Description from Flora of China
Mazus japonicus (Thunberg) Kuntze var. tenuiracemus Hayata ex Makino & Nemoto; M. taihokuensis Masamune.
Perennials, to 8-20 cm tall, white villous. Primary root short, fibrous roots numerous, tufted. Flowering stems not erect. Leaves all basal, rosulate; petiole broadly winged; leaf blade obovate-spatulate, 2-6 cm, papery, base tapering, margin coarsely serrate to double serrate or basally pinnately parted, apex obtuse. Flowering stems often branched, terete to somewhat ribbed, slender, ascending, leafless or with 1 or 2 small leaves. Racemes 3-15-flowered, lax; bracts ovate-triangular, 1.5-3 mm. Pedicel slender 1-2 cm. Calyx campanulate, 5-7 mm, enlarged to 1 cm in fruit; lobes as long as tube, lanceolate-triangular, apex acute, veins inconspicuous. Corolla light purple, 1.2-2 cm; lower lip ovate, middle lobe narrower and longer than lateral lobes; upper lip erect, lobes narrowly triangular. Ovary glabrous. Capsule globose, slightly longer than calyx tube. Seeds brown-yellow; seed coat finely reticulate. Fl. and fr. Apr-May.
Coastal cliffs; near sea level. N Taiwan [Japan].