To get rid of fungus gnats, let the top 1 to 2 inches of soil dry out, catch the adults with yellow sticky traps, and kill the larvae in the soil with a BTI drench (such as Mosquito Bits soaked in water). Keep it up for about three to four weeks to break their life cycle.
You water your plants, sit down with a coffee, and a tiny black fly drifts lazily past your face. Then another. By evening there is a small cloud hovering over the pothos. Fungus gnats are one of the most common houseplant pests, and one of the most irritating, but they are also one of the easiest to beat once you understand what they want.
Adult gnats are harmless to you and mostly harmless to plants. The real problem is underground: their larvae live in the top inch of damp soil, feeding on fungi and decaying matter, and in large numbers they nibble fine roots. That is why seedlings and young cuttings suffer most.
Fungus gnat or fruit fly?
Fungus gnats are tiny (about an eighth of an inch), dark gray to black, with long legs and long antennae. They look a bit like miniature mosquitoes and are weak, wobbly fliers that hang around plant pots. Fruit flies are rounder, tan, often have red eyes, and gather around fruit bowls and trash. If the flies rise up every time you water a plant, they are fungus gnats.
Why you have fungus gnats
Three things bring them in and keep them there:
- Constantly moist soil. This is the big one. Gnats lay eggs in damp potting mix and the larvae need moisture to survive.
- Soil rich in organic matter. Peat- and compost-heavy mixes feed the fungi the larvae eat.
- A fresh bag of potting mix or a new plant. Eggs and larvae often arrive in bags of soil stored damp, or in the pot of a new plant.
Winter makes it worse: plants use less water, soil stays wet longer, and gnats get a perfect nursery.
The 3-week plan to get rid of them
A gnat goes from egg to adult in roughly three to four weeks at room temperature. Kill only the adults and new ones keep hatching. Kill only the larvae and the adults keep laying. The plan below hits every stage at once.
- Day 1: let the soil dry. Water less. Let the top 1 to 2 inches dry out completely between waterings. Larvae die in dry soil. For most houseplants this alone cuts the population dramatically.
- Day 1: put up yellow sticky traps. Stick one flat trap in each affected pot, just above the soil. Adults are drawn to the yellow and get stuck before they can lay more eggs. The traps also show you how bad the problem is and whether it is improving.
- Day 1 and every watering after: treat the soil with BTI. BTI (Bacillus thuringiensis subspecies israelensis) is a natural bacterium that kills fly larvae but not plants, pets or people when used as directed. Soak a few tablespoons of Mosquito Bits in a gallon of water for 30 minutes, strain, and use that water for your plants.
- Weeks 2 to 4: keep going. Replace traps when they fill up, keep using BTI water, keep the top of the soil dry. By week three or four you should see few or no new flies on the traps.
Bottom watering helps. Set the pot in a tray of water for 15 to 20 minutes so the soil soaks up moisture from below. The surface stays drier, which is exactly where gnats lay their eggs.
Other remedies that work
- Beneficial nematodes (Steinernema feltiae). Microscopic worms watered into the soil that hunt gnat larvae. Excellent for big collections and greenhouses.
- A top layer of sand or fine gravel. Half an inch over the soil makes it hard for adults to reach damp mix to lay eggs.
- A hydrogen peroxide drench. Mix one part 3% hydrogen peroxide with four parts water and water the plant with it once the soil is mostly dry. It fizzes on contact and kills larvae near the surface. Use it as a one-off boost, not every watering.
- Fresh soil. For a badly infested pot, repotting into fresh, dry mix removes most larvae in one go.
What does not work very well
Cinnamon, apple cider vinegar traps and essential oil sprays come up a lot. Vinegar traps catch some adults but do nothing about larvae. Cinnamon may slow fungus slightly but will not break the cycle. They are not harmful, just not enough on their own.
Do not keep the soil soggy “to drown them”. Gnat larvae love wet soil. Overwatering makes the infestation worse and can rot your plant’s roots.
How to stop them coming back
- Check the soil before you water, every time.
- Store opened bags of potting mix sealed, somewhere dry.
- Quarantine new plants for a week or two and watch for flies.
- Use pots with drainage holes and empty saucers after watering.
- Keep one or two sticky traps in pots during winter as an early warning.
What we would buy to beat fungus gnats
Prices change often, so check the store for today's price.Summit Mosquito Bits (BTI)
The most effective way to wipe out the larvae hiding in your soil. One container treats a whole houseplant collection for months, and BTI targets fly larvae specifically.
- Kills larvae where they live
- Safe for plants and pets when used as directed
- One container lasts a long time
- Downside: Needs soaking before use
- Downside: Works over a couple of weeks, not overnight
How to use it: Soak 4 tablespoons in a gallon of water for 30 minutes, strain, and use the water every time you water for 3–4 weeks.
Yellow sticky traps
Catches flying adults before they lay eggs and gives you a clear picture of whether the problem is getting better.
- Instant results
- Cheap and simple
- Doubles as an early-warning system
- Downside: Does nothing about larvae on its own
- Downside: Not pretty
How to use it: Stick one trap per pot, close to the soil surface, and replace it when it fills up.
Beneficial nematodes (Steinernema feltiae)
Living predators that hunt gnat larvae through the soil. Ideal if you have dozens of plants, seed trays or a greenhouse.
- Natural biological control
- Treats many pots at once
- No residue
- Downside: Live product: use soon after delivery
- Downside: Needs moist (not soggy) soil to work
How to use it: Mix with water as directed and water into the soil in the evening. Repeat after two weeks for heavy infestations.
FAQ
Are fungus gnats harmful to humans or pets?
No. They do not bite, sting or spread disease to people or animals. They are a nuisance and can damage seedlings and fragile roots.
How long does it take to get rid of fungus gnats?
Usually three to four weeks, because that is how long one generation takes. You should see a big drop within the first week if you dry the soil and use BTI.
Can I put a plant with fungus gnats outside?
In warm weather, yes, but bring it back only after the soil has dried out and you have treated it, or you will carry the next generation inside.
Why do I have fungus gnats in winter?
Plants drink less in winter, so soil stays wet much longer. Water less often in the cooler months and the gnats usually fade.
