I have used play sand for the last 2.5 years and I haven't had a gnat since then. All you need to do is put a 2 inch layer of sand on top of the soil and its now 100% impossible for the gnats to lay their eggs. Takes about two weeks to get rid of all of them. Sand is $3 a bag. I throw out the sand when I transplant. Some dirt will get to the top but it's not enough for the gnats to lay eggs in because it dries out in between watering. I've been doing this for a while with 100% success. Be careful when pouring water onto the sand. You can dig a hole through the sand easily and get lots of dirt on top. Best thing to do is place a small bowl on top of the sand and poor the water into the bowl. This displaces the water and is an easy fix. I would never put any pesticide into the soil and I would never use a spray on the plants after the first week of bud. This technique stops all bugs from invading your soil and it's 100% safe.
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