Fungus gnats live and reproduce in the top inch and a half of soil so if you let the soil dry down that far between watering you should be fine

And....you can put yellow whitefly sticky strips on the surfaces around your pots. Gnats will be attracted to them along with other insects.

Pure potting SOIL is like candyland for fungus gnats. Ferns can be happy in a faster draining mix, especially when the room is humid. Fungus gnat larvae eat plant roots, so it's good to do something to control them.

There's also organic chemical control with BTi (Bacillus thuringiensis israelis) granules, which kill fungus gnats and mosquitoes. (BTk is for caterpillars & does nothing against gnats). Sprinkle on top, water in. There's also diatomaceous earth, yellow fly traps, shallow dishes of apple cider vinegar + a drop of dish soap, top-dressing with sand or gravel (if appropriate to the plant in the pot), etc.

As for my plants, it's way too hot here for most ferns but my water-loving tropicals do very well in a 1:1:1 mix of fir bark, perlite, and organic potting mix (very little of which is "soil"). This is, of course, specific to my very hot, very sunny area, where these particular plants like a more moisture-retaining medium than the 5:1:1. Do gnats happen? Sure. They're inevitable if you buy plants at big box stores that over-water their inventory but they're not that hard to control.

I use the 5-1-1 mix for all my container plants. But it doesn't matter what mix you use because if you're growing ferns the soil always has to be moist which means fungus gnats can still breed in there. And allowing just the top couple inches to dry out won't kill fungus gnats. I've gone through 2 weeks without watering citrus during the winter when it was indoors, and yes I actually checked to see if the soil was dry with my finger, and fungus gnats were still flying around like crazy. And yes I used those yellow sticky traps. 3 of them at once on each pot and my lord it was like a magnet to the fungus gnats. There were probably at least 150 on each strap.. YET the population still did not decrease. At all. I replaced it probably 5 times during that time totaling to 15 sticky straps. So then as sage mentioned I put in loads of mosquito bits in there and waited 2 months and yet still no decline of those gnats. NOTHING seemed to work. Whether it was in a 5-1-1 mix or allowing it to dry or using yellow sticky straps or applying mosquito bits. So when spring came I completely repotted my citrus and thankfully that got rid of the gnats and haven't seen them since.

This is why I'm asking because if even dry soil in a well draining mix won't work for killing off fungus gnats how in the world can a fern, a plant that needs consistent moisture in the soil, not have gnats? My answer most likely is 'not to grow them indoors period' but for people who do I don't get how you get no fungus gnats...

Depend on the type of fern. I don't grow Boston or maidenhair ferns simply because they require too much water and too much humidity to do well indoors. And I need houseplants that can survive on benign neglect :-) However, rabbit's foot fern, Davallia fejeensis, do not require the same degree of attention and will tolerate drier conditions. They do fine for me.

FWIW, all the rest of my houseplants excepting the orchids, cacti and succulents are in the 5-1-1 mix and I never have issues with fungus gnats. But I water with no degree of regularity either.........when I remember or think maybe it's about time. And they ALL spend the summer outdoors, when the need for watering is more critical and they receive water when all the rest of my containerized plants do.

I have 2 tiny adiantum capillus-veneris which is a southern maidenhair fern. I guess this is a fussy plant to grow then. I'll try looking for other ferns

Another factor may be the amount of organic/decaying material in the medium, which the larvae feed on.

Two plants I've had fungus gnats in had both been repotted into fast draining 511 type mixes, and allowed to dry down between waterings, but had some root rot issues I wasn't on top of and I'm sure that was key. Adjacent plants in moister finer soil were unaffected. Seemed bit backwards.

My ferns (ladder, button, doryoptis) are keep pretty consistently moist - I barely let the surface of the soil dry out - and the gnats haven't gone for them. So I wouldn't rule out a whole category of plants just because they like moist soil. I think there's more to it than that.

By the way, are the houseplant fungus gnats the same insect as the gnats out in the garden? Which we have clouds of?

Don't have any big issues with fungus gnats -- and some of my plants are swamp/marsh plants (constantly wet, very organic media). For me, gnat control is due to some of the plants I grow. I have a fair number of carnivorous plants-- Drosera (sundews) and Pinguicula (butterworts). They easily keep the gnat pop in check.

Try using mosquito dunks to infuse the water with Bt. If you do a search for them on this forum, you should find some previous posts outlining how to utilize them.

I resort to a pesticide in your situation. Are you able to tell which plants they are laying eggs in/coming out of. I usually start with the pots I think are the problem, so I don't have to treat them all. Azamax is very highly recommended on forums , it's basically just very strong neem oil. So no dangerous chemicals and very effective. It's sold online at the usual places, and at one of the big box stores. It is not cheap, but will last a long time. The gnats don't harm plants much.

Yea from now on whenever I pot up new plants I will include mosquito bits in the mix to prevent fungus gnats. Tried it before on my citrus but did not see any improvement after 2 months of waiting but I'll give it another try. For some reason I always get fungus gnats in my houseplants no matter what. Even if they are 5-1-1 mix and even if I let it dry out before the next watering.

Hmm azamax.. So can you both pour that in the soil and spray it on the leaves? Because it also says it treats spider mite issues in which they live on the leaves of the plant. Right now I have 3 in 1 organocide that I use for my citrus but the label says not for indoor use.. which Idk why.

I soaked the mosquito bits in water for a couple days then used that water to water the affected plants. Some of the pieces floated out onto the soil but I didn’t specifically add them to the soil. Maybe they don’t activate so well if they’re added directly to the mix, might not be wet enough that way - could explain your poor response?

I must be weird, because fungus gnats also colonize the soil inside the drain holes on my pots. After being free of them for several months I reintroduced them with an oregano plant I bought and kept inside for a while last spring, so mosquito dunks are now at the top of my shopping list.

Haha, no not weird, I had them (past tense, touch wood) on the bottom of the pots too, would lift the pots up to check for any to squash ;)

Mosquito dunks work. Just leave a small chunk in the watering can. You'll have to kill the adults by hand or sticky traps. The dunks WILL stop the larvae therefore future generations. It works.

An awesome way to keep their population down if you need moist soil is to go to a pet store and get a bag of small aquarium rocks and put it on your soil. the adults get confused because theyre like "..this isnt soil...". it also looks a lo tbetter than soil! it really compliments your plants, and the planter theyre in.

**this doesnt kill back the entire population, but it'll lower it drastically!

You might also look into a diluted hydrogen peroxide/water mix. I have heard people use this and it kills the eggs/larvae on contact. I've been nervous about trying it on my plants, but the gnats have killed 2 of mine, so it might be worth a try?