The water hyacinth is native to Latin America and is exotic to southern Africa where it does not have a natural predator. The weevils that eat it have been imported into the region, including Zimbabwe where results show that the weevils are helpful in controlling the weed.

The water hyacinth has been described as the " noxious beauty" and "the beauty killer". When in flower, the plant is colourful and is used by some people as a pond flower.

It is called noxious and killer because it can cause the death of fish and other water organisms largely by depleting oxygen in the water when the plant dies and rots in large quantities. Rotting uses oxygen.

The leaf stalks of the plant are swollen and spongy, making it light and allowing it to float on water. It reproduces by seed as well a vegetatively. This makes it difficult to destroy. Under favourable conditions, the hyacinth can double its mass and the area it covers every five days.

In a year, one plant can develop into millions of other plants. These can weigh about 28,000 tonnes and cover 140 hectares, nutrients and space permitting. It can grow to a metre in height. But the plant does not grow in temperatures below 10 or above 40C.

Seeds of the weed can lie dormant for years, buried under water. When the water level falls and the seeds are exposed, they are ready to shoot. Most of them will grow once the water level begins to rise as they are supplied with the necessary moisture. Most blooms frequently occur at the end of a dry period and the beginning of a wet one.

Ninety-five percent of the weight of plant is water. Loss of water through evapotranspiration by the plant is about three- and-half times that of evaporation from an open water surface.

The weed is found in much of southern Africa. It occours in more than 50 countries of the world, including Egypt, Sudan, Australia, Indonesia, SriLanka and India.

The hyacinth is a problem in the Limpopo river which is shared by Mozambique, South Africa and Zimbabwe, in the Zambezi shared by eight countries of the Southern African Development Community (SADC), in the Shire of Malawi, the Vaal, Orange, Crocodile and Letaba rivers of South Africa as well as in the Kagera river and Lake Victoria, Singi and Pangani rivers of Tanzania. Swaziland and Lesotho have no water hyacinth problems. And in Angola, the extent of the water hyacinth problem is not known.

The water hyacinth uses large amounts of water, leading to rapid reduction of surface water.

It inhibits the growth of other water plants that are food for fish and other aquatic animals. This reduces fish stocks and their ability to multiply. The "mats" formed by the weed reduce fish breeding space.

The weed can form thick dense mats that stop light from penetrating the water, reducing photosythesis in the water. The mats lower temperatures, disadvantaging species adpted to a warmer envirronment.

The weed provides an ideal habitat for bilharzia carrying snails and malaria carrying mosquitoes. The vectors anchor onto the plants and they are protected from waves. Fish fail to access mosquito larvae, allowing mosquito populations to grow rapidly. In addition, the mats help disperse the snails as they can comfortably float on them. New areas become infested by the disease carriers. The weed reduces the quantity of oxygen in the water by limiting space available for free air to mix with water. Rotting weeds use up to a lot of oxygen, lowering oxygen needed by other organisms.

The weed can affect the economy because it is expensive to control. In Zimbabwe, for example, about Z$7 million (US$875,000) was spent in a few months to reduce the spread of the hyacinth. The weed also inhibited boating in Lake Chivero, near Harare. In Mlawi, it has inhibited the use of the Shire for ferrying people and goods between Malawi and Mozambique.

The weed can block irrigation canals and pipes. When this happens, time that would otherwise be used to till the land is spent removing the weed. Meanwhile, the plants to be irrigated are deprived of water.

The problem of water hyacinth in Lake Chivero

Lake Chivero is 23 Kilometres from Harare. It receives treated sewage from the city and the water is, therefore, rich in nutrients. This, plus ideal temperatures make the water suitable for hyacinth growth and spread.

In 1952 when the lake was built on Manyame river, an outbreak ensued as the weed was already in the river. Herbicide 2,4-D was used to kill it. Following the 1968 drought, the dam level fell by 3.5 metres, exposing seeds that had been lying dormant for at least 20 years. When the water level rose again in 1971, the weed proliferated. The 1983-4 droughts were followed by similar weed population booms. Chemical and mechanical methods have both been employed to solve the problem. The success has always temporary.

When in January 1990 the Lake overflowed, between 50,00 and 100,00 tonnes of the weed passed the dam wall and smashed two pipes from the dam. It took eight months to remove the weed downstream from the dam.

The sheer weight of the weed as it moves with water can destroy bridges and dam walls, and these are costly to repair

It can also hamper fishing, a tourist activity, causing loss of revenue. Local fishermen lose a source of revenue and of protein.

In the Lower Shire river, fish production has decreased by half since the weed invaded the river. In 1991, Botswana’s Aquatic Vegetation Control Unit spent some 400,00 Pula (US$200,000) on controlling water weeds.

The weed can also cause health problems which impact on the economy. As the water hyacinth provides a good habitat for malarial mosquitoes and snails that carry bilharzia- causing parasites, governments are forced to import expensive drugs to cure people suffering from these diseases.

Another economic issue arises from the excess water loss caused by the plant. In a region where water is seasonally and spatially scarce, losing more of the resource has severe economic implications because water is less available for agriculture, mining and industrial production.

Basically there are two ways of dealing with the weed: eradication or utilisation.

Attempts to eradicate the water hyacinth have included manual and mechanical, chemical and biological means. Mechanical removal is the most inefficient and rarely succeeds. The plant grows faster than it can be removed.

Chemical control is easy to apply. The chemical that has been used is 2,4-D which, even though relatively cheap, can kill other non-target plants and pollutes the water. Chemicals reduce water quality, increasing the cost of purifying it.

The weevil Neochetina eichhorniae, originally from Latin America, was introduced in Lake Chivero in 1992 and has produced encouraging results partly because the temperatures remained high due to the drought.

Further east, Tanzania, Uganda, Rwanda and Kenya will release weevils in Lake Victoria this year to control the weed.

Biological control cannot eradicate the water hyacinth but can reduce the quantity of the weed to levels where it becomes easier and more effective to apply mechanical means or herbicides.

In India, water levels are manipulated to kill the weed. Since the weed is sensitive to arid conditions, it dies when water is flushed out of the river system or dam. This is effective where the plant has not produced seed.

The economic value of the plant renders it useless for cultivation. The plant cannot be used as feed for livestock because it has little protein. Its short fibres make it poor for paper-making. It could be used for purifying water by removing certain effluents. The plant can absorb nutrients from sewage and heavy metals from industrial wastes. However, the practice may lead to its spread to areas where it is unwanted.]

Countries in southern Africa sharing water bodies are jointly finding ways to control the spread of the weed. They exchange information about the weed and share their expertise and experience in dealing with it.

It is recommended that they further strengthen their current position by conducting regional training programmes, do cross-border weed surveys, prohibiting the transportation of the plant across borders, harmonising biological control methods, and establishing early warning systems on the occurrence of the weed.

Ministry of Mineral Resources and Water Affairs Water Research Commission

Department of Water Affairs P O Box 824, Pretoria

P Bag 0018, Gaborone South Africa

Botswana

Ministry of Works Ministry of Water, Energy and Minerals

Water Department P O Box 2000, Dar Es Salaam

P Bag 390. Lilongwe 3 Tanzania

Malawi

National Directorate of Water Department of Water Affairs

CP 1611, Maputo P. O. Box 50288. Lusaka

Mozambique Zambia

Ministry of Agriculture, Water and Rural Development Department of National Parks & Wildlife

Department of Water Affairs Management

P Bag X13184, Windhoek P.O. Box 8365, Causeway

Namibia Harare, Zimbabwe