Elodes japonica Blume, Mus. Bot. 2: 15. 1856 ["Elodea"]; E. virginica (Linnaneus) Nuttall var. asiatica Maximowicz; E. virginica var. japonica (Blume) Makino; Hypericum asiaticum (Maximowicz) Nakai; H. fauriei R. Keller; H. virginicum Linnaneus var. asiaticum (Maximowicz) Yatabe; H. virginicum var. japonicum (Blume) Matsumura; Triadenum asiaticum (Maximowicz) Komarov; T. japonicum f. asiaticum (Maximowicz) Y. Kimura.

Herbs, 15-50(-90) cm tall, erect from creeping and rooting base, with few or no branches below inflorescence. Stems terete, eglandular. Leaves sessile; blade oblong-lanceolate to ovate-oblong or oblong, (1-)2-5(-8) × (0.5-)1-1.7 cm; thickly papery, abaxially paler; laminar glands scattered dots; intramarginal glands ± dense to spaced; main lateral veins 4-paired, tertiary reticulation rather dense; base shallowly cordate-amplexicaul, margin revolute, apex obtuse to emarginate. Inflorescence 1-3-flowered, terminal, or sometimes with flowering branches from 1-3 nodes below, the whole narrowly cylindric to narrowly ellipsoid; peduncles 5-10 mm; bracts and bracteoles linear-lanceolate, persistent. Pedicels 1-3 mm. Flowers ca. 1 cm in diam., curved-funnel-shaped; buds cylindric, apex rounded. Sepals erect, ovate-lanceolate, 3-4 × ca. 2 mm; laminar glands 5, lines; marginal glands absent; margin entire, apex obtuse; veins 7. Petals flesh-pink, narrowly obovate to oblong, 6-7 × 2-3 mm, ca. 0.7 × as long as sepals; laminar glands few, dots, near apex; marginal glands absent; margin entire. Stamen fascicles ca. 4 mm, ca. 0.8 × as long as petals; filaments to 0.5 united. Fasciclodes orange, scalelike, ovate to orbicular, ca. 2 mm, not lobed. Ovary ovoid; styles 1.5-1.75 mm, ca. 0.5 × as long as ovary, appressed, eventually distally spreading. Capsule narrowly ellipsoid to narrowly obconic, 7-10 × 5-6 mm, 1.7-2 × as long as sepals, apex acute. Seeds dark brown, ca. 1 mm; testa finely foveolate. Fl. Jul-Aug, fr. Aug-Sep. 2n = 36.

Triadenum japonicum is very similar to the E North American T. virginicum (Linnaeus) Rafinesque, differing from it mainly in dimensions of floral parts.

Wet meadows and hillsides, marshes; lowland. Heilongjiang, Jilin [Japan, Korea, Russia (Far East)].