Salvinia molesta aka Giant Salvinia

Salvinia molesta is also known as giant salvinia. It belongs to Salviniaceae, the water-fern family. This freshwater fern lives for several years. Its native range spans southern and southeastern Brazil. It extends to Corrientes, Argentina. Kew places it in the wet tropics. It has spread beyond this range and become a worldwide weed. It is not native to Britain or Ireland. Other names include Kariba weed, waterfern, and aquarium watermoss.

This fern floats without roots in the ground. Its stems branch just below the water surface. They can be green or brown and grow across the water. Fronds grow in groups of three at stem nodes. The upper green fronds are oblong, with smooth edges. Hairs cover their surface, with tips joined in small loops. These hairs have a whisk shape and help fronds float. Upper fronds measure about 0.06–0.13 feet [0.02–0.04 metres] long. Lower brown leaves have deep cuts and act as roots. The lowest leaves reach 0.75 feet [0.23 metres]. These leaves help keep the plant steady.

Giant salvinia has no flowers. Its sporocarps grow in long chains among leaves beneath the water. These bodies form few spores. The source describes those spores as sterile. Most new growth comes from plant parts, not spores. This process is called vegetative reproduction. The plant grows fast and can form mats several feet thick. This depth is not the height of one plant. The evidence gives no mature spread for a single plant.

It lives in the fresh water of lakes and rivers. It grows in full sun or partial shade. It can withstand cold water, but not water that freezes. It grows in standing water, not a drained garden bed. North Carolina State rates its care needs as high. The source says not to grow it in gardens or aquariums. It also rules out ponds and other water features. The reason is harm to the environment.

Dense mats shade the water and lower its oxygen supply. Fish and native plants suffer beneath the mats. Mats also hinder boats, people who fish, and swimmers. They can block water intakes for crop watering and power production. Federal and North Carolina authorities class it as a noxious aquatic weed. The salvinia weevil feeds on this fern. Weevils brought to the United States help control it.