Annuals, 10-120 cm. Stems ± erect to prostrate. Leaves mostly cauline (at flowering); proximal opposite (often in winter-spring rosettes), most alternate; ± sessile; blades oblanceolate to linear or lance-linear, proximal usually 1-2-pinnatifid, ultimate margins toothed or entire (sometimes bristly-ciliate), (apices of distal leaves usually spine-tipped) faces glabrous, scabroso-hirtellous, ± hirsute, or villous, often glandular as well. Heads radiate, borne in glomerules or ± spiciform-paniculiform or ± umbelliform arrays. Peduncular bracts: pit-glands and tack-glands 0 (apices usually spine-tipped, sometimes apiculate). Involucres ± obconic or urceolate, 3-8+ mm diam. (subtended by calyculi of 5-12+ usually spine-tipped bractlets). Phyllaries falling or persistent, 5-75+ in 1 series (lanceolate to lance-attenuate or oblanceolate, herbaceous, each usually 1/2 enveloping subtended ray floret proximally, abaxially scabroso-hirtellous, hirsute, or villous and/or glandular, apices often spine-tipped). Receptacles flat to convex, setulose, paleate (paleae persistent, subtending all or most disc florets, distinct, phyllary-like, more scarious). Ray florets 5-75+, pistillate, fertile; corollas yellow. Disc florets 6-200+, usually functionally staminate, rarely bisexual and fertile; corollas yellow, tubes shorter than funnelform throats, lobes 5, deltate (anthers reddish to dark purple or yellow to brownish; styles glabrous proximal to branches). Ray cypselae ± compressed (abaxially gibbous, basal attachments basal or oblique, apices beaked or elevated adaxially, faces glabrous); pappi 0. Disc cypselae usually 0; pappi (of disc florets) 0 or of 3-12 linear, oblanceolate, or subulate scales. x = 13.
Following B. G. Baldwin (1999b), Centromadia is treated here as distinct from Hemizonia; Centromadia is more closely related to Calycadenia, Deinandra, Holocarpha, and Osmadenia than to Hemizonia in the strict sense (S. Carlquist et al. 2003). Taxa of Centromadia are self-incompatible and of low to high interfertility (C. S. Venkatesh 1958). Most occur in somewhat poorly drained or alkaline sites.
