1a. Heads 1-1.6 cm long, (1.5-) 2-2.5 cm wide, subsessile or short pedunculate, in a racemose inflorescence; petioles of larger leaves hollow; common weed.....A. minus – common burdock
1b. Heads 1.5-2.5 cm long, 1.5-3.5 cm wide, long pedunculate, in a corymbose inflorescence; petioles solid or hollow; rare weeds....2
2a. Heads 1.5-1.7 cm long, 1.5-2.1 cm wide, phyllaries densely cottony-pubescent; petioles of larger leaves hollow.....A. tomentosum – hairy burdock
2b. Heads ca 2.5 cm long, 3-3.5 cm wide, phyllaries glabrous; petioles of larger leaves solid.....A. lappa – great burdock
Adapted from Keys to the Asteraceae of Wisconsin. Assembled and editied by Robert R. Kowal, 11 February 2007.
Etymology: Arctium = from Greek arction, the name of a plant taken from arctos, "bear," because of the rough involucre.
Characteristics: Biennials or (monocarpic) perennials, 0.5- 3.0 m; herbage not spiny. Stems erect, openly branched, branches ascending. Leaves basal and cauline; long-petiolate; gradually smaller distally; blade margins entire or dentate (pinnately lobed or dissected), faces abaxially resin-gland-dotted, adaxially often tomentose. Heads discoid, in leafy-bracted racemiform to paniculiform or corymbiform arrays. Involucres spheric to ovoid. Phyllaries many in 9–17 series, outer and mid narrowly linear, bases appressed, margins entire, apices stiffly radiating, hooked-spiny tipped, inner linear, ascending or erect, straight tipped. Florets (5–)20–40+; corollas pink to ± purple, glabrous or glandular-puberulent, tubes elongate, throats campanulate, lobes narrowly triangular, ± equal; Cypselae obovoid, ± compressed, rough or ribbed, glabrous, attachment scars basal; pappi falling, of many bristles in 2–4 series.
Number of Species: 3 in Wisconsin (all non-native from N. Africa & Eurasia); 3 in US & Canada.
Robust biennials with large long-petioled ovate-cordate leaves pilose-villous and atomiferous beneath. Heads numerous, the purple flowers tubular and perfect, the involucre globose with the firm, lanceolate bracts terminated by a hook, the whole head breaking off as a bur. Achenes flattened, irregularly furrowed; pappus of minute coarse bristles. Old World genus; all our species introduced weeds.
Johnson, M.E. and H.H. Iltis. 1963. Preliminary reports on the flora of Wisconsin: No. 48. Compositae Family. Wisconsin Academy of Sciences, Arts & Letters. 52:255-342.