Perennials, 10-25(-120) cm (plants rhizomatous, polygamodioecious). Stems erect, not branched (± scapiform; stems of 'staminate' plants wither soon after flowering, stems of 'pistillate' plants elongate after flowering). Leaves basal and cauline; alternate; petiolate or sessile; basal (usually appearing after heads) palmately or palmati-pinnately nerved, mostly deltate to ovate or orbiculate, margins entire, denticulate, or toothed to lobed, abaxial faces ± tomentose, adaxial tomentulose and glabrescent or glabrous; cauline (sessile) bractlike (essentially expanded petioles, proximal sometimes bearing blades). Heads radiate, discoid, or disciform, usually in corymbiform, paniculiform, or racemiform arrays, rarely borne singly {'staminate' heads usually radiate, peripheral 1-20(-70) florets styliferous and sterile or neuter, inner 11-78 florets usually functionally staminate, rarely bisexual and fertile; 'pistillate' heads usually radiate, peripheral (1-)30-130+ florets pistillate and fertile, inner 1-12 florets functionally staminate}. Calyculi 0 or of 1-5+ bractlets. Involucres obconic to turbinate, 6-15+ mm diam. (expanding in fruit). Phyllaries persistent, mostly 12-15 in (1-)2 series (often purplish-tinged), erect, distinct or connate, narrowly oblong to linear (1-5-nerved), subequal, margins ± scarious (apices not black). Receptacles flat to convex, foveolate, epaleate. Ray florets 0 or (1-)30-130+, usually fertile (in pistillate heads), sometimes styliferous and sterile or neuter (in staminate heads); corollas whitish or pinkish to purplish [yellow] (tubes filiform, laminae linear to oblong; styles filiform to clavate, entire or shallowly 2-cleft, papillate). Peripheral (pistillate) florets usually 30-125 and fertile, sometimes 0; corollas whitish or pinkish to purplish [yellow] (filiform, usually 5-lobed, sometimes minutely bilabiate; styles filiform to clavate, entire or shallowly 2-cleft, papillate). Inner (functionally staminate or bisexual) florets 1-78, usually functionally staminate, rarely bisexual and fertile; corollas whitish [yellow] (tubes longer than ± campanulate throats, lobes 5, erect or recurved, lanceolate to linear; styles linear to clavate, branches usually 0 or short-conic and papillate, sometimes lanceolate to oblong and ± hispidulous). Cypselae narrowly cylindric to weakly fusiform or ± prismatic, 5- or 10-ribbed, faces glabrous [villous]; pa North American Petasites is taxonomically difficult as a result of low variability in reproductive morphology and a high degree of leaf polymorphism. The conservative reproductive structures across all taxa make identification almost impossible without foliage and, unhappily, flowering usually occurs prior to emergence of basal leaves. Further compounding the taxonomic confusion is the apparent intergradation of many of the different leaf forms as well as leaf morphologies being subject to environmental plasticity.

Using morphometric, isozymic, and chromosomal data as well as crossing studies and observations of field and herbarium specimens, relationships of the various entities have been interpreted by D. M. Cherniawsky and R. J. Bayer (1998, 1998b). North American Petasites has been shown to be loosely comprised of four polymorphic groups. Close relationships among the groups were evident in all analyses by Cherniawsky and Bayer and suggested rapid and recent morphologic and genetic divergence in North America. They postulated that groups within North American Petasites have not yet reached a level of differentiation characteristic of distinct species. On this basis, North American Petasites is treated here as one polymorphic species with four infraspecific taxa, one a hybrid.

Characters used in this treatment are those found to be the most taxonomically discriminating by D. M. Cherniawsky and R. J. Bayer (1998c), although there is a high degree of overlap of ranges across all taxa. Morphologic variation is continuous for most reproductive characters. Values for reproductive characters are mostly continuous with only a few extreme ranges providing some taxonomic demarcation. Although leaf characters were shown to be the most reliable, it is recommended that a combination of both flowers and leaves be used for accurate identifications.

In cooler regions of the northern hemisphere, Petasites glacialis and P. gmelinii, both with yellow corollas and 1[-3] heads per array, approach the Bering Strait in northeastern Siberia; they apparently do not reach Alaska (E. Hultén 1968).

Two relatively large-leaved exotic species of Petasites are sold as ornamentals: the European P. hybridus (Linnaeus) Gaertner, Meyer, & Scherbius, with purplish florets and reniform to orbiculate-cordate, shallowly angular-lobed, finely and unevenly toothed leaf blades 10-90(-100) cm wide; and the Asian P. japo