Fungus gnats are the least dangerous, most annoying, most persistent pest in indoor gardening. They don't hurt the plant much. They hurt your quality of life a lot — flying into your face while you read, hovering in the kitchen, showing up in your wine glass.

They also signal something real about how you're watering.

What they are

Small dark flies, about 1/8 inch long, that flutter weakly around houseplants. They lay eggs in wet topsoil. The larvae — tiny translucent worms in the top inch of soil — eat organic matter and, in bad infestations, feed on fine roots. Adults live about a week; each female lays around 200 eggs. Populations can explode from "one fly" to "swarm" in three weeks.

Why they're here

Fungus gnats need consistently wet topsoil to breed. If they've moved in, one or more of your plants is being watered more often than it should be, or the topsoil never dries between waterings for another reason (cover pot, low light, cool room, dense mix).

They come in from:

  • New plants from a nursery.
  • Bagged potting soil that wasn't stored dry.
  • Open windows in summer.
  • Outdoor pots brought in for winter.

The two-front attack

Adults are easy to see and easy to kill. Larvae are hidden and are the real problem. You have to attack both fronts at once, or the population comes back within a week.

Front one: dry out the topsoil

This is the single most effective move. Water only when the top two inches of soil are dry. Move mulch, moss, or decorative rocks off the topsoil so it can dry between waterings. Bottom-water plants for a couple of weeks so the top stays consistently dry.

Fungus gnat larvae need moist soil. Dry topsoil breaks the breeding cycle in about ten days, no chemicals needed.

Front two: kill what's already there

  • Yellow sticky traps. Place near the topsoil, not up in the leaves. They catch adults before they lay more eggs. Cheap, dramatic, satisfying.
  • BTI (Mosquito Bits or Gnatrol). Bacillus thuringiensis israelensis. A biological control that kills the larvae. Sprinkle on the topsoil and water it in, or steep it in water and use that to water your plants for two weeks. Safe for people and pets.
  • Hydrogen peroxide flush (3% household strength, diluted 1:4 with water). Water the soil with this once. Kills larvae on contact. It fizzes for a minute, then breaks down into water and oxygen. One-time nuclear option, not a routine tool.

Sticky traps catch adults. BTI kills larvae. You need both.

What doesn't work

  • Cinnamon on the soil. Popular on social media, negligible in practice.
  • Apple cider vinegar traps. Catch fruit flies well, fungus gnats poorly.
  • Repotting into fresh soil. Larvae hitch a ride on the root ball. Fresh soil doesn't solve it.
  • Squishing them one at a time. Feel free. It won't fix anything.

How long recovery takes

With the topsoil dry, sticky traps up, and BTI in the water for two weeks, most infestations are resolved in three weeks. Numbers drop noticeably in the first ten days. If you're still seeing adults after four weeks, one plant in your collection is still holding wet topsoil — find that plant.

Prevention going forward

  • Water when the topsoil is dry. This is the whole prevention strategy.
  • Bottom-water fussy plants that need moist soil, so the top stays dry.
  • Store potting soil in a sealed bag or tub, indoors and dry.
  • Quarantine new plants for two weeks and watch for adults.
  • Add a top layer of coarse sand, aquarium gravel, or perlite to the topsoil of chronically damp pots — the layer dries out fast and gnats won't lay eggs in it.

When it's actually a bigger problem

If a plant is declining despite good care and you're seeing gnats around it, the larvae may be feeding on fine roots. Signs: young plants growing slowly, wilting even when soil is moist, weak new growth. Unpot and check — if the root system looks sparse and there are visible larvae in the soil, the BTI + drying protocol is even more urgent.

For most people, though, fungus gnats are an inconvenience, not an emergency. A three-week protocol clears them, and the habit of watering only when the topsoil is dry keeps them from coming back.

The bottom line

Fungus gnats are a symptom of chronically wet topsoil. Dry the top, hang sticky traps, water in BTI for two weeks. Three weeks later they're gone, and if you keep watering the way you were, they'll come back. Change how you water; keep them gone.