Nov. Actorum Acad. Caes. Leop.-Carol. Nat. Cur. 14(suppl.): 691. 1829.
Shrubs or trees, 5–70 dm. Twigs appressed-pubescent, trichomes white. Leaves opposite or 3-whorled; petiole 0.6–2.4 cm, glabrous or sparsely hirsute; blade elliptic to ovate, (5–)6.4–15 × 3–6(–10) cm, unlobed, base rounded to truncate or cuneate, margins serrate to serrulate, apex usually acute, sometimes acuminate, abaxial surface green, sparsely hirsute, often densely hirsute along veins, trichomes at 40× smooth, 0.5–1 mm, adaxial surface green, glabrate or sparsely hirsute, sometimes moderately hirsute along veins. Inflorescences open or compact, 250–1000-flowered, usually conic, sometimes ovoid, 7–25 × 6–11.5 cm; peduncle 2–5.5 cm, sparsely to densely hirsute. Pedicels 1–4 mm, glabrous or sparsely hirsute proximally. Sterile flowers present, white or greenish white, sometimes tinged pink or blue, tube 9–16 mm, lobes 4(–5), elliptic to round, (6–)10–21 × (4–)8–15 mm. Bisexual flowers: hypanthium adnate to ovary to near its middle, 1–1.8 × 1.2–2.2 mm, weakly 7–10-ribbed in fruit, usually glabrous, rarely sparsely hairy; sepals shallowly triangular, 0.3–0.7 × 0.5–1.2 mm, margins entire, apex acute to acuminate, abaxial surface glabrous; petals tardily deciduous, spreading or reflexed, white or yellowish white, elliptic, ovate, or ovate-lanceolate, 3–4 × 1.2–1.5 mm; filaments 1.8–5 × 0.1–0.3 mm; anthers 0.3–0.6 mm; pistil 2–4-carpellate, ovary 1/2 inferior; styles 2–4, connate to middle or distally, 2–3.5 mm. Capsules oblong-ovoid, 3.5–5 × 2–3 mm. Seeds caudate at both ends, 1.7–3 mm. 2n = 72, 108 (Japan).
Phenology: Flowering Jul–Sep.
Habitat: Disturbed woods, roadsides, wooded swamps.
Elevation: 30–500 m.
Introduced; Ont., Conn., Maine, Mass., N.H., N.Y., N.C., Pa., R.I., Va., W.Va., e Asia.
M. A. Dirr (2004) listed 26 primary cultivars derived from Hydrangea paniculata.