Salvinia molesta

Salvinia molesta, commonly known as Giant salvinia or kariba weed after it infested a large portion of the reservoir of the same name, is an aquatic fern, native to south-eastern Brazil. It is a free floating plant that does not attach to the soil, but instead remains buoyant on the surface of a body of water. The fronds are 0.5-4 cm long and broad, with a bristly surface, and produced in pairs also with a third modified root-like frond that hangs in the water.

It reproduces by asexual reproduction only, but it is capable of growing extremely quickly, starting from small fragments and doubling in population size every few days, with the result that the surface of ponds, reservoirs, and lakes are covered by a floating mat 10-20 cm (in some rare cases up to 60 cm) thick. As a consequence, it has sometimes considered an invasive weed in some parts of the world. The plant's growth blocks sunlight needed by other aquatic plants and especially algae to oxygenate the water.

A tiny weevil, Cyrtobagous salviniae, found in the native habitat of S. molesta, is currently being studied as a biocontrol.