Source : Deccan Herald, Monday, May 16, 2005

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Weeds are defined as unwanted and harmful plants which hinder the growth of cultivated plants. This definition is based on man's needs rather than the functional role of a 'weed' in an ecosystem. Eichhornia crassipes (Mart.) Solms popularly known as “water hyacinth”, is the predominant weed at the Hebbal lake, the Nagavara lake and the recently cleared up Ulsoor lake. These floating plants have oval, shiny green upright leaves which act like sails. The spongy leaf stalk helps maintain buoyancy. They have showy blue-purple or lilac-coloured flowers with yellow spots.

Water hyacinths were native to Central and South America but were brought into the United States for the Cotton States Exposition held in New Orleans in1884. Because of their beauty, they were given away as gifts to visitors to the Japanese Pavilion at the expo.

Attendees took them home to add to their backyard ponds. By 1900, water hyacinth had become a serious pest, clogging waterways throughout the coastal states of America.

Killer weed

Water hyacinth is considered one of the world's worst aquatic plants. It forms a dense mat that interferes with navigation, recreation, irrigation, and power generation. It rapidly covers water bodies as it has well developed vegetative propagation in the form of offsets. The dense mat causes low oxygen regions beneath the mats which becomes good breeding grounds for mosquitoes. As hyacinths cover the water's surface, they restrict life-sustaining sunlight that submerged native plants need in order to grow. Eventually the shaded underwater plants die and decay. The decaying process depletes the amount of dissolved oxygen in the water. As the oxygen level declines, fish such as bass, perch, and bream, seek new habitat areas, leaving fish such as catfish, carp and gar, all of which can tolerate lower oxygen levels . Once oxygen levels become so low that even these less desirable fish cannot survive, the water below the hyacinth mass becomes devoid of most life.

Water hyacinth has a unique capacity to absorb and concentrate massive amounts of inorganic minerals dissolved in the water mass. This biomass serves as a productive resource for mushroom cultivation. Some people compress dried water hyacinth into logs and burn it for fuel. The fibrous root system of water hyacinth provides nesting habitat for invertebrates and insects. Leaf blades and petioles are occasionally used by coots. They also have a major role in plant succession as they provide the next generation of plants with humus by their death and decay. However, whatever benefits this plant provides to wildlife are greatly overshadowed by the environmental invasiveness of this noxious species.

Swamp Devil

Water hyacinth can be controlled by harvesting, aquatic herbicides, and biological control agents. Western countries use ‘Swamp Devil' — a mechanical harvester which is a heavy duty aquatic vegetation cutter that features tow blades measuring 2.4 meters across at the front. It has a 234 horsepower engine and can easily shred trees up to 15 cm in diameter. It collects and removing a portion of the chopped debris. The harvester has the ability to carry four tons of vegetation on board in a single load.

Depending on the weight and volume of the vegetation and the distance to the shore, the harvester can potentially remove 16 to 32 loads of chopped hyacinths in eight hours. But mechanical methods used to chop up the water hyacinth cannot reach all the water hyacinth in a given water body and this method results in fragmentation — one of water hyacinth's reproductive strategies. Herbicides are very effective in killing water hyacinth plants. Spray programmes are costly and in many cases are only used on waterways necessary for navigation or heavily used in recreation.

The use of natural predators, in particular a weevil species from Argentina and a carp species from the Soviet Union, both of which use the water hyacinth as a food source, would mean introducing other exotics species that may, in time, become as serious a pest as water hyacinth. Locally, the best way to manage water hyacinth is to prevent it from becoming established.

FLOATING GARDENS

In China and our country, Eichhornia crassipes is used to generate biogas and serves as a source of inexpensive energy to rural communities. The hyacinth is used as part of a filtration system for water purification systems. Water hyacinth is now sold by many nurseries for its unusual appearance, attractive flowers, and ability to remove nutrients from the water. People in Uganda use water hyacinth to make paper and mats. Water hyacinth is used as organic compost and is a cheap fertiliser and soil conditioner. Women in Fiji weave baskets with dried water hyacinth. Farmers in Bangladesh and Burma are said to make floating vegetable gardens by heaping mud on top of densely packed water hyacinth and growing all sorts of vegetables.

The possibility of growing mushrooms on a water hyacinth substrate was first demonstrated by Professor S.T. Chang of the Chinese University of Hong Kong, and was put into practice by Margaret Tagwira, a laboratory technician at the African University of Mutare in Zimbabwe. When the programme began, the only mushroom species being grown in Africa was the typical white button mushroom (Agaricus bisporus). Since then, thanks to the work of Margaret Tagwira, other species have been successfully cultivated, including the oyster mushroom (Pleurotus) and the wild Reishi mushroom (Ganoderma lucida).

HYACINTH FOR ANIMAL FEED

When water hyacinth grows out of control, the plant is a nuisance, but researchers suggest uses for this exotic plant that could make it a harvestable resource. It can be used as food for cattle. But care should be taken not to change cattle diet suddenly as it could cause illness. Water hyacinth should be mixed with other feed. Boiled water hyacinth is used in Southeast Asia as feed for pigs. The plants are chopped and sometimes mixed with other vegetable waste, such as banana stems, and boiled slowly for a few hours until the ingredients turn into a paste, to which oil cake, rice bran and sometimes maize and salt are added. The cooked mixture is good for only three days, after which it turns sour. A common formula for feed for pigs is 40 kg of water hyacinth, 15 kg of rice bran, 2.5 kg of fish meal and 5 kg of coconut meal.