Damasonium alisma aka Starfruit

Damasonium alisma is a wetland plant in the Alismataceae family. Its common names include starfruit, water star, and ray pod. It also goes by the name thrumwort. Kew places its native range from Europe to Siberia. The RHS confirms that it is native to Britain and Ireland. It grows in temperate regions, in places such as gravelly ditches and ponds. Plants For A Future describes its British populations as scarce and declining.

The plant reaches about 1 foot (0.3 metres) tall. Sources describe it as annual or perennial, so its lifespan varies. Kew also classes it as an annual or an emergent wetland plant. Its small size suits the shallow parts of a pond. The supplied accounts give no detail about leaf or stem shape. They also provide no mature spread or description of its fruit.

Flowers appear from June to August, according to Plants For A Future. Each flower has both male and female organs. Insects pollinate the blooms. The evidence gives no flower colour or flower size. Pond edges and bog gardens are listed as suitable settings. These uses fit its need for wet ground or shallow water.

Choose an open, sunny place for cultivation. The plant can also grow in partial shade. It accepts sandy soil, loam, and clay. Suitable soils range from mildly acid through neutral to mildly alkaline. Keep the soil wet, or grow it in still water. Water can reach about 0.8 feet (0.25 metres) deep. Plants For A Future lists USDA hardiness zones 6–9. Sow ripe seed promptly in a cold frame. Stand the seed trays in shallow water. Move seedlings into separate pots once large enough to handle. Keep their pots in shallow water before summer planting. Spring division offers another way to raise plants. Larger divisions can go straight into their final sites. Keep smaller pieces in pots until they root well.

Plants For A Future lists the root as edible. It gives no preparation method and rates food value as low. The source reports no known hazards, but gives no detailed safety assessment. It lists no medicinal uses or other practical uses. It does not flag the species as weedy. The supplied evidence identifies no specific pests or diseases.