Fungus gnats
Small dark flies that run over the compost surface and drift up when a pot is watered. The adults are a nuisance rather than a problem; the larvae live in the top few centimetres of permanently damp compost and can damage seedling roots, which is why the infestation is a symptom of watering habits more than of the insect.
Caused by: Sciaridae · also called sciarid fly, compost fly
Nothing available to a home grower will fix this
On an established houseplant there is often nothing worth doing beyond letting the compost dry. The larvae damage seedlings and cuttings, not a mature root system, and treating a harmless population is effort spent on the wrong problem.
source
GrowingHQ editorial positionmoderate confidence
Recorded because the visible flies provoke a stronger reaction than the actual damage justifies.
- GrowingHQ editorial synthesis — GrowingHQ
Checked 2026-08-13
What brings it on
Compost kept constantly moist, particularly peat-free mixes that hold water at the surface. Populations collapse when the top of the compost is allowed to dry between waterings.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Fungus gnats
Checked 2026-08-13
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Let the surface of the compost dry before watering again, and water from below where the plant tolerates it. Removing the damp surface layer removes the breeding site.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Fungus gnats — prevention
Checked 2026-08-13
- A layer of grit or fine bark on the surface makes it much harder for adults to lay in the compost.
source
Sourced guidancemoderate confidence
- Royal Horticultural Society: Fungus gnats — prevention
Checked 2026-08-13
Dealing with it once it is there
- Yellow sticky traps catch adults and monitor the population. On established plants the larvae rarely cause real harm, and the correct response is usually to change the watering rather than to treat.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Fungus gnats — control
Checked 2026-08-13
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.
Plant-specific guides
Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.