What you see
A finger goes an inch or two down and the soil is still wet there. Often there is water standing in the cachepot or the saucer as well.
Every measure that works goes at the larvae and at the damp surface they live in. Fight only the flying adults and you start over in two weeks.
Let the top layer dry out between waterings
Fungus gnats lay their eggs in damp soil surfaces. If the top layer dries out regularly, eggs and young larvae die and the population collapses on its own.
Before each watering, push a finger an inch or two into the soil. If it still feels wet down there, wait. Plants that cannot take a longer dry spell still get watered as needed, but pour into the saucer and tip the rest away after half an hour.
The weight of the pot is the second clue. Lift it right after watering and again a few days later. The difference is clearer than any look at the surface.
At the same time, check whether water is standing in the cachepot or the saucer. A layer that stays wet at the bottom keeps the soil damp on top and is often the real reason for the infestation.
Look at
- 01Small dark flies rising when you water
- 02Whether they stay low over the soil, not on the leaves
- 03How wet the top inch or two of soil is
- 04Whether seedlings, cuttings or fresh repots are involved
What to do now
Water only once the top layer feels dry. Water sensitive species through the saucer and tip the rest away after half an hour. Pot weight shows the difference more clearly than looking does.
Sticky traps show you the trend
Yellow sticky cards catch the flying females before they lay. Push them in low over the soil rather than up among the leaves, because the insects travel close to the surface.
On their own they are not enough, because the larvae in the pot stay untouched. Their real value is in the count. If each card catches fewer insects from week to week, what you changed at the soil is working.
Nematodes against the larvae in the mix
Nematodes of the species Steinernema feltiae are microscopic roundworms that attack the larvae in the mix. They are the targeted route against the generation already sitting in the pot.
You stir the powder into water and water every affected pot with it. Keep the soil evenly damp for the following weeks, because the nematodes only move in the film of water between the particles. A surface that dries out completely ends the treatment early.
This is where two measures contradict each other. Decide per pot: either keep it dry, or use nematodes and keep it damp for them. Both at once does not work.
Nematodes are living organisms. They ship chilled, they keep for a limited time, and they no longer work if the pack stood warm too long. Use them fresh and repeat the dose the way the pack says, because eggs keep hatching in the mix.
What you see
The larvae sit unseen in the mix. If flies keep rising, a generation is already established in the pot.
What to do now
Stir nematodes into water, water every affected pot with it and keep the soil evenly damp afterward. Per pot either keep it dry or use nematodes, because the two do not combine.
What you can change at the surface
A layer of coarse sand, fine grit or clay pebbles on the soil takes the laying surface away from the females. An inch is enough, but it has to lie across the whole surface and reach the rim.
Repotting only helps if you shake most of the old mix off the roots. Leave the colonized root ball intact and the larvae move house with it. With a weakened plant, wait with this until it has recovered.
Clear fallen leaves and old flowers off the soil while you are at it. That is the food the larvae live on besides the roots.
Which potting mix brings fewer fungus gnats
Some infestations arrive with the mix. A bag that sat outdoors at the store or stood open holds eggs and larvae more often than a dry, undamaged one.
Peat reduced and strongly mineral blends with a high share of sand, pumice or lava dry out faster on top and offer the larvae less organic material. A mix that rules fungus gnats out does not exist.
Close opened bags between repotting jobs and store them dry. A damp leftover amount in an open bag is an ideal nursery.
What you see
Some infestations come into the house with the mix. Bags stored open or outdoors hold eggs and larvae more often.
What to do now
Use dry stored, undamaged bags and close opened ones. Mineral blends with sand, pumice or lava dry out faster on top.
Remedies that do not deliver
Sticking matches head down into the soil is the best known tip and has no effect on the larvae.
Cinnamon, baking soda and dish soap in the watering can come up just as often. Cinnamon works at most against mold on the surface, and dish soap in the root zone harms the plant more than the larvae.
Carnivorous plants on the windowsill catch the odd fly. On the population in the pot they have no measurable effect.
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Common questions
How do I get rid of fungus gnats?
Through a drier soil surface combined with nematodes against the larvae. Sticky traps come on top, to catch the flying females and follow the trend. Anything that only hits the adults buys you a pause rather than a solution.
How do you use nematodes against fungus gnats?
The powder is stirred into water and poured over the soil. The mix then has to stay evenly damp for several weeks, or the nematodes do not survive. Repeat as the pack says, because new larvae keep hatching.
Do fungus gnat traps work?
Yellow sticky cards catch the flying females and tell you whether the population is falling. They reach nothing in the soil, so on their own they never end an infestation. Push them in low over the surface, where the insects fly.
Are fungus gnats harmful to my plants?
For a strong, well rooted houseplant they are mostly a nuisance. In seed trays, cuttings and freshly repotted plants the larvae eat the fine roots, and those plants stall or topple over.
How long does it take to get rid of fungus gnats?
Expect several weeks, because new larvae keep hatching in the mix. As long as fewer insects stick to the traps from week to week, the treatment is on track.
Should I treat every pot or only the infested one?
Treat every pot in the same room. The flies move between plants, and an untreated pot builds the population back up.
Last reviewed: 2026-09-07