Description from Flora of China
Diervilla japonica (Thunberg) Candolle.
Shrubs to 6 m tall. Petiole 8-12 mm, pubescent; leaf blade abaxially greenish, adaxially intense green, narrowly ovate to ovate-elliptic, rarely obovate, 5-15 × 3-8 cm, abaxially densely pubescent, adaxially sparsely pubescent, more densely so on veins, base broadly cuneate to rounded, margin serrate, apex acuminate to long acuminate. Flowers solitary or in threes in cymes. Calyx 1-1.2 cm, pubescent; tube 1-5 mm; lobes linear, 5-10 mm, pubescent. Corolla white or reddish, gradually turning red at anthesis, funnelform-campanulate, 2.5-3.5 cm, outside sparsely pubescent or subglabrous; base of tube narrowly tubular, abruptly enlarged above middle; lobes spreading, subregular, glabrous. Filaments white; anthers yellow-brown. Stigma discoid. Fruit 1.5-2 cm, crowned with a short beak, sparsely pubescent; seeds narrowly winged. Fl. Apr-May, fr. Aug-Sep.
Chinese populations are often segregated as Weigela japonica var. sinica (Rehder) Bailey (Gent. Herb. 2: 49. 1929; Diervilla japonica var. sinica Rehder, Mitt. Deutsch. Dendrol. Ges. 22: 264. 1913) primarily on the shape of the corolla, in which the lower half is narrowly tubular. In the typical variety, found in Japan and Korea, the corolla expands abruptly nearer to the base.
Forests, scrub; 400-1800 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, ?Hainan, Hubei, Hunan, Jiangxi, Sichuan, Zhejiang [Japan, Korea].