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.
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.
Perennial 3-10 dm from a tuberous-thickened, usually short root; stem simple or branched above, strigose-puberulent above, often glabrous below; lvs large and sometimes few, crenate-serrate, or sublyrate at base, scabrous to subglabrous, the basal long- petiolate, with lance-elliptic to broadly ovate blade 7-20 נ4-10 cm, the cauline progressively shorter- petiolate and generally reduced, the upper often sessile and clasping, but less conspicuously so than in the next 2 spp.; heads numerous in a flat-topped infl, the disk 4-7 mm wide; rays scarcely 2 mm; achenes obovate, black, 3 mm; 2n=72. Prairies and dry woods; Va. to se. Minn., s. to Ga. and Ark., and adventive in Mass. June-Sept.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
This is one of our typical prairie plants. Since all of our original prairies are under cultivation, this plant is found now only in prairie habitats along roadsides and railroads. I have a few specimens collected in "oak openings," which means that the plants are relicts. The range in Indiana is extended by published records from Clark, Floyd, Jefferson, and Marshall Counties.