Hypericum japonicum aka Matted St. John's Wort

Hypericum japonicum is in the St John's wort family. This family bears the name Hypericaceae. The plant's names include Japanese and small St John's wort. Its native range spans the Indian subcontinent, the Kuril Islands, and Australasia. It grows through much of east and southeast Asia. The RHS lists it as non-native to Britain and Ireland.

This small plant varies in both shape and lifespan. It can live as an annual or a short-lived perennial. Plants stand about 0.07–1.6 feet [0.02–0.5 metres] tall. Stems may stand upright, lean, or lie along the ground. They can grow roots at their bases. These forms give the plant quite different shapes as it grows. The sources give no separate height range for garden plants.

Plants For A Future lists flowers from July to August. Seeds ripen from August to September. Each flower has both male and female organs. Insects carry pollen between flowers. The plant is self-fertile: it can set seed with its own pollen. The supplied species accounts do not describe leaf shape or fruit form. The RHS gives these details only for the wider genus.

In the wild, this plant grows in rice fields and ditches. It also grows in marshes, grasslands, and ground that people have disturbed. Records include dry sites too, though the sites share an exposed setting. In gardens, it grows in sun or light shade. More sun helps it bloom well. It accepts sand, loam, and clay soils. It grows in slightly acid, neutral, and slightly alkaline soil. Soil should stay moist but let excess water drain out. Its taste for moist or wet ground suits a bog garden. The sources say it is easy to grow. They give no rating for how well it withstands frost. Sow seed in a cold frame in spring. Move young plants into pots, then plant out in late spring.

These sources record no food use for this plant. People gather wild plants for use in local medicine. They have used it to treat fever, wounds, asthma, and dysentery. These records do not prove that such treatments work or are safe. The safety evidence gives mixed results and needs care when read. Both plant-use sources report suspected sensitivity to sunlight after exposure. They also cite trials on livestock in New Zealand. Those trials found no toxicity. These findings do not settle its safety for people or pets. Plants For A Future lists no other uses. It also flags no risk of the plant becoming a weed.