I use Gnatrol to control the larvae in the soil, and yellow sticky traps to monitor the effectiveness.

If you can’t find a small amount of Gnatrol, it is the same ingredient that is in mosquito dunks - you can crumble those up and use them instead.

the problem is probably your base media ... and you arent going to solve that problem by putting anything on top of it ...

this is increased by your proclivity to overwatering ...

here are you.. big city name... can you get better media.. do the plants go outside for summer ...and.. what plants are we talking about ...

if it were me ... i would probably drag them all outside when the weather is proper.. bare root the plants.. sterilize some media.. wash out the pots .. repot them ... and then learn how to properly water the new media according to the plants needs ...

in other words.. i would get rid of the problem ... instead of trying to cure them ...

proper watering involves a sip of water.. and the proper media to shed all excess water the plants doesnt need.. and then near drying before watering again ...

and when i get a new plant.. i let it just barely wilt.. and note how long that took .. so you have some idea how long that plant.. in that pot.. needs to get dry before the plant actually needs water.... you might be surprised how long between watering it can be ...

over watering is a sign of trying to love your plants to death ...

ken

ps: houseplants go in full bright shade outside.. not in sun ...

https://tipnut.com/sterilize-soil/

Fungus gnats can occur with proper watering as well. I properly water and yet I do have a few fungus gnats that hang around.

I would not recommend sand as a top dressing. I can create compacted soil.

Controlling fungus gnats is best done with the two-angle approach that socalgal_gw suggests. Sticky traps for the adults and something to kill the larvae. For the larvae, you can use bacillus thuringiensis (several formulations available including gnatrol) or hydrogen peroxide in water or beneficial nematodes can also be effective.

I found that top dressing with pumice can be effective to prevent them. But first you have to eradicate them.

Good luck! You'll get it.

Ken is correct - fungus gnats can be controlled completely by using the right soil media and not overwatering. If you use a good, fast draining mix and keep the organics and the PWT to a minimum, you should never see them. In over 25 year of container gardening both indoors and out using Al's mixes, I have never had a problem with fungus gnats! It is all media and moisture control!

Cinnamon is used as a fungicide, not an insecticide! It will have no effect on the gnats or their larvae (btw, fungus gnats don't live on fungus - they eat decaying organic matter...a bit of a misnomer :-))

But topping the pots with a thin layer of coarse sand DOES work and it has no adverse effect on drainage nor does it encourage compaction. It just creates an inhospitable place for the gnats to lay their eggs.

Thanks all! I will take that into consideration. I forgot to mention I also have a neem oil spray that I can use. Just learned about how I can sterilize soil and reuse it (I was just tossing out so much before). Apparently I need to bake it and also thoroughly wash out old containers. Thanks Ken for your great suggestion to let new plants wilt a bit first and note the time it takes. Definitely going to implement some of these ideas!!

Have read that watering from the bottom works so the top layer of soil is dry which the gnats don’t like. Also use soapy water to spray and kill the adults when you see them.

A few months ago, I had a bad fungus gnat problem, which could probably be traced to a contaminated bag of succulent potting soil. I used a two-prong approach which seems to have worked. I got some Bonide Systemic Houseplant Insect Control granules and mixed the prescribed amount into the soil of my houseplants. That's to kill the larvae emerging from the eggs the gnats laid in the soil. And I coupled that with putting one of those yellow (they are attracted to yellow) sticky traps into each of my houseplants. Those caught a zillion adults while the Bonide was killing off their offspring. Now, I don't see fungus gnats. I have also become super picky about amending potting mixes to get better drainage, I don't water nearly as freely, and I recently even bought a moisture gauge to see if stuff needs watering, an adjunct to a finger inserted into the soil.

I associate the appearing of the adults with dank smells because of this insect's affinity to old potting medium that needs to be replaced.