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Key to Cirsium

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1 Heads several to many clustered together, usually touching, on peduncles 0-1 cm long
1 Heads solitary, or if several to many per plant, not clustered together and touching, and on peduncles mostly > 1 cm long.
  2 Plant colonial from creeping rhizomes; heads 13-20 (-25) mm high; phyllaries all lacking spine tips, or the outermost tipped with spines < 1 mm long; plant perennial; [exotic weeds, generally in altered habitats]
  2 Plant not colonial; heads 25-50 mm high (as small as 15-25 mm in C. carolinianum, C. nuttallii, C. muticum, and C. virginianum); phyllaries mostly spine-tipped, with at least some of the spines > 1 mm long (except sometimes mostly or entirely spine free in C. muticum); plant biennial (to weakly perennial); [native (except C. vulgare), in natural or some species also in disturbed habitats].
    3 Leaves decurrent onto the stem below, the decurrency extending as a wing at least several cm down the stem, and often to the leaf below; leaves scabrous-hispid above; phyllaries lacking a glutinous dorsal ridge; [exotic weed]
    3 Leaves not decurrent as a conspicuous wing, or the decurrency extending < 1 cm (sometimes more decurrent in C. lecontei); leaves not scabrous-hispid above; [native, sometimes in disturbed habitats].
      4 Phyllaries lacking spine tips (the outermost sometimes with a weak spine-tip to 0.5 mm long); leaves deeply lobed, to 55 cm long and 20 cm wide
      4 Phyllaries (at least the outer and middle) with well-developed spine-tips > 1 mm long; leaves lobed or merely toothed, generally < 30 cm long and < 10 cm wide (except in C. altissimus).
        5 Heads immediately subtended by several spiny-toothed leaves (appearing as a leafy involucre); flowers yellow, white, or purple.
          6 Involucres more-or-less densely tomentose; stems densely tomentose; [of the Coastal Plain and Piedmont]
          6 Involucres glabrous; stems glabrous or sparsely tomentose; [of the Coastal Plain].
             7 Leaves shallowly to deeply pinnatifid; main spines of the leaves 10-30 mm long; [of s. AL and Panhandle FL westward]
             7 Leaves spinose-dentate to shallowly pinnatifid; main spines mostly 5-10 mm long; [widespread in the Coastal Plain]
        5 Heads pedunculate (rarely with 1 or 2 reduced leaves below); flowers pink, purple, lavender, or white.
               8 Lower surface of the leaves thinly and loosely white-tomentose beneath, this sloughing off in age, the green surface visible through the tomentum except on very small, young leaves.
                 9 Heads 15-25 mm high; plants 5-35 dm tall, usually much branched and with numerous heads
                 9 Heads 25-50 mm high; plants 2-10 dm tall, usually strict or few-branched and with 1 or a few heads.
                   10 Heads on well-developed peduncles; [of moist to wet pinelands of the Coastal Plain from NC and SC south]
                   10 Heads on short peduncles; [of various habitats, mostly inland from the Coastal Plain, or of dry pinelands of the Coastal Plain].
                     11 Plants lacking well-developed basal leaves; cauline leaves with internodes mostly 0.5-2 cm long; [of dry pinelands of the Coastal Plain]
                     11 Plants generally with well-developed, persistent basal leaves; cauline leaves with internodes usually > 2 cm; [of various habitats, mostly inland from the Coastal Plain]
                       12 Spines of outer phyllaries slender, 1.5-3 mm long; plants simple or with a few short branches above; leaves usually shallowly lobed; roots often tuberous-thickened; cypselas usually 4.5-5 mm long; [midwestern U.S. and Canada distribution east to IN, MI]
                       12 Spines of outer phyllaries stout, 3-6 mm long; plants usually with several long branches; leaves usually lobed nearly to the midvein; roots rarely and only slightly tuberous-thickened; cypselas usually 3.5-4 mm long; [northeastern U.S. distribution inland to OH, PA]
               8 Lower surface of the leaves densely white-tomentose beneath, this persistent and entirely obscuring the green surface.
                          13 Heads 15-25 mm high; plants 4-20 dm tall; larger leaves 0.5-12 cm wide.
                            14 Cauline leaves 2-12 cm wide; peduncles 3-30 cm; corollas white to pale pink; [LA, MO, OK southwards and westwards]
                            14 Cauline leaves 0.5-5 cm wide; peduncles 1-15 cm; corollas pink to purple, rarely white; [collectively widely distributed].
                              15 Cauline leaves mostly 10-25; plants flowering Apr-Jun; [widespread, including extensively in the Inland provinces]
                              15 Cauline leaves mostly 30-70; plants flowering Aug-Oct; [moist to dry soils of the Coastal Plain (and very rarely in the lower Piedmont in association with other Coastal Plain species, such as Pinus palustris)]
                          13 Heads 25-45 mm high; plants 10-40 dm tall; larger leaves usually > 5 cm wide.
                                16 Stems uniformly and persistently tomentose; upper leaf faces tomentose when young, sometimes glabrate in age; cypselas 4-6 mm long.
                                  17 Cauline leaves with decurrent wing 1-5+ cm wide; plants 3-9 dm tall; phyllary spines 3-12 mm long
                                  17 Cauline leaves with decurrent wing 0-1 cm wide; plants 2-23 dm tall; phyllary spines 1.5-5 mm long
                                16 Stems thinly tomentose when young, later glabrate or tomentum persisting above; upper leaf faces green, glabrate; cypselas 6-9 mm long.
                                    18 Peduncle bracts much reduced; usually some roots with tuberlike enlargements; flowering May-Jul; [LA, OK, TX]
                                    18 Peduncle bracts leaflike; roots without tuberlike enlargements; flowering Jun-Oct; [collectively widely distributed].
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