You water a plant, and a small cloud of tiny black flies lifts off the soil. They are fungus gnats, and they are one of the most common houseplant pests. The adults are mostly a nuisance, flying around lamps and windows and drifting into your coffee. The real damage comes from their young, which live in the soil.
The good news is that you can clear them with gentle methods and a little patience. Below is how to confirm the problem, a step-by-step plan that avoids harsh sprays, and how to keep the gnats from coming back.
What Fungus Gnats Are and Why They Appear
Fungus gnats are small, dark, slender flies much smaller than a housefly. The adults lay eggs in moist potting soil. The larvae, thin white grubs with dark heads, feed on fungi and decaying matter in the soil and sometimes on fine roots. Seedlings and weak plants suffer most. Healthy mature plants usually shrug them off.
The cause is nearly always the same: soil that stays wet near the top. That is why the first step is to look at your watering. If you suspect you are giving too much, read the signs of overwatering houseplants.
How to Confirm It Is Fungus Gnats
Tiny flies near a pot could also be fruit flies, which are tan and drawn to ripe fruit and the kitchen. Fungus gnats are darker, weaker flyers, and stay close to the plants. Two easy tests help.
- Disturb the soil. Water lightly or tap the pot and watch for small flies rising.
- Try a potato slice. Lay a thick slice of raw potato flat on the soil. After a day or two, lift it. Tiny white larvae on the underside mean the soil is infested.
A Gentle Plan, Step by Step
Use these steps together. Each one deals with a different stage of the insect, so no single step is enough on its own.
- Let the topsoil dry. Larvae need damp soil. Water only when the top inch or two is dry, and empty any water from the saucer. This is the most important step.
- Set yellow sticky traps. Push the sticky cards upright into the soil. Adults fly to the yellow color and stick. The traps catch adults only, so they lower the egg-laying but do not remove the larvae. They also tell you how the problem is going. Keep the cards away from pets and children, and from your own hands.
- Water from the bottom. Stand the pot in a tray of water for fifteen to thirty minutes, until the soil is damp through, then let it drain well. The surface stays dry, which the gnats dislike for egg-laying.
- Treat the soil with BTI. BTI is short for Bacillus thuringiensis israelensis, a bacterium found naturally in soil that harms fly larvae. It is sold as mosquito bits or as a gnat-control product. Follow the label directions, which usually involve soaking the bits in water and then using the water to wet the soil. It is widely used because it targets the larvae. Repeat as the label says, since new eggs keep hatching.
- Add a dry top layer. A thin layer of coarse sand or fine gravel on the surface makes it harder for adults to lay eggs. Keep the soil beneath it from staying wet.
A few more habits help the plan along. Replace a sticky card when it is covered with insects, since a full card catches no more. Treat every pot in the room that has damp soil, not only the one where you saw the flies, because adults move easily from plant to plant. Be patient with seedlings and rooting cuttings, which stay damp on purpose. Water them from the bottom and keep a dry layer of sand on top of the soil.
Do not pour bleach, undiluted vinegar or strong cleaners into the soil. They can burn roots and make the problem worse. Keep BTI and any other product out of reach of children and pets, and do not leave it on a surface where an animal might drink or eat it.
When to Repot
If a pot is badly infested or the soil smells sour, replace the soil. Slide the plant out, shake off the old mix, trim away any soft roots, and repot in fresh, well-draining mix in a clean pot. Throw the old soil in the trash, not on the compost pile.
What Does Not Work Well
Spraying the flying adults can seem satisfying, but it does nothing about the larvae in the soil, and the cloud returns within days. Soil treatments, a dry surface and traps go to the source, and the source is the damp soil.
How to Keep Them Away
- Water carefully. Wait for the top of the soil to dry out between waterings.
- Use a good mix. A light, fast-draining mix dries more evenly than a dense one.
- Close the bag. Store opened potting mix tightly sealed, since gnats can arrive in a new bag.
- Clear the surface. Remove fallen leaves and old flowers from the soil, and keep fruit and compost away from your plants.
- Check new plants. Look at the soil of any plant before you bring it near the others.
Quick-Fire FAQ
Do fungus gnats harm people?
No. They do not bite or carry disease to people. They are annoying but not dangerous.
Why do fungus gnats keep coming back?
Usually because the soil is still wet or a new bag of soil brought eggs. Keep the topsoil dry, and stay with the routine long enough to break the cycle.
How long does it take to get rid of them?
Expect several weeks. Eggs and larvae in the soil keep emerging, so keep the traps up and the topsoil dry until you see no more adults.
Is BTI safe to use around pets?
It is made to target fly larvae and is widely used around homes, but follow the label and keep all products away from children and pets.
— Grandma Gigi
