Description from Flora of China
Perennials, to 40 cm tall, glabrous. Stems slender, often diffuse, much branched, basally creeping, quadrangular, narrowly 4-winged or not. Petiole 1-3 cm, slender; leaf blade ovate, ovate-triangular, or ovate-oblong, 1-3 X 0.4-1.5 cm, base truncate, margin sparsely serrate, apex acute; veins pinnate or palmate. Flowers axillary, solitary. Pedicel nearly as long as petiole or whole leaf. Bracteoles absent. Calyx cylindric to broadly campanulate, 5-10 mm or longer, saccate in fruit, sometimes narrowly winged, apex truncate or suboblique; lobes small, spinelike. Corolla yellow, throat with red spots; tube funnelform, 1.5-3 X as long as calyx, densely barbate. Stamens and style glabrous, included. Capsule ellipsoid, slightly shorter than calyx. Seeds ovoid, papillose. Fl. and fr. Jun-Sep.
The combination Erythranthe tenella (Bunge) G. L. Nesom (Phytoneuron. 2012-39: 40. 2012) appeared after publication of the family treatment for the Flora of China.
Beside water, wet places, forests, trailsides; 500-3000 m. Gansu, Guizhou, Hebei, Henan, Hubei, Hunan, Jiangxi, Jilin, Liaoning, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xizang, Yunnan, Zhejiang [India (Assam), Japan, Nepal, Sikkim, Vietnam].