Pistia stratiotes (Water Lettuce)

Scientific name

Pistia stratiotesL.

Synonyms

Pistia stratiotes var. cuneata Engl.; Pistia stratiotes var. linguiformis Engl.; Pistia stratiotes var. obcordata (Schleid.) Engl.; Pistia stratiotes var. spathulata (Michx.) Engl.

Common names

Water lettuce, Nile cabbage, water cabbage, tropical duckweed

Family

Araceae

Origin

South America (probably Brazil)

Naturalised distribution (global)

Now naturalised throughout the tropics and sub-tropics.

Introduced, naturalised or invasive in East Africa

Pistia stratiotes is invasive in parts of Kenya and Uganda (A.B.R. Witt pers. obs.) and Tanzania (Macdonald et al. 2003).

Habitat

Wetlands, dams, other waterbodies.

Description

Pistia stratiotes is a floating herb in rosettes of grey-green leaves, rosettes occurring singly or connected to others by short stolons. Roots numerous, feathery. Leaves often spongy near base, densely soft pubescent with obvious parallel veins, slightly broader than long, widest at apex, to 15 cm long (Ramey 2001).

Flowers inconspicuous, clustered on small fleshy stalk nearly hidden in leaf axils, with single female flower below and whorl of male flowers above. Fruit arising from female flower as a many-seeded green berry.

Reproduces rapidly by vegetative offshoots formed on short, brittle stolons. Varies seasonally in density of rosettes, from less than 100 to over 1,000 per square metre in south Florida. Seed production, once thought not to occur in North America is now considered important to reproduction and dispersal.

Economic and other uses

Pistia stratiotes has often been grown as an ornamental in lakes, ponds, aquaria and gardens. It is often used in tropical aquariums to provide cover for fry and small fish. It has medicinal properties and can be used as fodder for cattle and pigs. It can be beneficial in certain instances as it outcompetes algae for nutrients in the water, thereby preventing massive algal blooms. However, these uses cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Pistia stratiotes is a major weed of lakes, dams, ponds, irrigation channels and slow-moving waterways in tropical, subtropical and warmer temperate regions. It can completely cover water bodies, disrupting all life on the water. It clogs waterways preventing river travel, blocks irrigation canals, destroys rice paddies and ruins fishing grounds. It affects hydro-electricity production as its vast mats clog the turbines. P. stratiotes has been included in the Global Invasive Species Database (GISD 2005). It 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 most 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.

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 specially made machinery but the running costs are high and this method may not 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. 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.

Insects have been successfully used as biological control agents. Adults and larvae of the South American weevil Neohydronomous affinis feed on Pistia stratiotes leaves, and the larvae of moth Spodoptera pectinicornis has been reported to control P. stratiotes in Thailand. Both are proving to be useful tools in the management of this species.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

References

CABI Invasive Species Compendium online data sheet. Pistia stratiotes (water lettuce). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

GISD (2005). Global Invasive Species Database online data sheet. Pistia stratiotes (aquatic plant). www.issg.org/database. 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.

Ramey, V. (2001). Water Lettuce (Pistia stratiotes). Center for Aquatic and Invasive Plants, University of Florida. plants.ifas.ufl.edu/node/328.

Macdonald, A.W., Reaser, J.K., Bright, C., Neville, L.E., Howard, G.W., Murphy, S.J., Preston, G. (eds) 2003 Invasive Alien Species in South Africa: National Reports and Directory of Resources. GISP, Cape Town, SA.

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