Plants to 250 cm. Basal leaves: petioles hollow or solid, 10-15 cm, glandular-hairy; blades 30-40 × 16-28 cm, coarsely dentate to subentire, abaxially white-tomentose, adaxially green, sparsely short-hairy. Heads usually in corymbiform clusters, long-pedunculate. Peduncles 1.5-12 cm. Involucres 15-25 mm diam., densely cobwebby (rarely glabrate). Phyllaries linear to linear-lanceolate, inner usually purplish, margins with minute spreading or reflexed glandular hairs. Florets 30+; corollas rose-purple, (occasionally white), 9-13 mm. limb minutely glandular. Cypselae light brown, 5-8 mm; pappus bristles 1-3 mm. 2n = 36. Flowering summer-early fall (Jul-Oct). Waste places, roadsides, fields, forest clearings; 0-1600 m; introduced; Alta., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Sask.; Colo., Conn., Maine, Mass., Minn., Mo., N.H., N.Dak., Ohio, S.Dak., Vt.; Eurasia. Arctium tomentosum has been reported from Illinois, Iowa, Maryland, New Jersey, Oregon, Pennsylvania, South Carolina, and Wisconsin; I have not seen specimens.The involucres of Arctium tomentosum are usually very densely cobwebby.
Exceptional forms of A. tomentosum have nearly glabrous involucres. Forms of A. minus with especially cobwebby involucres have been misidentified as A. tomentosum; they lack the corymbiform capitulescence and glandular corollas of the latter.
Resembling no. 3 [Arctium lappa L.] but smaller, seldom over 13 dm; lower petioles mostly hollow; invol mostly 1.5-2.5 cm thick, ±strongly arachnoid-tomentose and the bracts only weakly or scarcely hooked; cor-limb glandular outside; 2n=36. Native of Eurasia, sparingly established in our range. June-Oct.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.