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Published In: Bulletin of the Torrey Botanical Club 70(6): 632. 1943. (Bull. Torrey Bot. Club) Name publication detailView in Botanicus

Project Name Data (Last Modified On 7/9/2009)
Acceptance : Accepted
Project Data     (Last Modified On 7/9/2009)
Status: Native


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1. Conyza canadensis (L.) Cronquist (horseweed, Canada fleabane, hog weed)

Erigeron canadensis L.

Pl. 232 a, b; Map 968

Stems solitary, very variable from less than 5 cm to more than 250 cm long, erect or strongly ascending, sparsely (in small plants) to densely branched above the midpoint, glabrous or more commonly sparsely to densely pubescent with mostly spreading, often broad-based white hairs. Leaves 0.5–10.0 cm long, sessile or short-petiolate, the blade linear to oblong-lanceolate or oblanceolate, glabrous or more commonly sparsely to moderately hairy, mostly along the margins and midvein. Inflorescences short (in small plants) to elongate (in larger plants) panicles, usually dense and well developed but in small plants appearing as short racemes and/or loose clusters at the branch tips. Involucre 2.5–4.0 mm long, the bracts glabrous or nearly so. Ray florets 20–40, the corolla white or less commonly pinkish-tinged. Disc florets 8–28. 2n=18. June–November.

Common throughout the state (throughout the U.S.; Canada, Mexico). Banks of streams and rivers, upland prairies, sand prairies, glades, and openings of mesic upland forests; also pastures, old fields, fallow fields, crop fields, gardens, railroads, roadsides, and open, disturbed areas.

Horseweed is an extremely variable plant, having been recorded to flower from a 3 cm stem as well as those nearly 3 m tall. On larger plants, the leaves are usually very numerous. In Missouri, the species mostly has been collected in highly disturbed habitats. It also is a serious crop weed, especially of corn and soy beans. In keeping with its variable and adaptive nature, strains that are resistant to various herbicides have been recorded (see http://www.weedscience.org), including races that are resistant to glyphosate herbicides such as Roundup (Van Gessel, 2001; Rogers, 2003).


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1 1. Stems sparsely to more commonly moderately or densely hairy; involucral bracts with green or white tips ... 1A. VAR. CANADENSIS

Conyza canadensis (L.) Cronquist var. canadensis
2 1. Stems glabrous or nearly so; involucral bracts with dark purple tips ... 1B. VAR. PUSILLA Conyza canadensis var. pusilla


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