Re sand, I've been using a quarter inch of sand atop the dirt, as apparently fungus gnats lay their eggs no deeper than that. The sand does get watered down into the soil over time, but it's easy to add another spoonful now and then. Caveat: sand lowers the pH of soil, so it's important to keep track and adjust if needed.

Thanks guys! What kind of sand do you use? And can it be used on any kind of plant? Right now I mostly have cacti/succulents, citrus, and the fig tree that started this problem.

Secondary question: What do I do with the infected Fig tree since there are apparently already eggs laid in the dirt. Should I just repot or do something else? Thanks!

What? ''...sand lowers the pH of soil...'' How?

Assuming the citrus & succulents are already in a citrus and succulents appropriate mix and their pH is okay, you can put a layer of sand on top of them with no problems. I use basic coarse grade contractors sand, and my lemon trees and aloe are gnat-free and healthy. Re the fig, though, I totally defer to the experts. Paul's advice to repot with a grittier mix sounds like a good idea, but the wisdom of doing that *now* depends on where exactly you are. If leaves are beginning to turn, wait til June. If you're in a low elevation subtropical latitude (like me), there's still time left.

@whypraytellwhy: sand's natural pH is fairly acidic. If you read about gardening in sandy coastal soils, you'll end up seeing a lot of info about adding compost and dolomitic lime to bring the soil into a more neutral pH range. Some plants adapt easily to acidic soils and others love them, but others are alkaline-only, so it's easiest just to test the soil pH and see where things are at.

Wouldn't there be a difference between using "sandy coastal soil " vs top-dressing with sand?

@rina One would hope, sure. But if you're using a well aerated mix, top-dressing sand gets watered down into it quite easily. It may not be enough to change the pH of the soil drastically/rapidly, but that depends on the size of the pot, the mix in the pot, the water, the fertilizer, and a zillion other potential factors. Since it's impossible to control for that many variables, it's FAR easier just to test the pH every once in a while. It's simple, cheap, takes no time at all, and gives you good data to make sure your potting mix matches your plant's needs.

A 2 pack of 80 litmus strips is less than $5 at Walmart, less on Amazon, more at CVS. Scoop out a spoonful of soil from your pot into a small cup, make a slurry with it and the water you use to water the plant, stick the strip in for a few seconds, pull it out, wait for result, compare to color key on package...and done! And 160 strips will last for ages if you store them in a ziploc in a dark place.

I really wouldn't know since I don't use sand...just very gritty mix that drains extremely well. Have not seen fungus gnats for few years...

If they're fungus gnats, they've laid eggs in all the pots by now, so you need to treat everything. Any plant with wet soil will harbor gnats, so there isn't anything wrong with the ficus.

And because fungus gnats are a sure indicator that your soil is staying wet all the time, I recommend against sand, which will only make the soil even more water retentive than it already is. Just get rid of them by taking all the plants with mosquito dunks and your problem is solved.

Ficuses (like a lot of house plants) really like their soil to dry out between waterings. That doesn't mean the top inch or two if the lower areas of the pot stay wet.

The composition of the sand will determine its affect on pH. A quartz based sand, for example, will be neutral and should not have any real impact. Calcareous sands, however, will tend to raise pH.