Greenie Get the app Small black flies around the pot
How to get rid of fungus gnats in houseplants
Fungus gnats breed in potting mix that stays damp, so the main fix is to let the surface of the soil dry out between waterings and empty saucers. Catch the adults with yellow sticky traps and, if larvae persist, treat the soil with a Bti-based drench labelled for fungus gnats, repeating it as directed. The adults are a nuisance; only large numbers of larvae harm roots, mostly on seedlings and young plants.
Check these first
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Check the flies: fungus gnats are small, dark and delicate, look like tiny mosquitoes with long antennae, fly weakly and rest on the soil or nearby.
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Push a finger into the soil. If the surface is always damp, the pot is a breeding site.
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Place a yellow sticky trap near the soil for a few days to see how many adults there are and which pots they come from.
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To find larvae, press a piece of raw potato, cut side down, onto the soil for a few days and look underneath for small white worms with shiny black heads.
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Look for standing water in saucers, cover pots or trays, and for bags of potting mix left open.
Causes and fixes
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Soil kept constantly moist
How to tellGnats rise when you water, the top of the soil never dries, and the pot is heavy for days after watering.
What to doLet the surface of the soil dry out between waterings, to the depth the plant's care sheet allows. For plants that like evenly moist soil, water as soon as the surface dries rather than keeping it wet. Empty saucers after watering and fix any leaks.
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Larvae living in the potting mix
How to tellAdults keep appearing after drying out the soil, and the potato test turns up larvae.
What to doTreat the soil with a drench based on Bti (Bacillus thuringiensis subsp. israelensis) labelled for fungus gnats. It does not last indoors, so repeat it at the interval on the label, about every five days, until no larvae turn up. A small, badly infested plant can simply be thrown away along with its mix.
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Adults laying more eggs
How to tellMany flies on sticky traps, around windows and lamps, and a new generation every few weeks.
What to doStand yellow sticky traps near or in the pots, or cut them into small squares on sticks, and replace them as they fill. Traps alone will not clear the soil, so combine them with drying out and Bti.
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Rich, damp or contaminated mix
How to tellGnats appear soon after repotting, or in pots with lots of decaying leaves or heavy, peaty mix.
What to doUse fresh, bagged potting mix and keep the bag closed. Clear dead leaves from the soil surface, and add perlite or grit for plants that like faster drainage.
Stop it coming back
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Water by checking the soil, and never leave pots standing in water.
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Store potting mix in a closed bag and use fresh mix when repotting.
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Pick dead leaves and plant debris off the soil surface.
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Keep a yellow sticky trap near your plants to spot gnats early.
When to worry
When to worry
A few gnats around a healthy plant are a nuisance, not a threat; the adults do not bite or damage plants. Larvae only harm roots when there are a great many of them, and seedlings and young cuttings suffer most. If a plant wilts or stops growing, check its roots: overwatering and root rot are far more common causes than gnats themselves. Insecticide sprays are rarely needed indoors; a badly infested small plant can simply be thrown away.
