Glacial. It is an extremely slow grower; it can take up to 20 years for a new plant to fully mature from spores.
"This species is a vital indicator of mountain health but is currently under threat. In some regions like Maine, it is documented in only a few counties due to its scarce habitat. It is also highly vulnerable to airborne nitrogen pollution from vehicle exhausts, which is devastating populations on mountain summits. Protecting its remaining habitats from heavy hiking use and pollution is critical for its survival." alpine clubmoss
Very High. This is not for the casual gardener. It is notoriously difficult to transplant and extremely sensitive to dry soil. Glacial
Diphasiastrum sitchense (Rupr.) Holub - Rare Plants - Maine.gov alpine clubmoss