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).