Click on images to enlarge
close-up of seeds (Photo: Steve Hurst at USDA PLANTS Database)
close-up of flower (Photo: Forest and Kim Starr, USGS)
infestation (Photo: Sheldon Navie)
in flower (Photo: Sheldon Navie)
infestation on the shore of Lake Nakuru, Kenya (Photo: John Mauremootoo, CC BY-NC)
adult feeding damage by Neochetina bruchi on the leaves of Eichhornia crassipes (Photo: Rüdiger Wittenberg, CABI)
Eichhornia crassipes (C.Mart.) Solms
Pontederia crassipes Mart.; Heteranthera formosa; E. speciosa Kunth.; Piaropus crassipes (Mart.) Raf.
Locations within which Eichhornia crassipes is naturalised include Africa, Australia, India and many other countries.
Water hyacinth is invasive in parts of Kenya, Tanzania and Uganda (Lyons and Miller 1999, Global Invasive Species Database). It is now found on virtually every large water body in the region and was renowned for invading the entire shores shoreline of Lake Victoria in the three countries.
Eichhornia crassipes grows and spread rapidly in freshwater. It can withstand extremes of nutrient supply, pH level, temperature, and can even grow in toxic water. It grows well in still or slow-moving water. Water hyacinth invasion is facilitated by water bodies that are enriched by agricultural chemicals, sediments from catchment erosion, domestic effluents and plant nutrients. Water hyacinth is widely considered to be the world's worst water weed.
Eichhornia crassipes is a large (up to 2 m above the water and 1 m below) floating freshwater plant with hollow, expanded leaf stalks (petioles) and "roots' (rhizoids) that trail underwater in a dense mat or in longing hanging strands.
Stems may be in the form of short runners (stolons) or upright (erect) flowering stems (up to 60 cm or more tall). The runners (about 10 cm long) are produced across the water surface and give rise to new plants called "ramets". These stems are hairless (glabrous), but dark coloured feather-like roots (2-100 cm long) are produced off the runners (stolons).
Plants generally produce leaves in basal clusters (rosettes). The leaf stalks (petioles) vary from being short, inflated, and spongy to being long and relatively slender (3-60 cm long). The leaf blades (2-25 cm long and 2-15 cm wide) range from being oval (elliptical) to egg-shaped in outline (ovate) or almost rounded (orbicular) in shape. They are hairless (glabrous) and glossy in appearance, with almost parallel veins following their outline.
The showy flowers (4-6 cm long and 3.5-5 cm wide) have six 'petals' (actually three petal-like sepals and three petals) that are fused at their bases into a short tube (1-2 cm long). They are purple, bluish or mauve (rarely white) in colour and the 'petal' lobes are 2-4 cm long. The uppermost of these 'petal' lobes has a yellow spot surrounded by a darker blue or purplish area. The flowers are arranged along an upright (erect) spike up to 15 cm long (containing 3-35, but normally 5-20, flowers). There are also two green bracts at the base of each flower (that resemble sepals) and they have six stamens.
The fruit are capsules (10-15 mm long) with three compartments and contain up to 300 seeds. The seeds (0.5-1.5 mm long) are egg-shaped (ovoid) and ribbed lengthwise (longitudinally). When all the flowers on a flower spike have withered, the stalk gradually bends into the water and later the seeds are released from the mature capsules.
The seeds can remain viable for 15-20 years but the main method of reproduction is vegetatively. They may also by spread by boats and wind A single plant under ideal conditions can produce 3,000 others in 50 days, and cover an area of 600 square metres in a year.
Eichhornia crassipes has sometimes been confused Pontederia cordata (pickerel weed), another introduced water weed of the same family. These species can be readily distinguished by the following differences:
Introduced as an ornamental plant, water hyacinth also has a number of other uses but this cannot compensate for its overall negative impacts. It can be used as a green manure and livestock feed (if mixed with dry feeds), for biomass production and for handicrafts among other uses. Water hyacinth at low density can increase fish catches due to its provision of niches for invertebrates that act as fish food.
Eichhornia crassipes is regarded as an environmental weed in many parts of the world and has been widely categorised as the "world's worst aquatic weed". It has been nominated as among 100 of the "World's Worst" invaders by the IUCN Invasive Species Specialist Group and it has been listed as a noxious weed in many countries and states including South Africa, Australia and Kenya. E. crassipes has a vast range of negative impacts across the world and in Africa in particular.
E. crassipes can disrupt waterbodies in many ways. It clogs waterways preventing river travel, blocks irrigation canals, destroys paddy rice fields and ruins fishing grounds. By shading the water, these plants can deprive native freshwater plants of sunlight and animals of oxygenated water. As the mats decay, there is a sharp increase in nutrient levels in the water, which spark off algal growths that further reduces oxygen levels. It affects hydro-electricity production as its vast mats clog the turbines of downstream hydropower installations. E. crassipes also increases water loss from a water body with its evapotranspiration far exceeding that of open pan evaporation (Howard and Harley, 1998, Hill et al., 1999). It is also able to accumulate suspended sediment amongst its rhizoids and later deposit it away from its origin.
The impacts of E. crassipes on Lake Victoria has become well known case study throughout the world. E. crassipes was first sighted in Lake Victoria in 1988.There, without any natural enemies, it became invasive around almost every available part of the lake in Kenya, Tanzania and Uganda, affecting fisheries, water supply, access to the lake (including lake transport), water quality and lake biodiversity, until it was largely managed by biological control, which became effective in the very early 2000s after much damage had been done (G.W. Howard pers. comm.).
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 harvesters but the running costs are high and this method is not sustainable in most cases. 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.
Several biological control agents, notably two weevils (Neochetina bruchi and Neochetina eichhorniae) have been released against water hyacinth and in many cases have given excellent and sustained control. Several other insects (moths, other beetles, grasshoppers), mites and fungi (rusts) have been successfully introduced for hyacinth control in various countries.
Eichhornia crassipes 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.
Henderson L (2001) Alien Weeds and Invasive Plants: a complete Guide to declared weeds and invaders in South Africa. Agricultural Research Council. Cape Town RSA.
Henderson, L. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.
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.
Hill, G., Cock, M. and Howard, G. (1999). Water Hyacinth, its control and utilization. A global review. SIDA Publications on Water Resources, No. 15, 54pp.
Howard G.W. and Harley, K.L.S. (1998). How do floating aquatic weeds affect wetland conservation and development? How can these effects be minimised? Wetland Ecology and Management, 5: 215-225.
Weber E. (2003). Invasive Plant Species of the World. A Reference Guide to Environmental Weeds. Wallingford, UK: CABI Publishing.
Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.
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).
BioNET-EAFRINET Regional Coordinator: [email protected]