Stems emersed, erect, robust, 70-120 cm tall or more. Leaves (20-)40-90 cm × 7-16 mm, upper flattened, lower keeled or trigonous on abaxial side. Panicles (8-)20-60 cm, with 3-7 lateral branches, each with (1 or)7-11 male heads and 1 or 2 female heads; main axis consisting of 3-5 or more male heads. Male flowers: perianth segments spatulate, lobate at apex; filaments ca. 3 mm; anthers obconic, (0.5-)1-1.2 × ca. 0.5 mm. Female flowers: perianth segments 5-7 × 1-1.5 mm; ovary sessile; styles forked or simple, ca. 1.5 mm; stigmas 3-4 mm. Fruit obconic, (3.5-)6-9 mm, ribbed. Fl. and fr. May-Oct.
Shallow water in lowland all over Japan. In lakes, streams, swamps and ponds at elevations up to around 3600 metres in northern China and Tibet.
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