5. Parthenium integrifolium Linnaeus, Sp. Pl. 2: 988. 1753.
Parthenium auriculatum Britton; P. hispidum Rafinesque; P. hispidum var. auriculatum (Britton) Rollins; P. integrifolium var. auriculatum (Britton) Cornelius ex Cronquist; P. integrifolium var. henryanum Mears; P. integrifolium var. hispidum (Rafinesque) Mears; P. integrifolium var. mabryanum Mears; P. radfordii Mears
Perennials, 30–60(–100+) cm. Leaf blades ovate to lanceolate, 30–350+ × 20–120+ mm, margins usually crenate to serrate, sometimes coarsely toothed or somewhat lobed (then mostly toward bases), faces hispid to hirtellous or ± scabrous, gland-dotted. Heads radiate, borne in corymbiform to paniculiform arrays. Peduncles 1–8(–12+) mm. Phyllaries: outer 5(–6) lanceolate to broadly ovate, 3–5 mm, inner 5(–6) ± orbiculate, 4–6 mm. Pistillate florets 5(–6); corolla laminae ovate to oblong or orbiculate, 1–2+ mm. Disc florets 15–35+. Cypselae ± obovoid, 3–4+ mm; pappus-like enations 0 or 2(–4), erect to spreading, ± subulate or threadlike, fragile, 0.3–0.6+ mm. 2n = 72.
Flowering May–Sep. Glades and barrens, prairies, disturbed sites; 10–500 m; Ala., Ark., Conn., Ga., Ill., Ind., Iowa., Kans., Ky., La., Mass., Mich., Minn., Miss., Mo., N.Y., N.C., Ohio, Okla., Pa., S.C., Tenn., Tex., Va., W.Va., Wis.
As evidenced by the synonymy, Parthenium integrifolium as here circumscribed has been variously partitioned by other taxonomists. For the present, I see no justification for segregating species from, or for formal recognition of infraspecific taxa within P. integrifolium.