Aquatic and Wetland Vascular Plants of the Northern Great Plains
49. Asteraceae, the Aster Family
6. Conyza Less1. Conyza canadensis (L.) Cronq. -- Horseweed
Slender, erect, weedy annual (1)2-15(25) dm tall, with a taproot, usually strongly hirsute with spreading to ascending hairs; stem simple or rarely branched below the inflorescence. Leaves numerous, alternate, ascending, mostly linear to linear-oblanceolate or lower ones elliptic-oblong to oblanceolate and often coarsely toothed, the latter often early deciduous, 2-10 cm long, 2-10 mm wide, acute, tapered to a sessile or short-petiolate base, usually hispid on the margins and at least on the midrib beneath. Inflorescence a terminal, elongate cluster with ascending racemiform branches; heads usually numerous (few on depauperate specimens), inconspicuously radiate; involucre 3-4 mm high, the bracts in ca. 3 series, the inner linear and much longer than the outer; receptacle smooth, slightly convex; rays minute, white or pink-tinged, about equal to or slightly exceeding the involucre; disk corollas yellowish to light pinkish. Achenes flattened, 2-ribbed, ivory to light brown, 1-1.2 mm long, appressed-puberulent; pappus of numerous, dull white capillary bristles. Late Jul--Oct. A weed of disturbed ground that often invades shorelines and drawdown zones; common throughout; (Throughout s Can. and the U.S. and widely intro. elsewhere). Erigeron canadensis L.
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