The Flies Are Not the Problem
The gnats you are swatting have about a week to live. They will die on schedule whether you intervene or not. Each female spends that week laying eggs in the top of your potting mix (up to 200 of them). That is where the population actually lives: larvae with shiny black heads on clear white bodies, sitting in the top two or three inches.
What those larvae eat is the part that matters, and it is the reason this page is about mold. They are fungus feeders first and root nibblers second, grazing on the fungi and algae that grow in permanently damp organic potting mix. A white film on your soil surface is not a coincidence that happened to show up alongside the flies. It is the food supply.
Which gives you the lever. Dry that top layer out and you remove the mold and starve the larvae in the same move. Watering habits end this problem. Fly spray does not.
Make Sure They Are Actually Fungus Gnats
Dark, a sixteenth to an eighth of an inch long, and bad at flying. They come up off the soil in a loose drift when you water and end up at the nearest window or lamp. The family is Sciaridae, mostly Bradysia species indoors. The common ones carry a Y-shaped vein in the wing, if you ever get close enough to look.
Two lookalikes change what you should do about them. Shore flies are stockier, with short bristle-like antennae, where a fungus gnat has long segmented antennae longer than its own head. Shore flies breed in algae on saucers and benches, not in the pot, so drying the mix does nothing for them. Fruit flies are tan with red eyes and hover around fruit bowls and drains, not soil. If the flies lift off the soil surface when you water and the mix is wet, you have gnats.
To confirm larvae without tipping the whole plant out, press a chunk of raw potato cut-side down into the soil surface and lift it after a few days. UC IPM specifies a chunk, not peelings. Larvae collect on the cut face, and counting them weekly tells you whether the population is actually falling.
A Four-Week Plan That Actually Ends It
One generation takes about 17 days at 75F: roughly three days as an egg, ten as a larva, four as a pupa. Warmer rooms run faster, cooler rooms slower. Any plan shorter than a month leaves one of those stages alive, so the problem returns for anyone who treats it over a single weekend.
Dry the top layer, and trap the adults while you do it
Let the top inch or two go properly dry before watering again. Every extension service that covers fungus gnats puts soil drying first, and on a mild case it is often the only step needed. Alongside that, yellow sticky cards catch the egg-laying adults: cut a card into squares and mount them on skewers so they sit just above the soil where the gnats actually are. Traps will not clear an infestation on their own, because the larvae are not flying anywhere, but they cut the next round of eggs and they give you a weekly count to judge progress by.
Drench for the larvae with Bti
Bacillus thuringiensis subspecies israelensis, sold as Bti, is the one treatment that reliably reaches the larvae. It is a bacterium that affects only fly larvae: gnats, mosquitoes, blackflies. The larvae eat the spores and the bacteria wreck their midgut. It does nothing at all to pupae or adults; it does not persist in the mix either. A single drench is close to pointless. Repeat it roughly every five days for a month. You can buy it as Gnatrol, or as mosquito-control bits you crumble and steep in water before watering it in. One thing to avoid: do not combine it with copper or chlorine-containing fertilizers or fungicides, which kill the spores before they reach anything.
Bring in nematodes if it is still going
Steinernema feltiae is a predatory nematode that hunts gnat larvae through the mix. It works between 60 and 90F, provided the soil stays moist. UC IPM rates it above the other nematode species sold for the job. They ship alive and deteriorate quickly. Apply them the day they arrive; a packet left in a drawer is a packet wasted.
Fix the mix so it stops being habitat
Larvae need wet organic matter. A peat-heavy or bark-heavy mix that holds water for a week is close to ideal gnat habitat. Blending in more perlite or coarse sand drains the pot faster and lets the surface dry between waterings. A dry surface is the one thing larvae cannot survive.
The pot matters as much as the mix. Terracotta loses moisture through the pot wall and dries its top layer noticeably faster than plastic or glazed ceramic, so a chronic gnat pot is often worth moving.
Two sourcing rules while you are at it. Skip blood meal and manure-based amendments in indoor pots, and do not use unpasteurized or part-composted material in a houseplant. Both give larvae exactly the decaying organic matter they feed on, straight out of the bag.
What Does Not Work, and What I Would Skip
Swatting, obviously. Also letting the entire pot go bone dry to kill the larvae off: that punishes the plant far more than the gnats, and anything surviving in the damp lower half of the pot repopulates the surface as soon as you water.
A hydrogen peroxide drench gets recommended constantly. It does kill larvae on contact at low concentrations (I am not going to pretend otherwise). But neither UC IPM nor Colorado State lists it among their control options, and it is indiscriminate about the rest of the soil biology. Bti is specific, labelled for this exact use, and cheaper per application. I would reach for the Bti first and keep peroxide as the thing I try when something has gone unusually wrong.
Cinnamon dusting and a sand layer over the surface are the two folk remedies I see most often. Cinnamon has measurable antifungal activity in lab conditions and may slow the mold down, but nothing from an extension service supports it as a way to kill larvae. Sand is more interesting: UC IPM endorses sand mixed into the medium to improve drainage, not spread across the top as a barrier. If a top-dressing does help, the likeliest reason is that it dries out fast, in which case you could have got there with watering habits and saved yourself the sand.
Pyrethrin sprays knock the adults down quickly and do nothing whatsoever to the larvae. They buy you about a week. If you use one, UC IPM advises moving the plant outdoors, spraying it there and leaving it about a day before bringing it back inside. Some neem oil products are also labelled for use as a soil drench; the soil is a more sensible target than the air.
When to Repot, and When the Gnats Were a Symptom
Two drying cycles and a month of Bti should finish a normal infestation. If larvae are still turning up on the potato test after that, repot: rinse the root ball clean, bin the old mix outdoors, and scrub the pot before it goes back into service. A moisture meter is worth having at that point, because a pot that bred gnats once was being watered on a schedule the plant did not need.
If you tip the plant out and the roots are brown and mushy rather than firm and pale, the gnats were a symptom and the real problem is underneath them. That is a different repair, and the distinction between root rot and harmless soil mold is where to start before you decide how much root to cut away.
Chronically soggy soil tends to come with company, so if you are finding other things on the plant as well, the guide to identifying common houseplant pests will sort out what you are looking at.
Fungus Gnat FAQ
Are fungus gnats actually harmful to plants?
The adults do no damage at all and cannot bite you. The larvae feed mainly on fungi, algae and decaying matter, with root hairs alongside. A small population is close to harmless. A heavy one will chew fine roots and visibly stunt seedlings and cuttings. On an established plant, though, UC IPM is clear that wilting comes far more often from watering problems or root decay than from gnat larvae. Check the roots before you pin it on the flies.
Will they spread to my other plants?
Yes, and quickly, because the adults fly and every pot with a damp surface is somewhere to lay. Treating only the plant you noticed them on is the most common way people end up doing this three times in a row. Work on every pot in the room at once, even the ones with no flies over them yet.
Why am I still seeing flies after a Bti drench?
Because Bti kills larvae and nothing else. Any gnat that had already reached the pupal stage when you drenched will still emerge as an adult, which takes about four more days, and those adults live another week or so. Seeing flies for ten days after the first application is normal. What you are watching for is whether the numbers on your sticky cards fall between one week and the next.
Are mosquito bits the same thing as Gnatrol?
Both carry Bacillus thuringiensis subspecies israelensis as the active ingredient, so the mechanism is identical. The formulations and the concentrations are not. Mosquito products are labelled for standing water, not potting mix, so read the label and work your own dilution out from it.
Is the white mold on my soil dangerous to me or the plant?
The fuzzy white growth on potting mix is usually a saprophytic fungus living on organic matter. It is not attacking the plant. Its presence tells you the surface stays wet long enough for fungi to establish; the gnats are reporting the same thing. I have gone through the causes in detail under what causes mold in houseplant soil.
Can I get rid of them without chemicals at all?
Often, yes. Drying the top two inches between waterings plus sticky cards for the adults clears a lot of mild infestations inside a month, with nothing applied to the soil. It takes discipline more than product, and it fails mainly when the plant is in a mix that stays wet no matter what you do. If you have been overwatering long enough for the roots to suffer, the recovery steps are in my guide to saving an overwatered plant.