Are fungus gnats harmful to my plant?

The adults are only a nuisance. The larvae feed on fungi and fine root hairs in the soil, which healthy plants shrug off, but seedlings and already stressed plants can be set back.

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

Plan on three to four weeks. Traps catch the adults while Bti soaks and drier soil break the egg and larva cycle. Stopping after one week is why they always seem to come back.

Fungus Gnats

Pests

What is going on

Tiny black flies around the soil. Annoying, mostly harmless, and entirely fixable. The full guide walks through the checks in order, fastest first, so you can find the cause before trying a fix.

The fix, in order

Start with the check that costs nothing: light, water, and where the plant actually sits. Most cases of fungus gnats trace back to one of those three, and the guide for your specific plant will narrow it further.

The step by step fix

Confirm the culprit. Brush the soil surface: adult fungus gnats flutter up in a little cloud. They are harmless to you and mostly harmless to the plant, but their larvae feed in wet topsoil, so the fix targets the soil, not the air.
Dry the surface out. Larvae need moist topsoil to survive. Let the top two to five centimetres dry fully between waterings, and water from the bottom where the plant allows it.
Trap the adults. Yellow sticky traps pushed into the pots catch the flying adults before they can lay more eggs. Replace them as they fill.
Clear the buffet. Pick dead leaves off the soil surface and out of the pot; decaying matter is larval food.
Block the door. Spread about a centimetre of coarse sand or fine grit over the soil. Adults cannot easily reach the soil to lay eggs through it, and the surface stays drier.
For a heavy infestation, water with a soak made from mosquito bits, the garden product carrying the bacteria Bacillus thuringiensis israelensis. It targets fly larvae and is safe for plants, pets, and people. Repeat weekly.
Keep going for three to four weeks. The life cycle takes about a month, so one good week is not a cure. When the traps stay empty for a fortnight, you have won.