Muscol. Recent., suppl. 4: 79. 1818,.
Plants forming dark or grayish to yellowish green, yellowish to blackish brown tufts or patches, usually hoary when dry. Stems to 15 cm or more, radiculose at the base or not. Leaves crowded, erect-appressed when dry, erectopatent when wet, straight to falcate-secund, linear-lanceolate, 3–5 × 0.6–0.9 mm; margins recurved to revolute for 1/2–2/3 the leaf length, entire proximally the echlorophyllose part formed by decurrencies from the awn; apices gradually tapering to a long and slender awn, not decurrent, canaliculate distally, concave to broadly carinate proximally, awn hyaline, densely papillose, broadly and evenly decurrent to 1/4–1/3 along the margins, with decurrencies consistently flat, erosodentate to nearly entire and short, to 30 µm, sharp or blunt teeth, distinctly papillose throughout, spreading at a 40–90º angle; costa in tranverse-section rectangular to reniform in outline, strongly convex abaxially, 75–100 µm wide basally; basal laminal cells long-rectangular, 40–90 × 7–8 µm, strongly nodulose, forming a broad orange strip along the insertion; medial cells becoming long-rectangular to linear, 50–60 × 6–8 µm wide; distal laminal cells short-rectangular, (10–) 15–40 × 7–9 µm. Seta 1–3 per perichaetium, brown to reddish-brown, 3–7 (–10) mm, erect, flexuose. Capsule brown, 1–1.7 mm, smooth, glistening; peristome teeth (300–) 500–700 (–900) µm, reddish-brown, arising from a low basal membrane, 2-fid almost to the base into filiform, terete, densely spiculate-papillose divisions. Spores 8–12 µm.
Habitat: Dry, exposed areas, mostly with high light intensity, acidic or seldom calciferous soil and rocks, boulders, cliffs, ledges, scree and in fellfields, polar tundra and tundra-like barrens in mountains, hummocks in peatland and moorland, over raw earth of bog margins
Elevation: low to high elevations (0-2300 m)
Greenland, Alta., B.C., Man., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Yukon, Alaska, Calif., Idaho, Maine, Mont., N.H., Oreg., Wash., Central America (Costa Rica), South America (Argentina), South America (Chile), South America (Colombia), South America (Ecuador), South America (Peru), Europe, arctic and temperate Asia (including Borneo), arctic and temperate Asia (Java), arctic and temperate Asia (New Guinea), arctic and temperate Asia (Sumatra), South Africa, Atlantic Islands (Azores), Atlantic Islands (Canary Islands), Atlantic Islands (Falkland Islands), Atlantic Islands (Gough Island), Atlantic Islands (Iceland), Atlantic Islands (Madeira), Atlantic Islands (South Georgia), Atlantic Islands (Tristan da Cunha), Indian Ocean Islands (Heard Island), Indian Ocean Islands (Îles Crozet), Indian Ocean Islands (Îles Kerguelen), Indian Ocean Islands (Prince Edward Islands), Indian Ocean Islands (Réunion), Pacific Islands (Hawaiian Islands), Pacific Islands (New Zealand), Australia, Antarctica (Deception Island)
Racomitrium lanuginosum is widely distributed throughout the Nearctic and Greenland, where it reaches the highest possible latitudes, becoming rare and scattered southwards. It usually occupies habitats of varying moisture regimes, but exhibits a tendency for growing in exposed, dry, and insolated situations. The often extensive and tumid patches of it found on rocks and soil are mostly hoary when dry. This is due to the very long hyaline awns that at once separate it from all other species of the broadly conceived Racomitrium. The shape of the hair-points is unique not only in the Grimmiaceae but among the mosses. Niphotrichum species often have a similar hoary appearance due to their long, papillose, hair-pointed leaves, but they differ from R. lanuginosum in having non-decurrent awns and tall, stout, conical papillae distributed over the leaf cell lumina, as well as large and often decurrent hyaline alar cells.
The laminal papillae of Racomitrium lanuginosum are identical to those of the genus Codriophorus, but species of that genus are usually readily distinguished by their muticous leaves. The only exception is C. varius, which often has pilose leaves but the awns are non-decurrent, smooth to faintly denticulate, and never papillose. Moreover, it has long-cylindric capsules, smooth setae, and very long, 1–1.8 mm peristome teeth, but in general sporophytes are produced infrequently in R. lanuginosum.
"um" is not declared as a valid unit of measurement for this property.