I used Jiffy seed starting mix last year with good results and no fungus gnats. I freeze my seed starting mix for several days during the winter to kill fungus gnat eggs and I have not had a single fungus gnat since I started doing this. I used to have them every year.
I was shopping for starting mix earlier this year and saw an organic brand that was not MG at Ace Hardware and there were fungus gnats flying around the bags right on the shelf!
These bags of mix have small holes for ventilation and the fungus gnats can easily fit through those holes and lay eggs in all of the bags.
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