I ran a humidifier for awhile and the water started condensing on my window over the winter, and the gnats started drowning in the condensation. That was pretty weird.

I didn't mention they are also in the bathroom (no plants there) and in the little drain for the dishwasher near the sink in the kitchen.

I tend to always have a few around, here and there, but nowhere near as bad as when I first started collecting plants again (about two years ago), and I have several times more plants now than I did then. Like, 383 plants now and 15 plants then.

It's usually a sign that the plants are overpotted (in pots too large for the root ball) or overwatered (being kept consistently too wet).[1] Better watering will take care of most of the problem without having to try anything more strenuous. In previous fungus gnat threads on Garden Web, people have advised pouring bleach down drains, because they will lay eggs down there occasionally; that could maybe be enough to sustain a population, though I'd be surprised. I'm also told that spreading a layer of sand on top of the soil will cut way back on them (adults can't get through the sand to lay eggs; larvae can't get through the sand upon adulthood). And there are sprays and stuff for them, though I really have a hard time imagining that sprays would actually be necessary for most people.

I'm a little curious about how many plants, and how many gnats, we're talking about: what you consider a lot and what I consider a lot may not be the same thing.

[1] I mean no disrespect by asking, but -- when you say you don't overwater, how do you know you're not overwatering? What happens when you water less often? Pothos in particular is easy to overdo.

First, please DON'T pour bleach down the drain. If the sewage in your area is even slightly acidic, it will become chloroform, and you can imagine what that will do to the aquatic life in your local stream, bay, or ocean.

I use a layer of aquarium rock over the soil; others on this forum feel that's a bad idea because it leads to overwatering. You can make this less likely by using unglazed clay pots which allow excess moisture to escape, and of course by erring on the side of underwatering.

There are always 5-10 flies in the living room (8 plants, appropriately sized containers) and 3-4 in the bathroom (no plants).

This is after 1 day next to the newly potted (2 weeks) pothos.
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I've come to believe that fungus gnats are a fact of life with plants grown in peat-based soils. Most readily available potting mixes contain peat in a greater or lesser degree. Even if you don't use those soils, virtually all plants we buy were grown in them.

The best controls in my opinion are BT (bacillus thuringiensis v. israelensis) based. This bacterium is the same thing found in those mosquito dunk donuts sold for ponds and the like. It kills larvae and is safe around pets and people. (Gnatrol and Knock Out Gnats are brands sold for houseplants.)

When I need to, I use yellow traps to get the adults and the KO Gnats to get the larvae. It works, but I have to repeat the treatments when I buy new plants or repot old ones.

I had fungas gnats last winter. Believe it or not, I poured VERY hot water on my plants. It instantly killed the gnats, and didn't hurt the plants. This beats using chemicals.

There may be something to the peat moss theory [braces self for jeannie7 attack]; I don't repot every single plant that comes into my home, but I do repot more of them, proportionally, than I used to, and I use a mix that contains a lot less peat than I used to. At the very least, the theory is consistent with what I've observed.

I don't think it's the peat, per se. I think it's just that the little buggers get so excited when they find a highly water retentive soil . . . or a friend who can't help but break out the irrigation gear every time it looks like the top 1/2" of the soil might dry out in a week or two, that that they go positively orgiastic and start a breeding frenzy.

Al

Actually, my reason for thinking that peat is the specific problem is that I don't remember ever having fungus gnats 25 years ago. Back then, the common potting mediums available to me were actually soil-based. Loamy mixtures have a whole host of problems in containers, including water retention and poor oxygenation, but they don't seem nearly as hospitable to gnats.

Peat moss and peat-based mixes seem to come with gnats almost as an added ingredient ;-)

I'm thinking that what some of you are seeing (like in the plant-less bathroom and drains) are not fungus gnats at all, but what are called drain flies or sewer flies.

Just a thought.