Why are there small flies around my houseplant?

Fungus gnats are two or three millimetres long, black or dark grey, with long legs and a slender body — more like a tiny mosquito than a fly. They fly badly. Disturb the pot and a few rise off the surface and settle again within seconds. That habit is the tell.

The adults do no real harm and live about a week. The problem is what they leave behind: eggs in the top layer of compost, which hatch into translucent larvae with black heads that feed on fungus, decaying matter and the finest roots. A mature plant shrugs that off. A tray of seedlings or a rooting cutting does not — it is the one situation where gnats genuinely kill.

The whole cycle runs about four weeks, which matters for the fix: anything you do has to hold for a month, not a weekend.

One cause, wearing different clothes: the surface of the compost stays damp. Larvae cannot survive in compost that dries out between waterings, so a plant with gnats is nearly always a plant that is watered before it needs it.

In autumn three things push the same way. The heating comes on, so you feel the dry air and water more. The light drops, so the plant drinks less. And the schedule that worked in summer — every Saturday, say — now waters into compost that is still wet from last time.

Two other ways in: a new bag of compost that was already carrying eggs, especially peat-based mixes left open and damp in a shed; and a self-watering pot, which keeps the surface moist by design and is the easiest home a gnat will ever find.

Three small flies get blamed for each other. Where they sit tells you which you have.

Something else — worth looking harder — if

• the insects are white and lift off the leaves in a cloud: whitefly, a foliage pest, treated on the leaves and not in the compost.
• they are on the leaf undersides and there is a sticky, shiny film: that is an aphid, scale or mealybug problem — the Check-up's Visible bugs or webbing card, not this one.
• the plant is also wilting, yellowing from the bottom and the compost smells sour: the gnats are a symptom of root rot from overwatering, and the rot is the thing to treat.

Let the top dry out, properly

The top three to five centimetres of compost should be dry to the touch before you water again — push a finger in. This alone breaks the cycle for most plants within a month, because the larvae have nowhere to live. It is the step people skip because it feels like neglect. It is not.

Water from below

Stand the pot in a saucer of water for twenty minutes and let it drink, then drain it. The roots get what they need and the surface stays dry, which is exactly the opposite of what the larvae want.

Yellow sticky traps at soil level

Adults are drawn to yellow. A trap pushed into the compost, or clipped low beside the pot, catches them before they lay. It also tells you whether the numbers are falling — the honest measure of whether the fix is working.

Top-dress with grit or sand

A centimetre of horticultural grit or coarse sand over the compost keeps the surface dry and stops adults reaching it to lay. It is the one step that keeps working after you have gone back to your normal routine, and it is the step that matters most in a self-watering pot.

For a heavy infestation, a nematode drench

Microscopic nematodes sold for fungus gnats are watered into the compost and hunt the larvae. They are a biological control, safe around pets and children, and available from any garden centre or online. Follow the pack; they are alive and do not keep.

Seedlings and cuttings

Move them to fresh, dry compost now rather than waiting a month. They cannot afford the root damage a mature plant can.

• Do not spray the leaves. Gnats do not live on the foliage; a leaf spray treats the wrong place and can mark the plant.
• Do not pour anything down the pot that is not sold for the purpose — washing-up liquid, vinegar, cinnamon, and the rest of the internet's advice at best does nothing and at worst harms the roots.
• Do not keep watering to the schedule. The schedule is the cause.
• Do not throw the plant away. Gnats are a compost problem, and compost can be dried, dressed or replaced.
• Do not leave an opened bag of compost damp. Fold it tight and keep it dry, or you reintroduce them with the next repot.

How do I get rid of fungus gnats for good?

Take away what the larvae live on: wet compost. Let the top three or four centimetres dry out fully between waterings and keep it that way; the adults die within days and the larvae cannot survive dry soil. Yellow sticky traps catch the adults while the compost dries, and a layer of grit or sand on the surface stops the next generation being laid. Sprays are not the answer; the water is.

Do fungus gnats harm plants?

The adults are harmless, only annoying. The larvae feed on fungus and decaying matter in wet compost and can nibble fine roots, which matters for seedlings and cuttings but rarely for an established plant. The real damage is what the gnats are telling you: the compost is staying wet enough to rot roots.

Where do fungus gnats come from?

Usually in the compost of a new plant, or in a bag of compost that has been standing damp. They breed wherever the surface stays moist, so one wet pot can seed every other pot in the room within a few weeks.

Fungus gnats or fruit flies — which do I have?

Fungus gnats are small, dark and delicate, fly weakly, and gather around the compost and on the windows; fruit flies are rounder, tan, and hover over fruit, bins and drains. If they lift off the soil when you water, they are gnats.

Will letting the soil dry out kill my plant?

Not for a few days, and most houseplants prefer it. The aim is the top few centimetres dry, not the whole pot bone dry; water when that depth is dry and the plant will not notice. Plants that genuinely need constant moisture — ferns, calathea — can be watered from below so the surface stays dry while the roots get their drink.

Anything kept moist all the time: peace lily, calathea, Boston fern, spider plant and any pothos that is watered on a schedule rather than when it is dry. The plant is not the problem — the routine is — but these are the ones to check first when you see a fly.

Treatments above name categories, not brands, on purpose. Availability and approval change; what works does not.

Gnats are a symptom of wet compost, so overwatering is usually the page underneath this one, and root rot is where it goes if it is left. In autumn the routine that was fine in July starts it — the autumn guide. The RHS's fungus gnats page is the reference.

Sabz's Check-up can name fungus gnats — and because it reads the watering you have actually logged, it can say whether your watering is what brought them, rather than guessing from a photo. Pick Small flies around the compost, answer the soil question honestly, and it gives you the plan above for your plant, including the version for a self-watering pot.