Plants 8–20 dm, glabrous, glaucous.
Leaves 10–40 cm, glabrous but occasionally with scattered hairs on veins on underside; rachises bent abaxially at bases of some pairs of primary leaflets; leaflets lanceolate to ovate, 3–11 cm.
Inflorescences peduncles 10–45 cm; rays 25–50, 2–8 cm; involucel bractlets conspicuous, 5–15 mm; pedicels 2–9 mm.
Flowers ovaries glabrous.
Fruits subglobose to ovate, flattened, 2–4 mm.
2n=22.
Wet meadows, bogs, streambanks. Flowering Jul–Sep. 0–1800 m. Casc, CR, ECas, Est, Sisk, WV. CA, WA; north to AK; East Asia. Native.
Both the common and species names refer to the bent leaves of this species resembling a bent knee.