Annuals, biennials, or perennials, 5-150 cm (sometimes with ± branched, woody caudices or stout rhizomes). Stems 1-30+, erect, unbranched or branched, green throughout to purple-red-tinted proximally or distally to purple-red-tinted throughout, glabrous or ± hairy. Leaves alternate; blades simple or 1-2-pinnately lobed, ultimate margins entire or toothed, faces glabrous or hairy, usually ± gland-dotted (often in pits). Heads radiate [discoid], borne singly or in paniculiform to corymbiform arrays. Involucres subhemispheric, hemispheric, globoid, campanulate, or urceolate, (2.5-)6-32 mm diam. Phyllaries persistent (or inner falling), usually (6-)16-30(-40) in 2 series and unequal, sometimes 28-50 in 2-3 series and subequal (usually spreading to erect in fruit). Receptacles usually hemispheric, globoid, ovoid, or conic (flat in H. ambigens), smooth or pitted, epaleate. Ray florets usually (3-)8-13(-16), sometimes 14-34 , pistillate, fertile; corollas (usually withering after flowering, falling early or tardily) yellow or yellow-orange to orange (laminae fan-shaped, lobes 3-5). Disc florets usually 25-150(-400+), usually bisexual and fertile (6-15, functionally staminate in H. ambigens); corollas yellow to yellow-brown proximally, yellow distally, tubes shorter than cylindric to cylindric-campanulate throats, lobes 5, ± deltate. Cypselae obconic or obpyramidal, glabrous or hairy; pappi 0, or persistent, of 2-11(-15) usually aristate scales. x = 15.
Subgenus Phileozera (Buckley) Cockerell includes Hymenoxys odorata and H. chrysanthemoides (Kunth) de Candolle (Mexico) (M. W. Bierner 2001). Subgenus Plummera (A. Gray) Bierner comprises only Hymenoxys ambigens (Bierner 1994, 2001). Subgenus Dugaldia (Cassini) Bierner includes Hymenoxys hoopesii, H. integrifolia (Kunth) Bierner (Mexico and Guatemala), and H. pinetorum (Standley) Bierner (Mexico; Bierner 1994, 2001). Subgenus Rydbergia (Greene) Bierner comprises Hymenoxys brandegeei, H. grandiflora, and H. insignis (Mexico) (Bierner 2001, 2005). Hymenoxys bigelovii is the sole member of subg. Macdougalia (A. Heller) Bierner (Bierner 2001, 2004). Hymenoxys richardsonii, H. subintegra, H. cooperi, H. lemmonii, H. rusbyi, H. jamesii, and H. brachyactis belong to subg. Picradenia (Hooker) Cockerell (Bierner 2001). J. L. Anderson et al. (1996) and M. W. Bierner and R. K. Jansen (1998) provided evidence that Hymenoxys helenioides is a hybrid between H. hoopesii and H. richardsonii var. floribunda. Bierner (2001) recognized H. helenioides as a species because he was unable to determine whether all of the plants were F1 hybrids or at least some of them had given rise to breeding populations. Plants identified as Hymenoxys anthemoides, a discoid, South American annual that resembles H. odorata, were reported as 'adventive on ballast,' Mobile County, Alabama, by C. T. Mohr (1901).
Heads mostly radiate, the rays pistillate and fertile, broad, yellow, mostly 5-35; invol bracts permanently erect, partly or wholly herbaceous, in 2-3 similar or sharply differentiated series; receptacle naked, hemispheric to merely convex; disk-fls yellow, perfect and fertile; style-branches flattened, with introrsely marginal stigmatic lines, truncate, minutely penicillate; achenes turbinate, hairy, mostly 5-angled; pappus of a few (typically 5) hyaline, often awned scales; aromatic herbs with alternate or all basal, entire to ternate or pinnatifid, commonly punctate lvs. 20, New World.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.