Catmint, fungus gnats love moisture. Keeping your soil on the dry side will help a lot. I think I've read that some people sprinkle cinnamon on top of the soil. Bayers 3 in 1 is a good systemic. Neem is also effective. I found this info for those who choose not to use insecticide: You can also place inch-long pieces of raw potato on the soil surface. If fungus gnat larvae are present, they will migrate to the potato within four hours for a free lunch. Look for the tiny black-headed larvae both under and on the potato piece, then dispose of the potato.
I also found that fungus gnat larvae will die on contact with hydrogen peroxide. Once the top layer of soil is dry, mix one part hydrogen peroxide with four parts water. Using this solution, water your plants as you normally would. The fungus gnat larvae will die on contact with the hydrogen peroxide, but the solution will not hurt your plants so long as it is mixed correctly. I hope this helps.
