Hygrophila polysperma (Roxb.) T. Anders.: Hygrophila, East Indian hygrophila, Indian swampweed, Miramar weed. Acanthaceae (acanthus family).
Exotic perennial. A robust rooted plant growing densely in slow-moving water to a length of 6 ft (1.8 m). Can generate unattached but solid-appearing mats that float just below the surface The plant is mostly submersed but stem tips can emerge to a height of about 6 in (15 cm). Stems change from cylindrical when underwater to almost four-square when above water; they are brittle, and root and branch readily at the nodes. Leaves are opposite, oval, broadly pointed, 1.5 in long by 0.5 in wide (4 by 1.25 cm), with short stalks when submersed and none when emergent. Leaves contain tiny granules of calcium carbonate (cystoliths) that feel rough. Flowers are two-lipped, white to light blue, but inconspicuous, borne on the upper stem. Small cylindrical fruits split apart to shed a couple of dozen seeds each. Invasive and weedy, forming dense mats that hinder water use and aquatic wildlife, this species is listed as a U.S. Federal noxious weed. Note that the native lake hygrophila is almost entirely emergent, with larger leaves.
