Rumex japonicus

Rumex japonicus is a perennial in the knotweed family, Polygonaceae. Its native range runs from the Russian Far East to Vietnam. It also spans temperate parts of East Asia. It is not native to Britain or Ireland. Plants reach about 3.3 feet (1 metre) in height. The supplied sources give no spread or detailed species-level leaf description. They also lack clear details on its stems and fruit.

This plant grows in wet meadows and lowland ditches across Japan. In China, it grows beside fields, streams, and wet valleys. Its Chinese range extends from sea level into high mountains. These records place it in damp sites across a wide range. Flowers appear from May to June. Each flower has both male and female organs. Wind carries the pollen. Seeds ripen from June to July, soon after the flowering season.

Plants For A Future lists sun and light shade as suitable. It reports that the plant prefers moist, well-drained soil. Sandy, loamy, and clay soils can all suit it. It accepts mildly acid, neutral, and mildly alkaline ground. The source lists it as a plant for bog gardens. However, its more detailed growing notes draw on the wider genus. Those notes do not establish this species' exact needs. British hardiness remains uncertain in that source, despite its broad native range. Sow seeds where they will grow in spring. Spring division offers another way to raise plants.

The leaves and seeds have recorded food uses after cooking. Cooks use the leaves as greens or add them to soups. They can also dry the leaves for later use. Cooked seeds can accompany rice. Ground seeds provide flour for dumplings. Plants For A Future gives its food value two out of five. It lists no known medicinal use for this species. The supplied evidence gives no species-specific pest or disease details.

Food use comes with an oxalic acid warning. Large servings of leaves may reduce the body's uptake of calcium. Cooking lowers the oxalic acid content but does not remove the caution. People prone to kidney stones or gout should take extra care. The source also warns people with arthritis, rheumatism, or hyperacidity. These cautions matter even though the plant has recorded food uses.