Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads radiate, the rays 3–13, pistillate and fertile (rarely neutral), white, sometimes pink or yellow, short and broad; invol bracts imbricate in several series, dry, with scarious or hyaline margins and often greenish midrib; receptacle conic or convex, chaffy; disk-fls mostly 10–75, perfect and fertile; style-branches flattened, truncate, penicillate; achenes compressed parallel to the invol bracts, callous-margined, glabrous; pappus none; perennial herbs with alternate, subentire to pinnately dissected lvs and several or many relatively small heads in a ± corymbiform infl. 75, N. Hemisphere, mainly Old World.
In addition to the following spp. [A. ptarmica; A. sibirica; & A. millefolium], A. tomentosa L., 2–4 dm, with bright yellow fls, casually escapes from cult.