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. Flowering year round, mostly summer-fall. Disturbed places; 0-2000 m; Alta., B.C., Man., N.S., Ont., P.E.I., Que., Sask.; Ala., Ariz., Ark., Calif., Colo., Conn., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; Central America; introduced in South America, Europe, Asia, Africa. 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.
Annual or winter-annual, 1-15 dm, simple or nearly so to the infl; lvs numerous, ±pubescent, or at least coarsely ciliate at the base, oblanceolate to linear, acute, toothed (especially the lower) or entire, gradually reduced upwards, the cauline ones to 8 cm נ8 mm, the basal ones larger and broader but generally deciduous; heads, except in depauperate plants, numerous m a long and open infl; invol 3-4 mm, glabrous or with a few small scattered hairs, the bracts strongly imbricate, brown or with a brown or pale midvein and greener sides; rays mostly 25-40, commonly ca 34, white or sometimes pinkish, 0.5-1.0 mm, equaling or shortly surpassing the style and pappus; 2n=18. A weed in waste places and old fields, throughout the U.S. and s. Can. and to trop. Amer. Late summer and autumn. (Leptilon c.; Erigeron c.) Var. canadensis, our common phase, has the stem coarsely spreading-hirsute, and lacks purple tips on the bracts. Var. pusilla (Nutt.) Cronquist, chiefly along the coast or on the coastal plain, from Conn. to trop. Amer., has the stem glabrous or nearly so, and some of or all the invol bracts are minutely purple-tipped. (Leptilon p.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.