Causes
Why do I have fungus gnats?
Written by Mustafa Ayan Fact-checked against university extension/IPM sources Last reviewed: September 19, 2026
Direct answer
Fungus gnats show up because something in your home offers moist, organic material for long enough to complete a life cycle — most commonly overwatered or moisture-retentive potting mix, a new plant that arrived already infested, or a saucer that never fully drains. Potting mix is the usual source, but they can occasionally breed in other damp organic sites too, including drains with a buildup of debris. Fix the moisture at the actual source and you remove the reason they're there at all.
Soil that never fully dries
This is the single biggest driver. Overwatering, poor drainage, or a moisture-retentive mix (heavy on peat) keeps the top inch of soil damp enough for eggs to survive and larvae to feed, week after week.
Organic matter breaking down in the mix
Larvae feed on fungi and decaying organic material — bark, peat, and root debris in ordinary potting mix. Compost-heavy or well-used mix gives them more to eat and a more established fungal food source.
New plants or bagged soil brought in already infested
Fungus gnats hitchhike in nursery pots and in bagged potting mix that was stored somewhere damp. A new plant is a common, easy-to-miss entry point.
Standing water in saucers or cachepots
A saucer that holds water after every watering, or a decorative cachepot with no drainage, keeps the base of the pot wet even when the surface looks dry — a second nursery you can't see.
Warm, stable indoor temperatures
Fungus gnats breed year-round indoors because homes don't get cold enough to interrupt the life cycle the way outdoor winters do. Without a dry spell, each generation overlaps the next.
Once you know why, fix it
Identifying the cause tells you which habit to change. If the cause was a recent arrival, see what to do after a new plant. Otherwise, the treatment itself is the same process either way: confirm the pest, manage moisture, trap the adults, and treat larvae if needed.
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