Canadian Horseweed

Conyza canadensis is thought to be native to North America and is now widely adventive, e.g., in South America, Europe, Asia, and Africa. Plants with stems glabrous and phyllaries red-tipped are sometimes treated as var. pusilla; similar plants with stems glabrous and phyllaries stramineous (not red-tipped) are sometimes treated as var. glabrata.

Plants erect, (3–)50–200(–350+) cm, branched mostly distally. Leaves: faces usually glabrate (proximal margins ± ciliolate, hairs usually stiff, spreading and hispid on nerves, hairs erect); proximal blades oblanceolate to linear, 20–50(–100+) × 4–10(–15+) mm, toothed to entire; distal similar, smaller, entire. Heads usually in paniculiform, sometimes corymbiform arrays. Involucres 3–4 mm. Phyllaries usually glabrous, sometimes sparsely strigose (margins chartaceous to scarious); outer greenish to stramineous, lanceolate to linear, shorter; inner stramineous to reddish, lance-attenuate to linear. Receptacles 1–1.5(–3) mm diam. in fruit. Pistillate florets 20–30(–45+); corollas ± equaling or surpassing styles, laminae 0.3–1 mm. Disc florets 8–30+. Cypselae uniformly pale tan to light gray-brown, 1–1.5 mm, faces sparsely strigillose; pappi of 15–25, white bristles 2–3 mm. 2n = 18.

A cosmopolitan weed of N. American origin.

450-2500 m

Erigeron canadensis Linnaeus, Sp. Pl. 2: 863. 1753 (as canadense); Conyza canadensis var. glabrata (A. Gray) Cronquist; C. canadensis var. pusilla (Nuttall) Cronquist; C. parva Cronquist; E. canadensis var. pusillus (Nuttall) B. Boivin