Small dark flies drifting up from a pot every time you water are most likely fungus gnats. Iowa State names them among the most common flies breeding in homes, usually in houseplants. They are annoying rather than dangerous. University of Maryland Extension calls them harmless and says they do not significantly damage indoor plants.

The problem lives in the potting mix. UC IPM explains that fungus gnats spend most of their lives as larvae and pupae in moist organic matter, so control works best on those stages. Swatting adults does little. Changing the moisture in the pot does a lot.

Confirm they are fungus gnats

UC IPM describes adults as dark, delicate, mosquito-like flies, usually 1.5–3 mm (1/16–1/8 in) long, with long legs and antennae longer than their heads. They are weak fliers and tend to stay near pots, running over the mix and leaves. Adults are drawn to light, so you may first see them at windows or around screens at night.

To check for larvae, use the potato test. Colorado State suggests pushing 6 mm (1/4 in) slices of raw potato into the mix. After a few days, turn them over and look for larvae: translucent, legless, with a shiny black head.

To find the source pot, Maryland suggests covering a suspect plant with screen or cheesecloth and watching for flies over the next few days. Iowa State suggests watching the soil surface as you water: flies running over the mix and up the pot sides are breeding there.

UC IPM notes that shore flies look similar but are more robust, with short, bristle-like antennae. They feed on algae rather than in the root zone.

The life cycle

StageWhat happensSource
EggLaid in moist organic debris or potting mix; hatches in about 3 days at 24 °C (75 °F)UC IPM
LarvaFour larval stages over about 10 days at 24 °C (75 °F); usually in the top 5–8 cm (2–3 in) of mixUC IPM, Colorado State
PupaAbout 4 days in or on the mix before adults emergeUC IPM
AdultLives 7–10 days; females may lay up to 200 eggsColorado State

A full generation can take about 17 days at 24 °C (75 °F), according to UC IPM. Colorado State gives three to four weeks at typical room temperatures of 18–24 °C (65–75 °F). Either way, generations overlap and breed year-round indoors. That is why a single treatment rarely ends an infestation.

Why they breed in your pots

  • Wet mix. Iowa State says overwatered houseplants are the most common breeding site in homes.
  • Peat-rich mix. Clemson and Colorado State both say moist mix high in peat moss is especially attractive.
  • Old mix. Colorado State notes that mix holds more water as it ages and breaks down.
  • Slower winter growth. Colorado State says shorter days slow water use. If you keep watering at the summer rate, the mix stays wet.
  • New or summered plants. UC IPM says infestations are common in newly bought or repotted plants. Colorado State says plants kept outdoors in summer often bring gnats in.

Step 1: dry the surface

This is the step every source agrees on. Colorado State calls it the most important strategy: let the mix dry between waterings, especially the top 2.5–5 cm (1–2 in). Dry mix reduces egg and larva survival and makes the pot less attractive to egg-laying females. Maryland and Clemson both say dry conditions kill the larvae.

  • Water only when the plant needs it, and let excess drain.
  • Empty saucers and cachepots; Clemson says not to let water stand under the pot.
  • Remove fallen leaves and decaying matter from the surface (UC IPM, Colorado State).
  • Repot into fresh mix when the old mix has broken down (Colorado State, Iowa State). UC IPM recommends sterile or pasteurized potting mix.

Plants that need constantly moist mix make this harder. Dry the surface as far as the plant tolerates, and lean on the steps below.

Step 2: yellow sticky cards

Adults are attracted to yellow. UC IPM suggests cutting yellow sticky traps into small squares, mounting them on skewers and pushing them into the pots. Colorado State says cards placed under the canopy or on the pot edge can mass-trap females and reduce the next generation. Maryland recommends them for monitoring.

Treat cards as a gauge as much as a fix. If catches keep falling week by week, your watering change is working.

Step 3: Bti for the larvae

Bacillus thuringiensis subspecies israelensis (Bti) is a bacterium that kills fly larvae. UC IPM says it is toxic only to fly larvae such as mosquitoes, black flies and fungus gnats. Maryland says it is the active ingredient in soil-drench products labeled for fungus gnat larvae in indoor plants.

  • Mix with water and apply as a drench to the potting mix, following the label (UC IPM).
  • Expect to repeat it. UC IPM says Bti does not persist indoors, so applications about five days apart may be needed.
  • Use the right Bt. UC IPM says Bt sold for caterpillars is not effective against fly larvae.

UC IPM also lists Steinernema feltiae nematodes, applied as a drench, which work at 16–32 °C (60–90 °F) in moist mix. Predatory Hypoaspis mites probably do not perform well in individual houseplants.

What does not work, or is not worth it

  • Spraying the flies. Colorado State says short-lived contact sprays such as pyrethrins, soaps, oils and neem do not give lasting control of adults. They need repeating every couple of days, and they do not touch larvae in the mix.
  • Fogging or chasing flies with a spray. UC IPM says not to fog indoors or spray gnats in flight.
  • Routine insecticides. Iowa State says insecticides are not necessary and not effective for fungus gnats in houseplants. UC IPM says they are rarely warranted in homes.
  • Traps alone. Colorado State notes trapping will not eliminate pests whose young stages stay in or on the plant.
  • Kitchen remedies. None of the extension pages we used recommends hydrogen peroxide drenches or cinnamon for fungus gnats. Stick to the methods above.

When gnats point to a bigger problem

UC IPM notes that a wilting plant with gnats more often has another root problem, such as overwatering, root rot or poor drainage. Larvae and adults can also spread plant pathogens. If the plant is declining, check the roots: see our root rot causes guide and houseplant pest guide.

Plant-specific guides: pothos fungus gnats, peace lily fungus gnats and Boston fern fungus gnats. If you see white fuzz rather than flies, read mold on houseplant soil.

References