Dryas octopetala, the mountain avens, eightpetal mountain-avens, white dryas or white dryad, is an Arctic–alpine flowering plant in the family Rosaceae. It is a small prostrate evergreen subshrub forming large colonies. The specific epithet octopetala derives from the Greek octo (eight) and petalon (petal), referring to the eight petals of the flower, an unusual number in the Rosaceae, where five is the normal number. However, flowers with up to 16 petals also occur naturally.
The stems are woody, tortuous, with short, horizontal rooting branches. The leaves are glabrous above, densely white-tomentose beneath. The flowers are produced on stalks 3–10 cm (1.2–3.9 in) long, and have eight creamy white petals - hence the specific epithet octopetala. The style is persistent on the fruit with white feathery hairs, functioning as a wind-dispersal agent. The feathery hairs of the seed head first appear twisted together and glossy before spreading out to an expanded ball which the wind quickly disperses.
Dryas octopetala has a widespread occurrence throughout mountainous areas where it is generally restricted to limestone outcrops. These include the entire Arctic, as well as the mountains of Scandinavia, Iceland, the Alps, Carpathian Mountains, Balkans, Caucasus and in isolated locations elsewhere. In Great Britain it occurs in the Pennines (northern England), at two locations in Snowdonia (north Wales), and more widely in the Scottish Highlands; in Ireland it occurs on The Burren and a few other sites. In North America it is found in Alaska, most frequently on previously glaciated terrain, and through the Canadian rockies  reaching as far south as Colorado in the Rocky Mountains. It grows in dry localities where snow melts early, on gravel and rocky barrens, forming a distinct heath community on calcareous soils.
The Younger Dryas, Older Dryas and Oldest Dryas stadials are named after Dryas octopetala, because of the great quantities of its pollen found in cores dating from those times. During these cold spells, Dryas octopetala was much more widely distributed than it is today, as large parts of the northern hemisphere that are now covered by forests were replaced in the cold periods by tundra.
D. octopetala is cultivated in temperate regions as groundcover, or as an alpine or rock garden plant. It has gained the Royal Horticultural Society's Award of Garden Merit. The leaves are occasionally used as a herbal tea.
- Govaerts R. "Dryas octopetala L.". Plants of the World Online. Board of Trustees of the Royal Botanic Gardens, Kew. Retrieved 14 December 2020.
- BSBI List 2007 (xls). Botanical Society of Britain and Ireland. Archived from the original (xls) on 2015-06-26. Retrieved 2014-10-17.
- "Dryas octopetala". Germplasm Resources Information Network (GRIN). Agricultural Research Service (ARS), United States Department of Agriculture (USDA).
- Harrison, Lorraine (2012). RHS Latin for gardeners. United Kingdom: Mitchell Beazley. p. 224. ISBN 9781845337315.
- "Plants of Canada Database - Dryas octopetala". Government of Canada. Archived from the original on 7 March 2016. Retrieved 21 February 2016.
- "Dryas octopetala". Royal Horticultural Society. Retrieved 16 February 2020.
- "AGM Plants - Ornamental" (PDF). Royal Horticultural Society. July 2017. p. 33. Retrieved 6 February 2018.
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- Elkington, T. T. (1971). "Dryas Octopetala L.". Journal of Ecology. 59 (3): 887–905. doi:10.2307/2258146. JSTOR 2258146.
- Fisher, P. J.; et al. (1995). "Fungal Endophytes of Dryas octopetala from a High Arctic Polar Semidesert and from the Swiss Alps". Mycologia. 87 (3): 319–323. doi:10.2307/3760828. JSTOR 3760828.
- Skrede, Inger; et al. (2006). "Refugia, differentiation and postglacial migration in arctic-alpine Eurasia, exemplified by the mountain avens (Dryas octopetala L.)". Molecular Ecology. 15 (7): 1827–1840. doi:10.1111/j.1365-294X.2006.02908.x. PMID 16689901. S2CID 44667191.