Like many 400-million-year-olds, moss is particular about its environment. Shade or semi-shade is usually a necessity. So are a consistent source of ambient moisture and vigilant maintenance to keep it free of weeds and debris (because mosses are nonvascular—no roots—they rely on their leaves for transportation of nutrients and moisture). Experts suggest setting down netting on top of moss in the fall and regularly emptying it of fallen leaves.
Hair cap moss (Polytrichum commune) prefers medium shade to partial sun, and sandy, acidic soils. If the soil is sufficiently moist, it can tolerate almost full sun. This variety can handle light foot traffic.