Salvinia molesta (Kariba Weed)

Scientific name

Salvinia molesta D.S.Mitch.

Synonyms

Salvinia adnata Desv. (although this has been disputed by Moran and Smith (1999) based on the assumption that the original description probably related to another species)

Common names

Kariba weed, Kariba-weed, karibaweed, salvinia, water fern African payal, African pyle, aquarium water moss, aquarium water-moss, giant salvinia, giant water fern.

Family

Salviniaceae

Origin

Native to South America (south-eastern Brazil).

Naturalised distribution (global)

Locations within which Salvinia molesta is naturalised include Africa, the Indian Sub-continent, south-eastern Asia, warmer parts of Australia and New Zealand, southern USA and several Pacific islands.

Introduced, naturalised or invasive in East Africa

Salvinia molesta is invasive in water bodies in parts of Kenya, Tanzania and Uganda (Lyons and Miller 1999).

Habitat

Salvinia molesta is found in rivers, streams, lakes, dams, swamps, irrigation channels, drainage lines and other bodies of water and is also a pest of paddy rice crops around the world. Mainly a weed of tropical, subtropical and warmer temperate regions.

Description

A free-floating, green-brown, freshwater fern with branching horizontal stems and has submerged feathery 'roots'. It can easily break apart and it can also form dense mats of foliage on the water surface. Younger plants which are mostly bright green, generally have smaller, flat, 'leaves' that are more spread apart (the primary growth form).

As the plants get older, larger 'leaves' are produced that are slightly folded and borne closer together along the stems (the secondary growth form). Eventually the 'leaves' become very folded in nature.

Plants produce slender, branching runners and form mats of vegetation very quickly. These slender stems (1-2 mm thick) are much-branched and grow up to 30 cm long (usually only 6-25 cm long) before separating to form new plants. The length of stem between the joints (nodes), that is the internode length, varies depending on the density of the weed. When plant density is low the 'internode' length is relatively long, but as plant density increases, the 'internode' length may become very short.

The 'leaves' (fronds) are oval or folded, borne on short stalks (petioles), and are green or yellowish-green to brown in colour. These 'leaves' (20-60 mm long and 10-15 mm wide) have a covering of water-repellent waxy hairs (papillae) on their upper surface. These hairs (1-3 mm long) are arranged in distinct rows and are tipped with distinctive egg-beater shaped structures that aid buoyancy. The undersides of the leaves are covered in densely matted brown hairs. The 'roots' (submerged fronds) are brown in colour and highly divided into many filaments (2-50 cm long).

Reproduction and dispersal

This plant reproduces vegetatively, with the floating branches readily breaking apart and forming new plants. Individual plants (ramets) can consist of as little as a piece of stem with two floating 'leaves' (fronds) and a third 'leaf', which is modified into feathery 'roots' and remains submerged.

Dispersal of these plants occurs during floods and also during water movement caused by water currents or wind and through dumping of pond waste. It may also be spread by animals (through long distances by water birds), vehicles and boats.

Similar species

This a very distinctive floating plant and is rarely confused with other species. However, it has been confused with Azolla filiculoides (red azolla) and Azolla nilotica (ferny azolla) when young. The azollas (Azolla filiculoides and Azolla pinnata) can be distinguished from Salvinia molesta by having tiny overlapping scale-like leaves, not having egg-beater shaped hairs on the upper surface of their leaves, and by the fact that individual plants rarely grow larger than 2 cm in size. A more likely cause of confusion is with Salvinia hastate, which is a native species.

Economic and other uses

Salvinia molesta has been widely introduced as an ornamental plant and has been used as a mulch for crops in dry areas near water bodies where it grows. However, these uses cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Salvinia molesta grows rapidly and can quickly cover the entire surface of (small and medium-sized) waterbodies with a thick mat of vegetation, shading out any submerged plant life. Dense infestations can also impede oxygen exchange and light availability in the water column below, reducing water quality and causing the death of primary producers and disrupting the freshwater food chain. Dense mats impede navigation, fishing and recreational activities and provide breeding places for vectors of malaria and bilharzia.

S. molesta  has been included in the Global Invasive Species Database (GISD 2010). It is on the Federal Noxious Weeds List in the USA and has been listed as a noxious weed in South Africa (prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment) and in all Australian states.  

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Even the smallest plant fragment can develop into a plant so it is important to examine boats, fishing gear and other materials moved from an infested water body to prevent the weed affecting uninfested water bodies.

Salvinia molesta growth is greatly enhanced in eutrophied water bodies. Where possible the flow of nutrients from the surrounding catchments should be minimised through such measures as processing sewage and minimising runoff from agricultural lands.

Small scale infestations can be controlled manually. Larger infestations have been tackled with harvesters but the running costs are high and this method is very unlikely to be sustainable. Chemical control has been widely used and can be effective in the short term but needs to be reapplied over a long period, as does manual control. Other problems include tainting of drinking water (when using 2,4-D) and non-target damage (using any herbicide).

When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

The salvinia weevil (Cyrtobagus salviniae) has proved to be a very effective biological control agent for S. molesta in many countries and is an essential component in any integrated management effort. It has been released with positive consequences in East Africa but S. molesta is still considered to be invasive.

Legislation

Salvinia molesta has been declared a noxious weed in Kenya under the Suppression of Noxious Weeds Act (CAP 325). Under this act the Minister of Agriculture, can compel land owners who have such declared noxious weeds growing on their land to remove or have it otherwise removed. However, this species is not declared in Uganda and Tanzania.

References

Agnew, A. D. Q. and Agnew, S. (1994). Upland Kenya Wild Flowers. A flora of the Ferns and Herbaceous Flowering Plants of Upland Kenya. 2nd Ed. EANHS, Nairobi-Kenya.

CABI Invasive Species Compendium online data sheet. Salvinia molesta (Kariba weed). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Germplasm Resources Information Network (GRIN). www.ars-grin.gov/npgs/index.html. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA. Accessed March 2011.

GISD (2010). Global Invasive Species Database online data sheet. Salvinia molesta (aquatic plant, herb). www.issg.org/database. Accessed March 2011.

Global Compendium of Weeds. www.hear.org/gcw. Hawaiian Ecosystems at Risk Project. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Henderson, L. and Cilliers, C.J. 2002. Invasive aquatic plants-a guide to the identification of the most important and potentially dangerous invasive aquatic and wetland plants in South Africa. PPRI Handbook No. 16, Agricultural Research Council, Pretoria.www.arc.agric.za/uploads/images/0_SAPIA_NEWS_No._17.pdf.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

Lyons, E.E., Miller, S.E. (eds)1999 Invasive Species in Eastern Africa: Proceedings of a Workshop held at ICIPE, July 5-6, 1999.

Pacific Island Ecosystems at Risk (PIER). Salvinia molesta D.S.Mitchell, Salviniaceae: plant threats to Pacific ecosystems. www.hear.org/pier/species/salvinia_molesta.htm. Institute of Pacific Islands Forestry, Hawaii, USA.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke