Juniper hair-cap moss is adapted to dry and acidic habitats, often found on exposed south-facing hillsides and burned areas. Plants can grow individually, or form large hummocks. Mosses in the genus Polytrichum have semi-vascularized stems, allowing them to grow taller than many other mosses. Its overall appearance is bristly, looking like a bottlebrush or conifer seedling. Plants have a single stem, 3-4 cm tall, with 1 cm long leaves held out in all directions (especially when wet). The leaves are narrow-lanceolate. When dry, leaves are tightly inrolled, leading to a long white line along the center of the leaf (look with a handlens). The white line is the key character separating it from other members of that genus, but it is also present in the small, bog-loving Polytrichum strictum. That species is covered in a mat of white rhizoids and the base of the stem. Plants often have reproductive structures, either a stalked capsule with a hairy cap (female) or a brown cup-like structure at the tip of the plant (male).
Plants are dioecious—male and female reproductive structures produced on separate plants. Frequently fertile, the female capsules release spores into the wind to establish new plants.