Plant care guide
Fungus gnats in Denver houseplants: why they show up in winter and how to end them
Those tiny black flies drifting up from your pots are not a plant problem. They are a wet soil problem, and Denver winters create it the same way every year.
By Kyle Lutze, Certified Colorado Gardener · 20 September 2026 · 6 minute read
Why Denver gets them in November
Every year the calls start a few weeks after the furnace comes on. The pattern is the same in Highlands, Lakewood, Wash Park and Cherry Hills: the air in the house drops to 20 percent humidity, leaf edges go crisp, and people respond by watering more. At the same time the days are shorter and the plants are barely growing, so they drink a fraction of what they did in July. The pot stays wet for a week, and wet peat is exactly what a fungus gnat is looking for.
The adults are harmless. They do not bite and they do not eat leaves. The larvae live in the top inch or two of soil, feeding on fungus and decaying matter, and in a heavy infestation they chew fine roots. The damage you see is usually the overwatering itself: yellow lower leaves, soft stems, a sour smell from the pot. The gnats are the symptom that finally gets noticed.
Gnats or fruit flies?
Fungus gnats are slim, dark gray to black, and weak fliers. They hover near the soil and walk on the surface when you water. Fruit flies are rounder, tan, with red eyes, and they hang around the kitchen, not the plants. If they are coming up out of the pots, they are gnats.
What actually works, in order
Check the soil with a meter, not a finger
A finger tells you the top half inch. A cheap moisture meter tells you what is happening at the roots, which is where the problem is. Most gnat-ridden pots read wet all the way down. Stop watering until the top two inches read dry, and for most plants in a Denver winter that means every two to three weeks, not weekly.
Drench with BTi
BTi is a soil bacterium that kills gnat larvae and nothing else. It is sold as mosquito bits or mosquito dunks at any hardware store in the metro. Soak a handful of bits in your watering can for half an hour, then water with it. Repeat every time you water for a month so you catch each new hatch.
Trap the adults
Yellow sticky traps stuck into the soil or clipped to the pot catch the adults before they lay again. They also give you a count: when a trap stays clean for a week, you are done.
Repot the chronic offenders
If a plant never dries out no matter what, the mix is the problem. Bagged potting soil is heavy on peat, and peat holds water for weeks in a cool room. Repot into something chunkier, with bark and perlite, and check that the pot drains. This is the step people skip and the reason the gnats come back every winter.
Quarantine new plants
A surprising number of infestations arrive in the bag. New plants and new soil sit apart from the collection for two weeks with a sticky trap beside them before they join the others.
What to skip
Hydrogen peroxide drenches kill larvae on contact and also kill the soil life that keeps roots healthy, and the gnats return within a month. Cinnamon on the surface does nothing to larvae an inch down. Dish soap sprays hit a few adults and stress the leaves. A layer of sand or fine gravel on top of the soil does help a little by stopping adults from laying, but only after the watering is fixed.
When to call someone
One or two pots with a few gnats is a weekend job. Twenty pots, a plant that is already yellowing from the bottom up, or a gnat cloud in an office where clients notice is a different situation. We treat it as a plant health visit: readings on every pot, a BTi program, repotting where the mix has failed, and a written plan so it does not return in January. Plant health and pest treatment starts with a $65 in-home consultation anywhere in the Denver metro.
Related: Houseplant care in Colorado's dry climate and how we water by meter readings instead of a schedule.
If the gnats are winning
Send a photo of the pot and tell us how often you water. The phone consultation is free, and most gnat problems can be diagnosed from that alone.