Eupatorium perfoliatum L.
Family - Asteraceae
Stems - To 1.5m tall, erect, herbaceous, villous to hirsute, branching above, from thick rhizomes.
Leaves - Opposite, decussate, to 20cm long, 4cm broad, crenate-serrate, lanceolate, densely pubescent below, pubescent above, rugose, acuminate. At least the middle and lower cauline leaves typically perfoliate.
Inflorescence - Terminal flat-topped to subdomed cymes dense with flower heads. Peduncles villous to densely pubescent. Flower heads with +/-15 flowers (florets).
Involucre - To 6mm tall(long), 3mm in diameter. Phyllaries imbricate, pubescent to villous externally, green, 3.5mm long, 1mm broad, linear to linear-oblong, acuminate. Margins near apex slightly scarious.
Ray flowers - Absent.
Disk flowers - Disk florets 9-23 per head. Corollas white to pinkish, 5-lobed, to 5mm long. Lobes acute. Stamens 5, included. Anthers connate around style, purplish. Style bifurcate, exserted. Pappus of capillary bristles. Achenes black when mature.
Flowering - July - October.
Habitat - Moist to wet ground.
Origin - Native to U.S.
Other info. - Steyermark lists two varieties for this species. Variety perfoliatum, (pictured above), has leaves which are perfoliate. Variety cuneatum Engelm has leaves with distinct bases which are not perfoliate.
Photographs taken along the shores of the Current River, Shannon County, MO., 8-16-03 (DETenaglia); also at Crowley's Ridge Conservation Area, Stoddard County, MO, 10-5-2011 (SRTurner).