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Key to Cirsium

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1 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; [alien weeds, generally in altered habitats]
1 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].
..2 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; [alien weed]
..2 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].
....3 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
....3 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).
......4 Heads immediately subtended by several spiny-toothed leaves (appearing as a leafy involucre); flowers yellow, white, or purple.
........5 Involucres more-or-less densely tomentose; stems densely tomentose; [of the Coastal Plain and Piedmont]
........5 Involucres glabrous; stems glabrous or sparsely tomentose; [of the Coastal Plain].
..........6 Leaves shallowly to deeply pinnatifid; main spines of the leaves 10-30 mm long; [of s. AL and Panhandle FL westward]
..........6 Leaves spinose-dentate to shallowly pinnatifid; main spines mostly 5-10 mm long; [widespread in the Coastal Plain]
......4 Heads pedunculate (rarely with 1 or 2 reduced leaves below); flowers pink, purple, lavender, or white.
............ 7 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.
............ ..8 Heads 15-25 mm high; plants 5-35 dm tall, usually much branched and with numerous heads
............ ..8 Heads 25-50 mm high; plants 2-10 dm tall, usually strict or few-branched and with 1 or a few heads.
............ ....9 Heads on well-developed peduncles; [of moist to wet pinelands of the Coastal Plain from NC and SC south]
............ ....9 Heads on short peduncles; [of various habitats, mostly inland from the Coastal Plain, or of dry pinelands of the Coastal Plain].
............ ......10 Plants lacking well-developed basal leaves; cauline leaves with internodes mostly 0.5-2 cm long; [of dry pinelands of the Coastal Plain]
............ ......10 Plants generally with well-developed, persistent basal leaves; cauline leaves with internodes usually > 2 cm; [of various habitats, mostly inland from the Coastal Plain]
............ ........11 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]
............ ........11 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]
............ 7 Lower surface of the leaves densely white-tomentose beneath, this persistent and entirely obscuring the green surface.
............ ..........12 Heads 15-25 mm high; plants 4-20 dm tall; larger leaves 0.5-12 cm wide.
............ ............ 13 Cauline leaves 2-12 cm wide; peduncles 3-30 cm; corollas white to pale pink; [LA, MO, OK southwards and westwards]
............ ............ 13 Cauline leaves 0.5-5 cm wide; peduncles 1-15 cm; corollas pink to purple, rarely white; [collectively widely distributed].
............ ............ ..14 Cauline leaves mostly 10-25; plants flowering Apr-Jun; [widespread, including extensively in the Inland provinces]
............ ............ ..14 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)]
............ ..........12 Heads 25-45 mm high; plants 10-40 dm tall; larger leaves usually > 5 cm wide.
............ ............ ....15 Stems uniformly and persistently tomentose; upper leaf faces tomentose when young, sometimes glabrate in age; cypselas 4-6 mm long.
............ ............ ......16 Cauline leaves with decurrent wing 1-5+ cm wide; plants 3-9 dm tall; phyllary spines 3-12 mm long
............ ............ ......16 Cauline leaves with decurrent wing 0-1 cm wide; plants 2-23 dm tall; phyllary spines 1.5-5 mm long
............ ............ ....15 Stems thinly tomentose when young, later glabrate or tomentum persisting above; upper leaf faces green, glabrate; cypselas 6-9 mm long.
............ ............ ........17 Peduncle bracts much reduced; usually some roots with tuberlike enlargements; flowering May-Jul; [LA, OK, TX]
............ ............ ........17 Peduncle bracts leaflike; roots without tuberlike enlargements; flowering Jun-Oct; [collectively widely distributed].

Key to Asteraceae, Key E: herbaceous composites with leaves spiny, leaves alternate or basal, and heads discoid

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1 Heads appearing spherical (the ‘visual head’ actually a secondary head aggregated of numerous 1-flowered heads); disk flowers blue (rarely almost white)
1 Heads not spherical; disk flowers pink (rarely yellow).
..2 Stem winged, the wings armed with spines.
....3 Pappus of plumose bristles
....3 Pappus of barbellulate bristles.
......4 Cypselas borne on the surface of the receptacle, intermixed with setiform scales; foliage glabrous or more loosely and sparsely pubescent
......4 Cypselas borne in the pits of the naked honeycombed receptacle; foliage densely white-tomentose
..2 Stem not winged.
........5 Leaves green with white mottles
........5 Leaves green.
..........6 Disk flowers pink (rarely white)
..........6 Disk flowers yellow.
............ 7 Pappus of numerous plumose bristles
............ 7 Pappus absent or of scales or barbellulate bristles.
............ ..8 Cypselas 4-angled; receptacles bearing subulate scales
............ ..8 Cypselas terete, 20-ribbed; receptacles bearing flattened bristles
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