Oclemena is a genus of flowering plants in the tribe Astereae within the family Asteraceae. All species in the genus are native to eastern North America. The species were originally included in the genus Aster so they are often referred to simply as asters. Since their flower heads are nodding in bud, they are sometimes called nodding-asters.
Oclemena species are perennial, herbaceous plants that propagate via a swollen tuber at the tip of a slender, elongated rhizome. Stems are erect and unbranched, with dense long hairs. Stem leaves are sessile (or short-petiolate) and alternate. Leaf blades are sparsely dotted with short glandular hairs, each with a yellow to orange resin head. Leaf margins are either serrate (O. acuminata) or both entire and revolute. The inflorescence is either a single flower head (O. nemoralis) or a corymb of 2âÂÂ46 flower heads on long, slender peduncles, nodding in bud (except O. reticulata). A flower head has 7âÂÂ25 ray flowers, white or pink, and 14âÂÂ35 disc flowers, pale or pinkish yellow, reddening at maturity. The chromosome base number is x=9.
The hybrid Oclemena ÃÂ blakei is intermediate in appearance between its parents, Oclemena acuminata and Oclemena nemoralis.
In 1903, the American botanist Edward Lee Greene established genus Oclemena by segregating two species, Aster acuminatus and Aster nemoralis . Greene, initially drawn to this group of plants by the nodding habit of their flower heads in bud, had been using the name Oclemena acuminata on the labels of herbarium specimens since 1897. In 1995, the American botanist Guy L. Nesom segregated two additional taxa, Aster nemoralis var. blakei and Aster reticulatus . Nesom considered Oclemena to be monophyletic but closely related to Doellingeria . A major treatment of genus Oclemena appeared in Flora of North America in 2006:
, the generic name Oclemena is widely accepted.
Oclemena belongs to the North American clade of the tribe Astereae, as a basal member of one of its main branches.
Oclemena species are native to eastern North America. Oclemena acuminata is the most wide-ranging species, from the Appalachian Uplands of Newfoundland to the southern tip of the Appalachian Mountains in the U.S. state of Georgia. In contrast, Oclemena reticulata îs restricted to the extreme southeastern United States where other species of Oclemena are not found. Oclemena nemoralis and Oclemena àblakei are boreal species that prefer the cold acidic bogs of eastern Canada and northeastern United States.