Flora of the Southeastern United States
2022 Edition

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Key to Parthenium

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1 Leaves pinnatifid to bipinnatifid, the primary sinuses extending 9/10 or more of the way to the midrib; leaves thin in texture; pappus of 2 petaloid scales; [section Parthenium].
..2 Annuals (rarely persisting); leaf blades (1–)2-pinnately lobed, scabrellous below and seldom accompanied with erect hairs 1-2 mm long
..2 Biennials (sometimes flowering first year or persisting); leaf blades ± pinnately lobed, strigillose below and usually accompanied with erect hairs 1-2 mm long
1 Leaves toothed (pinnatifid or sinuous-margined in forms of P. integrifolium var. mabryanum, the sinuses extending up to 3/4 of the way to the midrib); leaves somewhat thick in texture; pappus of 2-3 weak awns; [native perennials]; [section Partheniastrum].
....3 Plants arising from a short caudex surmounting a system of fleshy-thickened but tough roots; stems glabrous or with short, appressed pubescence < 1 mm long; cauline leaves seldom auriculate-clasping, the upper cauline leaves typically sessile or petiolate, the lower cauline leaves petiolate, the petioles winged or not; blades of basal leaves (4-) 6-21 (-27) cm long, (1.4-) 2-12 (-13.5) cm wide.
......4 Blades of basal leaves ovate-lanceolate, (4-) 6-12 (-20) cm long, (3-) 4-8 (-9.5) cm wide, coarsely toothed but never undulately lobed; heads mostly 5-7 mm across, usually (18-) 90-180 (-400) per inflorescence; [widespread in our area]
......4 Blades of basal leaves linear-lanceolate to ovate-lanceolate, (6-) 7-12 (-13.5) cm long, (1.4-) 2-4 (-4.5 cm) wide, coarsely toothed and sometimes also undulately lobed throughout their length; heads mostly 4-6 mm across, usually (30) 40-75 (-85) per inflorescence; [of Coastal Plain and eastern Piedmont of VA, NC, and SC]
....3 Plants arising from a long, even-width, 3–5(–6) mm wide, branching, woody or lignescent rhizome; stems either with coarse, spreading pubescence 1-3 mm long or puberulous or strigose with hairs < 1 mm long; cauline leaves all auriculate-clasping, the upper cauline leaves sessile and auriculate-clasping, the lower cauline leaves with winged petioles, the wings expanded at the base; blades of basal leaves 11-18 (-20) cm long, 5-8 cm wide.
........5 Leaves stiffly to loosely, sparsely to densely villous on both sides with thin, white hairs arising from a slender, non-pedestaled base; stems hirsute-pilose with white hairs 1–3 mm long; heads 4-7 mm in diameter
........5 Leaves hispid with short, basally erect hairs, each arising from a “pedestal” of 2–4 concentric rings of glassy cells; stems either sparsely to densely hirsute-villous with white hairs 1–2 mm long, or merely puberulous or strigose with minute, appressed hairs 0.1–0.8 mm long; heads 7-10 mm in diameter