I grow all my semps outdoors in raised beds. The top 4" of the beds are bank-run road gravel - no additives. I'm seeing the beginnings of problems with moss growths and have had no luck controlling it.
I'd love to have suggestions beyond hand-picking it out. That's what I've been doing and it's abominably labor-intensive and only somewhat effective.
Kate, I'm adding a photo to your post so it will have a thumbnail showing in the list of threads. If you have another photo you would rather have let me know, it's easy to change. Valleylynn : )