For indoors seed stating, "fine" is what I would use for very fine seeds needing to be sown on the surface. For anything else, I usually assume that coarse vermiculite chunks will provide enough humidity, and allow plenty of air to get in. I feared that fine verm iculite might encouage rot, but that may be a bogus fear.
I just know that I killed an entire tray of petunias - 128 cells and 200-300 petunia seeds - by trying to sown them "on top" of a bark mix that was too coarse. They fell into the cracks and were never seen again.Next year I'll be using a layer of fine vermiculite under (and maybe over) petunia, alyssum and lobellia seeds started indoors in propagation cells.
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