Eichhornia crassipes, commonly known as Common Water Hyacinth, is an aquatic plant native to the Amazon basin, and is often considered a highly problematic invasive species outside its native range.

Description

Its habitat ranges from tropical desert to subtropical or warm temperate desert to rainforest zones. It tolerates annual precipitations of 8.2 dm to 27.0 dm (mean of 8 cases = 15.8 dm), annual temperatures from 21.1°C to 27.2°C (mean of 5 cases = 24.9°C), and its pH tolerance is estimated at 5.0 to 7.5. It does not tolerate water temperatures >34°C. Leaves are killed by frost and salt water, the latter trait being used to kill some of it by floating rafts of the cut weed to the sea. Water hyacinths do not grow when the average salinity is greater than 15% that of sea water. In brackish water, its leaves show epinasty and chlorosis, and eventually die.^[1]

Because of E. crassipes invasiveness, several biological control agents have been released to control it, including two weevils, Neochetina bruchi Hustache (Coleoptera: Curculionidae) and Neochetina eichhorniae Warner (Coleoptera: Curculionidae), and the moth Niphograpta albiguttalis (Warren) (Lepidoptera: Pyralidae).^[2] Neochetina eichhorniae causes "a substantial reduction in water hyacinth production" (in Louisiana); it reduces plant height, weight, root length, and makes the plant produce fewer daughter plants. N. eichhorniae was introduced from Argentina to Florida in 1972.^[3]

Azotobacter chroococcum, an N-fixing bacteria, is probably concentrated around the bases of the petioles. But the bacteria do not fix nitrogen unless the plant is suffering extreme N-deficiency.^[4]

Fresh plants contain prickly crystals.^[1] This plant is reported to contain HCN, alkaloid, and triterpenoid, and may induce itching.^[5] Plants sprayed with 2,4-D may accumulate lethal doses of nitrates,^[6] as well as various other nocive elements in polluted environments. See further down.

Invasive species

E. crassipes has been introduced in various countries, and in some locations it has become serious invasive plant species. In New Zealand it is listed on the National Pest Plant Accord which prevents it from being propagated, distributed or sold.

Africa

The plant was introduced by Belgian colonists to Rwanda to beautify their holdings and then advanced by natural means to Lake Victoria where it was first sighted in 1988.^[7] There, without any natural enemies, it has become an ecological plague, suffocating the lake, diminishing the fish reservoir, and hurting the local economies. It impedes access to Kisumu and other harbors.

The water hyacinth has also appeared to the north in Ethiopia, where it was first reported in 1965 at the Koka Reservoir and in the Awash River, where the Ethiopian Electric Light and Power Authority has managed to bring it under moderate control at the considerable cost of human labor. Other infestations in Ethiopia include many bodies of water in the Gambela Region, the Blue Nile from just below Lake Tana into Sudan, and Lake Ellen near Alem Tena.^[8]

Uses

Bioenergy

Because of its extremely high rate of development, Eichhornia crassipes is an excellent source of biomass. One hectare of standing crop can thus produce more than 70,000 m³ of biogas.^[9] According to Curtis and Duke, one kg of dry matter can yield 370 liters of biogas, giving a heating value of 22,000 kJ/m³ (580 Btu/ft³) compared to pure methane (895 Btu/ft³)^[10]

Wolverton and McDonald report only 0.2 m³ methane per kg, indicating requirements of 350 MT biomass/ha to attain the 70,000 m³ yield projected by the National Academy of Sciences (Washington).^[11] Ueki and Kobayashi mention more than 200 MT/ha/yr.^[12] Reddy and Tucker found an experimental maximum of more than a half ton per day.^[13] Bengali farmers collect and pile up these plants to dry at the onset of the cold season; they then use the dry water hyacinths as fuel. They then use the ashes as fertilizer. In India, a ton of dried water hyacinth yield circa 50 liters ethanol and 200 kg residual fiber (7,700 Btu). Bacterial fermentation of one ton yields 26,500 cu ft gas (600 Btu) with 51.6% methane, 25.4% hydrogen, 22.1% CO₂, and 1.2% oxygen. Gasification of one ton dry matter by air and steam at high temperatures (800°) gives circa 40,000 ft³ (circa 1,100 m³) natural gas (143 Btu/cu ft) containing 16.6% H3, 4.8% methane, 21.7% CO, 4.1% CO₂, and 52.8% N. The high moisture content of water hyacinth, adding so much to handling costs, tends to limit commercial ventures.,^[11][14] A continuous, hydraulic production system could be designed, which would provide a better utilization of capital investments than in conventional agriculture, which is essentially a batch operation.,^[1][15]

The labour involved in harvesting water hyacinth can be greatly reduced by locating collection sites and processors on impoundments that take advantage of prevailing winds. Wastewater treatment systems could also favourably be added to this operation. The harvested biomass would then be converted to ethanol, natural gas, hydrogen and/or gaseous nitrogen, and fertilizer. The resulting byproducts of water and fertilizer can both be used to irrigate nearby cropland.^[1]

Phytoremediation, waste water treatment

The roots of Eichhornia crassipes naturally absorb pollutants, including lead, mercury, and strontium-90, as well as some organic compounds believed to be carcinogenic, in concentrations 10,000 times that in the surrounding water.^[16] Water hyacinths can be cultivated for waste water treatment.^[1]

Edibility

The plant is used as a carotene-rich table vegetable in Taiwan. Javanese sometimes cook and eat the green parts and inflorescence.^[1]

Medicinal use

In Kedah (Java), the flowers are used for medicating the skin of horses.^[1] The species is a "tonic."^[17]

Other uses

In East Africa, water hyacinths from Lake Victoria are used to make furniture, handbags and rope.^[18] The plant is also used as animal feed and organic fertilizer although there is controversy stemming from the high alkaline pH value of the fertilizer.^[19] Though a study found water hyacinths of very limited use for paper production,^[20] they are nonetheless being used for paper production on a small scale.

Gallery

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  Floating

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  Flowers

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  A pond covered with water hyacinth

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  Inflated petiole

References