Cypripedium arietinum, the ram's head lady's slipper, is a rare orchid that grows in lightly shaded areas with calcareous soils. It is characteristic of the alvars around the Great Lakes in North America, as well as in New England. In Canada, it is found from Quebec to Saskatchewan, plus an isolated population in Nova Scotia, where it grows on gypsum based soils, 330 km away from the nearest population in Maine.
Cypripedium arietinum is a small lady's slipper which typically has 3, but sometimes 4-5, leaves. This species has a single flower on each stem having divided lateral sepals and a unique hairy pouch shaped like an funnel. The purplish-red flower has light venation and is white at the lip.
The plant grows to 10–40 cm (4–16 in), and the flowers may reach 1–2 cm (1⁄2–3⁄4 in). It flowers from May to June, producing brownish-green flowers with a pink and white lip.
The ram's head lady's slipper is difficult to cultivate, and rarely survives transplantation to a garden from the wild. It should never be removed from any natural area.
Cypripedium arietinum is considered rare to extremely rare in all locations where it occurs. More specifically, it is rare in Ontario and rare in Manitoba. The ram's head lady's slipper is a threatened plant species in other areas within its range, including Wisconsin, Minnesota, Michigan, and Saskatchewan. It is believed to be extirpated in Connecticut. This species is listed as an endangered species in Nova Scotia.
- "Cypripedium arietinum". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 1 July 2016.
- Catling, P.M.; Brownell, V.R. (1999). "Alvars of the Great Lakes Region". In Anderson, R. C.; Fralish, J. S.; Baskin, J. M. (eds.). Savannas, Barrens and Rock Outcrop Communities of North America. Cambridge: Cambridge University Press. pp. 375–391. ISBN 9780511574627.
- Blaney, S.; Mazerolle, D. (2007). Nova Scotia Provincial Status Report on Ram's-Head Lady Slipper (Cypripredium arietinum R. Br.). Nova Scotia Department of Natural Resources.
- "Cypripedium arietinum". County-level distribution map from the North American Plant Atlas (NAPA). Biota of North America Program (BONAP). 2014.
- Oldham, M.J.; Brinker, S.R. (2009). Rare Vascular Plants of Ontario (4th ed.). Peterborough, Ontario: Natural Heritage Information Centre, Ontario Ministry of Natural Resources.
- "Species at Risk Overview". Nova Scotia Department of Natural Resources. Retrieved 15 July 2018.