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 caused by the hair-like strands that join at the end to form eggbeater shapes. They are used to provide a waterproof covering. These fronds are produced in pairs also with a third modified root-like frond that hangs in the water.
Favoured environmental conditions
S. molesta prefers to grow in slow moving waters such as those found in lakes, ponds, billabongs (oxbows), streams, ditches, marshes and rivers. It prefers nutrient rich waters such as those found in eutrophic water or those polluted by waste water. It does not usually grow in brackish or salty waters but has been reported in streams with a tidal flow in southeast Texas. It copes well with dewatering and while it prefers to grow in moderate temperatures it will tolerate temperatures that are low or very high. the United States Geological Service believes that it could grow in Zones 7a, 8, 9, and 10 of the USDA Plant Hardiness Map.
Methods of introduction
The plant originated in southeast Brazil and was exported as part of the pet industry to be used in aquariums and garden ponds. From there it escaped or was deliberately released into the wild. It is possible that it may also have been brought in with fresh, iced fish. Once in a waterway it can be spread by infested boats which not only spread it to new areas but also break the plant up which allows it to propagate. It also is spread by waterflow.
It reproduces by asexual reproduction only, but it is capable of growing extremely quickly, starting from small fragments and doubling in dry weight every 2.2-2.5 days. It grows from fragments that have broken off or dormant buds that have been detached from the main plant. Each node has five buds so potential for great & rapid spread is high. It also produces spores but they are genetically defective and do not produce viable offspring.
The result of the rapid growth rate of Salvinia molesta has resulted in its classification as an invasive weed in some parts of the world such as Australia, New Zealand and parts of America. Surfaces of ponds, reservoirs, and lakes are covered by a floating mat 10–20 cm (in some rare cases up to 60 cm) thick. The plant's growth clogs waterways and blocks sunlight needed by other aquatic plants and especially algae to carry out photosynthesis thereby oxygenating the water. As it dies and decays, decomposers use up the oxygen in the water. It also prevents the natural exchange of gases between the air & the body of water the plant has invaded causing the waterway to stagnate. This can kill any plants, insects or fish trapped underneath its growth. Its ability to grow and cover a vast area makes it a threat to biodiversity. Large infestations covering a wide area may also pose a problem to migratory birds as they may not be able to recognise an infested waterway when flying over head and so may not stop at it. S. molesta also provides ideal conditions for the breeding of mosquitoes that carry disease. The growth habit of Salvinia also is problematic to human activities including flood mitigation, conservation of endangered species & threatened environments, boating and irrigation.
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