Erigeron canadensis L. (syn. Conyza canadensis), horseweed. Annual, taprooted, rosetted, 1—several–stemmed at base, unbranched or with a cluster of principal axes unbranched below inflorescence, erect (to ascending), in range 20—100+ cm tall; shoots with basal leaves and cauline leaves, leafy throughout, above midplant often with tufts of leaves from unexpanded axillary shoots, sparsely stiff–strigose to hirsute.
Stems cylindric, to 10 mm diameter, with 3 veins descending from each leaf but scarcely raised, greenish striped with yellow–green to tannish white or orangey tan veins.
Leaves helically alternate, simple, sessile and indistinctly petiolate (rosette and lower cauline leaves), without stipules; petiole to 7 mm long, weakly differentiated form tapered leaf base; blade linear to oblanceolate–linear, 7—70 × 1—6 mm, tapered at base, entire to remotely serrate on 1 margin + stiff–strigose to coarsely ciliate on other margin, pinnately veined with midrib raised on lower surface, strigose especially along principal veins.
Inflorescence heads, in terminal, open, paniclelike arrays, array sometimes with lower branches overtopping main axis, 120—500+ × 40—260 mm, several—many branched from rachis, head obscurely radiate, ca. 3 mm across, in range with 24—40 pistillate peripheral flowers in 2+ series and ca. 10—15 bisexual disc flowers, bracteate, sparsely hairy; bract subtending lateral branch leaflike, 10—40 mm long; lateral branches producing ascending cymelike clusters of several—many heads; bract subtending peduncle, decreasing upward; peduncle 1—15 mm long, often low–ridged, strigose, with 1—several linear–lanceolate bracts (= bracts subtending future peduncles); involucre bell–shaped, 2.7—4 × ± 2.5 mm becoming wider in fruit, phyllaries 23—30, unequal and graduated in 3—4 series, linear, 0.7—3.5 × 0.25—0.4 mm, the shortest in outer series, green with translucent midvein, white–membranous on margins, fringed at acute tip, glabrous or sparsely strigose along midvein, strongly reflexed in fruit; receptacle flat, 1—1.5 mm diameter, without bractlets (paleae), pitted with short teeth surrounding each ovary.
Peripheral flower pistillate, bilateral, < 0.2 mm across; calyx (pappus) of 19—22 capillary bristles in 1 whorl, fine, 2.2—2.5 mm long, tawny; corolla 2–lobed or acute at tip; tube cylindric, 2 mm long, light green, with a few hairs at tip; limb erect, slender, ca. 1 × 0.25 mm, white (sometimes at tip aging reddish) but greenish at base; stamens absent; pistil 1; ovary inferior, oblanceoloid–ellipsoid compressed side–to–side, 1 mm long, finely short–hairy, 1–chambered with 1 ovule; style exserted, 2—2.5 mm long, white, 2–branched, the branches spreading.
Disc flower bisexual, radial, ± 0.4 mm across, 2.5—2.8 mm long; calyx (pappus) of 20—25(—29) capillary bristles in 1 whorl, 2.2—2.8 mm long, white; corolla (4—)5–lobed, 2.2—2.8 mm long; tube + throat cylindric, greenish white; lobes acute, 0.1—0.25 mm long, pale greenish yellow with greenish marginal vein; stamens (4—)5, fused to corolla at top of tube, included; filaments < 0.5 mm long; anthers fused into cylinder surrounding style, basifixed, dithecal, ± 0.7 mm long, pale yellow, longitudinally dehiscent; pollen pale yellow; pistil 1; ovary inferior, oblanceoloid–ellipsoid compressed side–to–side, 1 mm long, finely short–hairy, 1–chambered with 1 ovule; style 2—2.5 mm long, 2–branched, the branches exserted.
Fruits cypselae, monomorphic, with spheroidal array of pappus ca. 8 mm diameter, monomorphic; fruit body oblong to oblanceoloid, 1—1.3 mm long, pale tan to light gray–brown, sparsely and finely sericeous on faces; pappus of 19—25(—29) whitish, ascending, fine capillary bristles, 2—2.7 mm long.
A. C. Gibson & B. A. Prigge