Pistia is a genus of aquatic plant in the family Araceae, comprising a single species, Pistia stratiotes, often called water cabbage or water lettuce. Its native distribution is uncertain, but probably pantropical; it was first described from the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways.
It floats on the surface of the water its roots hanging submersed beneath floating leaves. It is a perennial monocotyledon with thick, soft leaves that form a rosette. The leaves can be up to 14 cm long and have no stem. They are light green, with parallel veins, wavy margins and are covered in short hairs which form basket-like structures which trap air bubbles, increasing the plant's buoyancy. The flowers are dioecious, and are hidden in the middle of the plant amongst the leaves. Small green berries form after successful fertilization. The plant can also undergo asexual reproduction. Mother and daughter plants are connected by a short stolon, forming dense mats.
The growth habit can make it a weed in waterways. It is a common aquatic weed in the United States, particularly in Florida where it may clog waterways. It has the potential to reduce the biodiversity of a waterway. Mats of Pistia block gas exchange at the air-water interface, reducing the oxygen in the water and killing fish. They also block light, killing native submerged plants, and alter immersed plant communities by crushing them.
Pistia can be controlled by mechanical harvestors that remove the water lettuce from the water and transport it to disposal on shore. Aquatic herbicides may also be used. Two insects are also being used as a biological control. Adults and larvae of the South American weevil, Neohydronomous affinis feed on Pistia leaves, and the larvae of moth Spodoptera pectinicornis from Thailand. Both are proving to be useful tools in the management of Pistia.
Water lettuce is often used in tropical aquariums to provide cover for fry and small fish. It is also helpful as it outcompetes algae for nutrients in the water, thereby preventing massive algal blooms.