[Wikipedia] Flower Info, Range, Habitat: The plant is dioecious, with male and female flower types occurring on separate individuals.[6][4][5][7][8] In the British Isles it is possible that only female plants occur. It flowers in mid-summer.[9] Najas marina has a wide, almost circumglobal distribution in temperate and tropical regions. It occurs in mesoeutrophic water over deep peat or mud. It was first recorded in the British Isles in 1883 at Hickling Broad in Norfolk where it had become established. Populations declined in the 1960s because of pollution, but action has been taken to reduce the level of nutrients in the Norfolk Broads and the water quality has improved.[11] (link added by Mary Ann Machi)