Fungus Gnats in Houseplants: Why They Appear and How to Get Rid of Them

What fungus gnats are

Fungus gnats are tiny, dark flies with long legs, often mistaken for fruit flies. They are drawn to moist potting soil. The adults don't bite and don't harm plants. Their larvae live in the top layer of soil, and large numbers can feed on root hairs, which matters most for seedlings, cuttings and young plants.

Why gnats mean the soil stays wet

N.C. Cooperative Extension calls them a common problem of over-watered houseplants. Consistently wet soil is where they lay eggs. They are often worse in winter, when plants need less water but get the same amount. They usually arrive in contaminated potting soil or new plants.

Step 1: let the soil dry

Let the top inch of soil dry between waterings. That breaks the larvae's life cycle, and drying the soil thoroughly kills larvae. Drain any water left in the saucer, then add a day or more between your usual waterings. Watering from the bottom keeps the surface drier.

Step 2: yellow sticky traps

Yellow sticky traps near the pots catch adults, so fewer eggs get laid. They also show you whether the problem is shrinking. Replace them as they fill up.

Step 3: Bti for the larvae

Bti (Bacillus thuringiensis subsp. israelensis), sold in products like mosquito dunks or Gnatrol, targets gnat larvae. It's mixed with water and applied to the soil. Beneficial nematodes (Steinernema feltiae) are another option. Extension advises that insecticides usually aren't needed indoors. Whatever you use, read and follow the label.

If they keep coming back

Top-dress the soil with coarse sand or gravel so adults are less likely to lay eggs there. For stubborn cases, repot in fresh, well-draining mix and throw out the old soil. Use pots with drainage holes (see pots without drainage holes), and check new plants before you bring them home.