Salvinia adnata  Desv. 1827

pronounced: sal-VIN-ee-uh ad-NAH-tuh

(Salviniaceae – the salvinia family)

synonym: Salvinia molesta  D.S.Mitch. 1972

pronounced: sal-VIN-ee-uh mol-ESS-tuh

common names:  Giant Salvinia, Kariba Weed

salvinia adnatasalvinia adnataBoth the family and the genus are named for Antonio Salvini, a 17th century Italian scholar. The only other genus in this family is Azolla. This is also a genus of floating ferns, but generally they are not as detrimental to the environment as the Salvinia, and, indeed, usually do more good than harm. Adnata is from the Latin adnato, to swim; molesta is from molestus, troublesome
The Giant Salvinia has just about taken over the system of lagoons in the Horseshoe Bay wetlands. Its other common name, Kariba Weed, was given to the plant after it infested a large portion of the reservoir of the same name, a hydroelectric dam in the Kariba Gorge of the Zambezi river basin between Zambia and Zimbabwe, and one of the largest dams in the world. Salvinia molesta is an aquatic fern, 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 egg-beater shapes. These provide a waterproof covering. The fronds are produced in pairs, but with a third modified root-like frond that hangs in the water. The plant was exported from Brazil as part of the pet industry, to be used in aquaria and garden ponds. From there it has escaped into, or been introduced into the wild in many parts of the world.

salvinia adnatasalvinia adnataIt reproduces by asexual reproduction only, but is capable of growing extremely quickly, starting from small fragments and doubling in size every 2–3 days. It grows from fragments that have been broken off, or from dormant buds that have become detached from the main plant. Each node has 5 buds, so there is high potential for great and rapid spread. It can also produce spores, but they are genetically defective and do not produce viable offspring. In Australia, salvinia has never been known to produce spores, and is considered to be a sterile clone. The floating mats of the weed shade out any submerged plant life and impede oxygen exchange, making the water unsuitable for fish and other animals. An infestation can reach up to 400 tonnes of wet weight per hectare. Infestations block irrigation channels, cause flooding, pollute drinking water, and prevent recreational activities such as swimming, fishing and boating. Infestations can be dangerous to animals and people because the mats look like solid ground, and they also provide an ideal breeding environment for disease-carrying mosquitoes. Another quirky effect is that migratory birds may not be able to recognize an infested waterway when flying overhead, and so may not stop at it.

Salvinia grows best in still or slow-moving fresh water. It can survive being frozen, and water temperatures up to 43ºC, and salinates up to one-tenth that of sea water. Additionally, plants protected as part of dense mats can survive for many months if the water dries up, particularly in shaded environments or on soils that retain water. Salvinia also thrives in nutrient-rich waters such as agricultural run-off or wastewater. It is thought that the run-off from the new housing estates near the Horseshoe Bay wetlands may have contributed to the phenomenal spread of the weed on Magnetic Island.

Salvinia is a native of south-eastern Brazil. It has become a serious weed throughout Africa, India, Sri Lanka, south-east Asia, the Philippines, PNG, New Zealand, Fiji, Hawaii and mainland USA, as well as Australia.
In Australia it was recorded as a weed near Sydney in 1952, and a year later near Brisbane. Since then it has infested most coastal streams from Cairns to Moruya on the south coast of NSW. It has spread from back yards in all capital cities, and remote infestations affect the Top End of the Northern Territory and regions of Western Australia. There is a belief that many infestations have been deliberately spread in order to harvest the plants for sale in the aquarium and horticulture industries.

Large infestations are virtually impossible to eradicate manually. Biological control with the salvinia weevil Cyrtobagous salviniae seems to be very effective in certain conditions. Following research by the CSIRO, the 2 mm long weevil was released in 1980 into Lake Moondarra near Mount Isa. It was a spectacular success – an estimated 800 hectares infestation weighing tens of thousands of tonnes was reduced to less than 1 tonne in a little over a year. This weevil has since been released from Sydney to Kakadu and elsewhere around the world, providing very successful control of salvinia in specific climates.

The weevil larvae feed inside the stems and the adults feed on buds, both contributing to plant death. Eventually the whole mat turns brown and begins to sink and decompose, the process generally taking between one and three years to control an infestation. Let us hope that these voracious weevils do not find other, more useful plants to attack after they have finished with the salvinia!

The caterpillars of the Salvinia Moth Samea multiplicalis also feed on the plant.

Photographs taken Horseshoe Bay lagoon 2010

Page last updated 19th February 2018







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