coming back

Keep bags of compost, coco and soil closed, and use fresh growing medium for new plants.
Don't overwater seedlings and cuttings in propagators: they are the plants gnats hurt most.
Clear up fallen leaves and old growing medium, and clean the room between grows (see our cleaning products).
Leave a sticky trap up all the time as an early warning.

Quick answers

Do fungus gnats bite?

No. They don't bite people or pets.

Are fungus gnats the same as fruit flies?

No. Fruit flies are drawn to ripe fruit and kitchen waste; fungus gnats breed in damp compost, so you find them around plant pots.

Are sciarid flies and fungus gnats the same thing?

Yes. "Sciarid fly" is the name more often used in the UK; you'll also hear "compost fly".

Whatever you use, read the label first. It gives the dose, how often to repeat it, whether it can go on plants you'll eat, and how close to harvest you can use it.

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