I’m in trouble with my 6-month-old cannabis plant. Unfortunately, I may have overwatered it, leading to moist soil that attracted a severe fungus gnat infestation. I need some good suggestions for safe treatments that will provide quick results without harming my plant. Any advice would be greatly appreciated!
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Let the soil dry up before watering again. Mosquito bits work well. They can be used to make a “tea” to drench and/or apply to top layer and water in over time. Probably won’t eradicate them, but will definitely help.
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I second mosquito bits. I mixed them like the directions and watered with the water. After 3 times, problem gone. I used fly paper for the adults.
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You can add a layer of garden sand or perlite (about 2 or 3 inches) to the top of your pot which will stop the adults from laying their eggs deep enough to survive in combonation with mosquito bits (described above) to kill the larvae. We even use them at work, just drop several into the 500 gallon tanks, and drip feed it into the pots. Sticky pads to catch the adults.
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I used 2-3 inches of perlite on the top of the soil and purchased some gnat traps( little sticky glue traps that look like plant markers) for plants. I also tried many other things, but the above is what worked for me.
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Perlite has too many gaps. The sand is tried and true and almost immediate. It suffocates and prevents a life cycle continuance. Blocks out, and traps within. No escape, no entry.
Mosquito bits are fantastic. Good to amend directly into the medium before grows. There are also powdered bti products on the market, which step it up a notch in effectiveness and time.
I’ve had severe outbreak with gnats before. Like a dozen black stickies, severe. They don’t necessarily hurt the plant, but these little a-holes to get stuck to trichomes. Do a good bud wash after chop. 
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If you have a fungus gnat infestation, they have been around for more than a month because they take around 3 weeks to complete their lifecycle.
Yellow sticky papers only catch adults and usually after they have bred and left the soil. Liquid traps with soap, vinegar and whatever else you want to add do the same as the sticky paper and only catch the adults.
If you use pots with holes for drainage, the fungus gnats simply go in through there instead of the top of the potting mix. So adding sand or perlite or whatever you want to the top won’t make much difference.
Drying the top few inches of potting mix out doesn’t do anything if you have an infestation. If you have any other plants in the house, the fungus gnats will be in them and outside, and they come inside every time you open the door or window. They are small enough to go through flyscreen so if you have fungus gnats inside, you probably have them outside too.
I have spent the last few years living with fungus gnats and have gone around the bend because of them. The only thing I found that got rid of them was a nematode that you water into the soil and the worms eat the baby fungus gnats. It breaks the lifecycle and gets rid of the fungus gnats. You can Google search for fungus gnat nematodes and order them online. They are safe and live in the soil for as long as the fungus gnats breed there. The only issue with the nematodes is they don’t work if the temperature is below about 16C. If the temperature is over 18C, they work great.
You can get a bacteria called BT (Bacillus thuringiensis), which is a natural occurring bacteria found in soil all around the world. It kills fungus gnats and doesn’t affect the plant. It works in cooler temperatures than the nematodes or mites are active in, and also works in warm weather.
There are also mites that kill fungus gnats and can be ordered online. They like warmer temperature similar to the nematodes (18C plus).
These 3 control methods take a couple of weeks to work and you don’t see any change for at least a week.
If the plant is half way through flowering, I probably wouldn’t bother doing anything except trying to control the adult insects (sticky paper). If it has just started flowering or you have a number of plants, get the nematodes or bacteria or both. And treat all your plants at the same time otherwise the gnats will reinfect the treated potting mix a few months later when the worms have died off from lack of food.
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And it could be organic