that should be fine for the rhyzome. As long as it's mostly out of the gravel, it'll be good 
The dots on the underside of the fern leaves are spores. They use them to reproduce. Their "baby plants" and sex cells (sperm, eggs) actually need to swim, even the terrestrial ferns have this limitation! They're a very ancient plant.

The dots on the underside of the fern leaves are spores. They use them to reproduce. Their "baby plants" and sex cells (sperm, eggs) actually need to swim, even the terrestrial ferns have this limitation! They're a very ancient plant.