Plants 10-60(-120) cm. Rhizomes much branched, creeping, often forming long-lived, spreading clones. Basal leaves: blades mostly reniform, orbiculate-cordate, cordate, ovate, triangular, or sagittate, margins subentire to coarsely dentate to wavy, or shallowly to deeply pinnipalmately or palmately lobed, lobes obtuse to acute, often again lobed or toothed, abaxial faces densely white-tomentose to lanate, adaxial faces densely lanate or floccose, glabrescent, or glabrous. Staminate heads 2-53 (ray florets: 0 or 1-70, styliferous and sterile, corolla laminae 1.1-12 mm; disc florets 11-78, functionally staminate; style branches 0-2.3 mm, papillate or hairy). Pistillate heads 5-60 (ray florets 30-139, pistillate and fertile, corolla laminae 0.6-6.3 mm; disc florets 1-12, functionally staminate, style branches 0-2.3 mm, papillate or hairy). Phyllaries: staminate 3.6-12.8 × 1-4.4 mm, pistillate 1-9.5 × 0.7-3.3 mm, margins entire or serrate. Cypselae 1.7-3.5 mm; pappi (longest, pistillate heads) 7-17.3 mm.
Rhizomatous perennial; basal lvs long- petioled, glabrous or sparsely hirsute above, loosely white-tomentulose beneath, large, 5-40 cm wide, palmately or pinnipalmately veined and lobed and also ±toothed; stem 1-5 dm, with parallel-veined bracts 2.5-6 cm, the lower sometimes with an abortive blade at the end; heads campanulate in a corymbiform or racemiform infl; invol 5-9 mm; fls whitish, the outer 10-25 in pistillate heads with short rays 2-7 mm; 2n=60(-62). Meadows, swampy places, and moist woods; circumboreal, s. in Amer. to Mass., Mich., Minn., and Calif. Apr.-July. Nearly all our plants belong to var. palmatus (Aiton) Cronquist, with the lvs palmately cleft well over half way to the base. (P. palmatus) A few plants from n. Minn. and n. Mich. (and northward) with the lvs palmately or pinnipalmately cleft not more than about half way to the base, may reflect hybridization with no. 2 [Petasites sagittatus (Banks) A. Gray]. The name P. ضitifolius Greene is available for these.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.