How to Get Rid of Fungus Gnats: 3-Week Plan

What Fungus Gnats Actually Are

The small dark flies drifting up whenever you water a houseplant are fungus gnats — usually Bradysia species — and they are a symptom of wet compost, not dirty houses. Adults live about a week, fly badly, and are harmless: they don't bite, don't spread disease and don't eat leaves. The problem is underground. Each female lays up to 200 eggs in the top 2–3cm of moist compost, and the translucent larvae feed on fungi, decaying organic matter and, when populations are high, fine root hairs. On seedlings and cuttings that root damage is real and can stall or kill them; on an established monstera it's mostly a nuisance. The whole life cycle takes three to four weeks at UK room temperature, which is exactly why partial measures fail — kill the adults alone and a fresh batch emerges a fortnight later.

The Three-Week Elimination Plan

What Actually Works — and What Doesn't

Why They Appeared in the First Place

• ✓ Overwatering — by far the most common cause, especially in low-light UK winters when plants use far less water
• ✓ Peat-free composts, which hold more moisture and organic matter than old peat-based mixes
• ✓ Bringing a new plant home already carrying eggs in its compost — quarantine new arrivals for two weeks
• ✓ Open bags of compost stored damp in a shed or garage
• ✓ Pots without drainage holes, or decorative covers that trap water
• ✓ Dead leaves and spent flowers left on the compost surface, feeding the fungi larvae eat

Fungus Gnats in Greenhouses and Seed Trays

Indoors it's an irritation; in a propagator it's a genuine crop risk. Seedlings and cuttings have tiny root systems, and larval feeding on them shows up as inexplicable damping-off, stalled growth and wilting despite wet compost. In a UK greenhouse the answer is prevention: sow into fresh, sterile seed compost rather than reused mix, water from below wherever possible, ventilate whenever temperatures allow to dry the surface, and hang a sticky trap over each bench from February so you spot the first adults. If you're propagating seriously, apply nematodes preventatively at the start of the season rather than waiting for damage. Keep the greenhouse floor clear of old compost spills and abandoned pots — those are the reservoir the flies keep re-emerging from.

Common Mistakes

• ✗ Spraying the adults with fly killer and calling it done — the larvae are untouched
• ✗ Treating one plant and not the others in the same room; the adults simply move next door
• ✗ Letting compost dry so severely the plant wilts, which stresses roots and helps nothing
• ✗ Using nematodes past their expiry date or storing them at room temperature
• ✗ Repotting into the same wet, exhausted compost that caused the problem
• ✗ Giving up after two weeks — the cycle runs three to four weeks, so treatment must too

What to Buy to Clear an Infestation

A complete fungus gnat kit costs less than a decent houseplant: a pack of yellow sticky traps, a sachet of Steinernema feltiae nematodes or BTi granules, and a bag of horticultural grit or fine perlite for top-dressing. If several plants are affected, add a bag of good peat-free houseplant compost with extra perlite so you can repot the worst offenders rather than fighting them for months. A cheap moisture meter is optional but genuinely useful if you're the sort of plant owner who waters on a schedule instead of by feel.

Frequently Asked Questions