Naturalized Giant salvinia, Kariba weed

Latin molestus, troublesome, irksome, annoying, disturbing, in reference to the very aggressive weedy nature of this fern.

Blades broadly elliptic, up to 3 x 2.4 em, apex obtuse to emarginate, hairs on upper (abaxial) surfaces with 3-4 branches above base that reunite at apex to form an eggbeat-erlike structure. Sporocarps often absent, empty (sterile) when present.

Noxious floating fern of fresh or stagnant water in ponds, lakes, ditches, reservoirs, lakes, marshes, and taro and rice paddies, usually at elevations below 260 m. First noted in Hawai 'i at the Hawaii Tropical Botanical Garden north of Hilo and found in April 1999 to be rapidly spreading on Enchanted Lake in Kailua and Lake Wilson in central O'ahu.

Salvinia molesta was first noted exploding in Lake Kariba in Zimbabwe in 1959 and is now a serious pest in many parts of the Tropics. A sterile hybrid of unknown origin, but probably originating from South American species, it spreads by proliferation and fragmentation and not by sexual reproduction.

Salvinia molesta is used ornamentally in aquariums and garden ponds, from which it has surely escaped. It is occasionally terrestrial on continuously wet sites and it sometimes grows in slightly brackish water. It can form large, floating mats in drainage ditches, streams, and rivers, reducing water flow and impeding navigation. It may completely cover the surfaces of water bodies. In favorable situations, mats up to 25 cm thick can be formed, preventing sunlight from reaching plants and animals that need it and reducing oxygen content, and secondarily inhibiting the growth of fish. Cyrtobagus salviniae, a snout beetle that feeds upon and eventually kills the fern, has been helpful in controlling the species in other parts of the world.

Salvinia molesta may be recognized as forming large, floating mats composed of clusters of paired, upright, oval to heart-shaped blades up to 3 x 2.4 cm, and one submerged, multibranched rootlike leaf.