Why does my spider plant have small flies rising from the compost?

On a spider plant this is a cloud of small dark flies, two or three millimetres long, that rise weakly from the pot when you water it or brush the leaves, and walk across the surface of the compost. They are harmless to a healthy plant, and they are a nuisance in a room.

The most likely cause is fungus gnats breeding in compost that stays wet, as the entry says. The larvae need damp compost, and a spider plant that is watered more often than its 7 day growing season rhythm, or kept on that rhythm into the 14 day winter one, offers it. This is a plant that wants to be forgotten for a fortnight rather than watered every other day.

To confirm it in a minute, push a finger into the top 2 to 3 cm of compost. If it is still dark and damp days after watering, and the pot is heavy, that is the cause. The flies are worst on the pots that are watered most. Then look at the surface for fallen leaves, trimmed runners or old flower stalks, which hold water at the top.

Two other causes are worth ruling out. A new plant or an opened bag of compost brought them in, and the flies appeared within a fortnight of it arriving. And a compost rich in peat holds water at the top and keeps the surface damp, so the peat-free mix with a handful of perlite that the entry recommends is the better choice.

The page on small flies says that letting the top of the compost dry ends most infestations within a month. The plant itself is not harmed while this happens.

What this plant wants

The fix

Let the top third of the pot dry between waterings and cover the surface with a centimetre of grit. The larvae need damp compost.

Read on

Small flies rising from the compost

The same sign on any plant, with the causes in the order they are likely.

Spider plant care

Light, water, feeding, repotting and the rest of the entry.

Check it in the diagnoser

Opens with the plant, the part and the sign already chosen.