Herbs, clump-forming, not rhizomatous, not stoloniferous; caudex stout. Flowering stems erect, leafy, 150-200 cm, stipitate-glandular with unicellular and multicellular hairs. Leaves basal and cauline, usually ternately decompound or 3-pinnately compound, never simple, leaflets short-petiolulate; stipules present; petiole present, glandular-stipitate, hairs uniseriate and multiseriate; leaflet blades ovate, largest terminal leaflet usually 3-lobed, base of leaflet oblique, truncate, or cordate, margins coarsely and irregularly double-serrate, teeth mucronate, apex acuminate, surfaces short stipitate-glandular on veins abaxially, glabrous adaxially; venation of terminal leaflet pinnately-palmately netted. Inflorescences relatively large, plumose panicles, rarely congested and spikelike, terminal from terminal bud of caudex, (branched), 500-2000-flowered, bracteate, (elongate-triangular); (branches stipitate-glandular). Flowers usually unisexual, rarely bisexual, plants usually dioecious, rarely polygamous, (becoming reflexed in fruit, short-pedicellate); hypanthium 1/4 adnate to ovary proximally, free from ovary 1 mm, (shallow), greenish whitish; sepals 5, white [pink or red to purplish]; petals absent or (1-)5, white [pink or red to purplish]; staminate flowers: stamens 8-10, (erect), inserted below rim of hypanthium, filaments filiform, (anthers cordate-orbiculate), ovary abortive, surrounded by distinct nectary disc; pistillate flowers: stamens abortive, pistils 2(-3)-carpellate, carpels connate proximally, ovary superior, barely adnate proximally to hypanthium, 2- or 3-locular, with poorly differentiated nectary disc, placentation axile, styles 2(-3), stigmas 2(-3), capitate. Capsules 2(-3)-beaked. Seeds brown, winged due to presence of loose seed coat, often twisted at both ends, oblong-cylindric, striate or lightly wrinkled, lustrous. x = 7.

Astilbe japonica (C. Morren & Decaisne) A. Gray has been reported (1891 in Rhode Island, 1945 in New York) as spreading from cultivation; it probably has not naturalized. Aside from being much to the north of the range of A. biternata, it is much smaller, 15-80 cm tall; has cuneate-oblanceolate leaflets that are rarely lobed or cleft; and has perfect flowers with the petals longer than the stamens. Petals can be pink to purplish in A. japonica versus white in A. biternata. There are many garden hybrids and cultivars involving several species of southeastern Asian Astilbe species.

Polygamo-dioecious; fls regular, 5-merous; hypanthium saucer-shaped; pet narrow; stamens 10; ovary superior; carpels 2(3), ±connate (except the styles) and with axile placentas, separating into erect follicles at maturity; tall perennial herbs with large, alternate, decompound lvs and large panicles of small, white or yellowish fls. 25, N. Amer., e. Asia.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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