Small flies rising from the compost
Fungus gnats. Harmless to a healthy plant, a nuisance in a room, and a reliable sign that the compost is being kept wetter than it needs.
What it usually is
1 Compost kept permanently damp
| How to tell | Small dark flies, two or three millimetres, that walk on the compost and fly weakly when disturbed. They are worst on the pots that are watered most, and the top of the compost stays dark and damp between waterings. |
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| The fix | Let the top two or three centimetres dry between waterings, which kills the larvae without touching the plant. Water from below where you can. This alone ends most infestations within a month. |
2 A layer of organic matter on the surface
| How to tell | Fallen leaves, spent flowers or a mulch of bark on the compost surface, or a peat rich compost that holds water at the top. |
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| The fix | Clear the surface, remove fallen leaves as they land, and top the pot with a centimetre of horticultural grit or fine gravel. The larvae need damp organic matter at the surface and a dry mineral layer denies it to them. |
3 A new plant or a new bag of compost brought them in
| How to tell | The flies appeared within a fortnight of a new plant arriving or a bag of compost being opened, and are worst on that pot. |
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| The fix | Isolate the new plant, let its surface dry, and treat it as above. Store opened compost sealed and dry rather than in an open bag in a shed. |
Through the year. Worst in winter, when the plants use less water, rooms are warm and the compost stays damp for far longer than it does in July.
Fungus gnats are the least serious pest in the journal and the most irritating to live with. The adults do no damage at all: they cannot bite, they do not feed on the plant, and they live about a week. The larvae, small translucent maggots with black heads, feed on fungi and decaying matter in the top centimetre or two of compost, and on a healthy plant they take almost nothing. On a seedling or a cutting with few roots they can do real harm.
What matters is what their presence tells you. Fungus gnats need damp organic matter at the surface of the compost, continuously. A pot that dries out at the top between waterings does not support them. So a plant with a permanent gnat population is a plant being watered more often than it needs, and dealing with the flies and dealing with the watering are the same job.
The routine that works, in order of usefulness. Let the top two or three centimetres of compost dry before watering again, which breaks the larval cycle without any chemical. Clear fallen leaves and spent flowers off the surface. Top each pot with a centimetre of horticultural grit, which stays dry and gives the females nowhere to lay. Put up yellow sticky traps to catch the adults, which reduces the next generation and, more usefully, tells you whether the population is falling.
Two things that people try and that mostly disappoint. Letting a plant wilt to dry it out stresses the plant more than the gnats. And sprays aimed at the adults do nothing about the larvae in the compost, so the population returns in a week.
For a persistent case on a plant that must stay damp, a drench of the bacterium Bacillus thuringiensis israelensis, sold for gnat and mosquito control, kills the larvae specifically and harms nothing else. Two applications a week apart clears it.
Entries that name it
What the journal says about this symptom on each plant.
| Coriander | Fungus gnats in compost kept permanently wet. Let the very top of the compost dry between waterings and cover it with a little grit. |
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| Heartleaf philodendron | Fungus gnats breeding in permanently damp compost. Let the top third of the pot dry between waterings and top the surface with grit. |
| Inch plant | Fungus gnats in permanently damp compost. Let the top of the pot dry between waterings and cover the surface with grit. |
| Spider plant | Fungus gnats breeding in compost that stays wet. Let the top third of the pot dry between waterings and cover the surface with a centimetre of grit. The larvae need damp compost. |
Questions from the margin
- What are the tiny flies around my houseplant?
- Fungus gnats. The adults are harmless and live about a week; the larvae feed on fungi and decaying matter in the top centimetre or two of compost. Their presence means the compost surface never dries, which is worth fixing for the plant's sake as much as your own.
- How do I get rid of fungus gnats?
- Let the top two or three centimetres of compost dry between waterings, which breaks the larval cycle without any chemical. Clear fallen leaves off the surface and top each pot with a centimetre of grit. Yellow sticky traps catch the adults and show you whether the population is falling.
Checked against RHS, Fungus gnats or sciarid flies; Michigan State University Extension, Fungus gnats in houseplants.