Perennials, 5-90(-100) cm (in ± extensive, open clones or in dense clusters); long-rhizomatous, white, with swollen apical buds, and/or with branched, woody caudices (O. reticulata), roots fibrous or thick and fleshy (O. reticulata). Stems erect, simple, densely villosulous (hairs long, flattened, with ± colored cross-walls), sometimes stipitate-glandular distally. Leaves cauline; alternate; sessile or short-petiolate; blades (1-nerved, abaxial veins raised, ± reticulate), ovate- or obovate-elliptic to elliptic-oblong, obovate to oblanceolate, or linear-lanceolate to linear, proximal reduced to scales, mid largest, gradually reduced to bracts distally, margins entire (then revolute) or serrate (mainly distally), faces sparsely hispid, villosulo-puberulent, or villosulous, sometimes scabrous adaxially, sparsely short-stipitate-glandular (hairs with yellow to orange, resin heads). Heads radiate, in ± loose corymbiform arrays or borne singly (nodding in buds). Involucres cylindro-campanulate, (5-10 ×) 6-12 mm. Phyllaries 25-50 in 3(-4) series, 1-nerved (slightly keeled) lance-ovate (outer) to linear (inner), unequal, membranous, bases not indurate, margins hyaline, chartaceous, erose, with green zones along midveins, lance-linear distally (apices acute), abaxial faces glabrous or sparingly puberulent, sparingly or not stipitate-glandular. Receptacles plano-convex, pitted or smooth, epaleate. Ray florets (5-)7-25, pistillate, fertile; corollas white or pink (laminae not coiling). Disc florets (10-)14-35, bisexual, fertile; corollas pale or pinkish yellow, reddening at maturity, abruptly or gradually ampliate, tubes shorter to longer than tubular to narrowly campanulate throats, lobes 5, spreading or reflexed, triangular to lanceolate; style-branch appendages narrowly lanceolate. Cypselae fusiform or fusiform-obconic (bases stipitate), slightly compressed, 5-8-nerved with 2 thicker lateral nerves, faces ± densely gland-dotted (sessile); pappi persistent, of 40-55+ tan, barbellulate, (inner) apically attenuate or ± clavate bristles in (2-)3 series (outer usually shorter, seldom present in O. nemoralis, inner longer). x = 9.

G. L. Nesom (1994b) summarized the taxonomic history, composition, and affinities of Oclemena, which he considered a monophyletic group. He suggested affinity to the eastern North American Doellingeria. Differences with Doellingeria include heads nodding in bud, 1-nerved, acute phyllaries not proximally indurate, triangular disc corolla lobes, fusiform and slightly compressed cypselae shorter than involucres, with 5-8 non-resinous nerves, and faces with sessile glandular hairs. Xiang C. and J. C. Semple (1996), using RFLPs on cpDNA data, showed O. reticulata associated with Doellingeria instead of Oclemena. The current composition of Oclemena is supported by phylogenetic analysis of ITS data (Semple et al. 2002). Oclemena appears to be among the basal lineages of the North American clade of Astereae. Genetic proximity between these basal lineages is illustrated further by the discovery of an F 1 hybrid between O. nemoralis and D. umbellata in upper Michigan (L. B. Gerdes 1998; Nesom 2001b). This suggests that these genera have preserved the symplesiomorphic ability to hybridize. Their relative morphologic conservatism may have facilitated survival of the F 1 progeny. Given its frequent occurrence in northeastern North America and occasional confusion with other taxa (e.g., Eurybia radula), the hybrid O. ×blakei also is described below.