Description from Flora of China
Hydrangea involucrata Siebold var. longifolia (Hayata) Y. C. Liu; H. strigosa Rehder var. longifolia (Hayata) Chun.
Shrubs erect. Branchlets, petioles, leaf blades, and inflorescences densely appressed hairy with both simple and 2-branched hairs. Branchlets dark brown-red, terete or slightly obtusely angled near apex. Petiole thin, 1.5-2 cm; leaf blade lanceolate, 10-20 × 3-4.5 cm, papery, adaxially with more 2-branched hairs than simple hairs, abaxially with fewer 2-branched hairs than simple hairs, secondary veins 8-10 on both sides of midvein, slender, abaxially prominent, base obtuse to cuneate, margin aristate serrulate, apex caudate-acuminate. Inflorescences corymbose cymes, ca. 9 × 11-14 cm; bracts ovate, ca. 2 × 1.5 cm, densely puberulous and enveloping immature inflorescence before falling. Sterile flowers with sepals 4, elliptic to broadly ovate, 1.5-1.8 × 1.1-1.5 cm in fruit. Fertile flowers unknown. Capsule campanulate, ca. 3 × 3.5-4 mm, with simple hairs and a few 2-branched hairs, apex truncate; persistent calyx teeth triangular, ca. 0.5 mm; persistent styles 2, erect to recurved, 1.5-2 mm, distally enlarged. Seeds brown, ellipsoid, compressed, ca. 0.5 mm, winged at both ends; wings 0.2-0.4 mm; seed coat striate veined. Fl. Jan.
This species is closely related to Hydrangea involucrata Siebold, from Japan. However, the distinctive, 2-branched hairs of H. longifolia easily differentiate the two species.
* Forests in high mountains. Taiwan.