In India, over 140 plant species act as aquatic weeds. Of these, the following species are particularly notorious: 1. Eichhornia crassipes 2. Hydrilla verticillata 3. Potamogeton spp. 4. Salvinia spp. 5. Typha spp. 6. Pistia stratiotes 7. Nymphaea (waterlily) and Nelumbo spp. 8. Vallisneria spiralis 9. Ipomoea carnea 10. Lemnids 11. Alternanthera philoxeroides.
Aquatic Weed # 1. Eichhornia crassipes (waterhyacinth):
Waterhyacinth is a free-floating, surface aquatic weed with large, buoyant petioles and highly attractive, mauve or lilac flowers. The plant produces copious seeds which germinate on the shore lines. Each hyacinth seedling after reaching the main waters multiplies fast through its offsets into as many as 600 plants just in four months.
Gradually a thick hyacinth mat is formed on the water surface which drifts with water currents. In warm waters the hyacinth biomass may increase at the rate of up to 20 g m-1 day-1. Waterhyacinth mats frequently weigh 350 to 1700 t ha-1.
Waterhyacinth is native to Brazil. In India it was introduced in 1886 in Bengal as an ornamental, pond plant. Since then it has spread throughout India as an obnoxious aquatic weed covering about 2 million ha of water surface in various moving and confined waterbodies. Fish and rice crops worth millions of rupees are damaged each year at the hands of this weed.
In shallow drainage and irrigation ditches, waterhyacinth becomes a rooted plant to hinder the free flow of water there.
Several kinds of waterhyacinth harvesters and mat lifting devices have been developed and used to remove this weed from waterbodies in different countries, including India. Simultaneously, very effective and economic control of the weed has been found possible by spray application of 2,4-D (and MCPA). The treated mats sink in about 6-8 weeks. Other effective herbicides against hyacinth are paraquat, diquat, and Amitrole-T.
The biological control of waterhyacinth has met with field success particularly in Florida using certain insect bioagents (Table 16.1).
Aquatic Weed # 2. Hydrilla verticillata (hydrilla):
Hydrilla is a rooted, submersed weed growing in up to 10-12 m deep waters. Once infected, the ponds, lakes, and canals are filled with it fast. It propagates through its axillary and underground turions and plant fragments. The weed offers great resistance to the movement of water and fish in the waterbody. Also, its rotting masses emit highly offensive odour. In confined waterbodies hydrilla may fill these so badly that eventually they become dry.
Under-water weed cutters are employed to cut hydrilla shoots from up to 1 m depth. The harvested vegetation can be collected down the stream at convenient points. Besides these, very effective herbicides are now available to control hydrilla. Of these, a mixture of diquat (or paraquat) at 1 ppm and CSP at 4 ppm Cu, is found most feasible.
Grasscarp is a voraceous consumer of hydrilla. It can be very effectively used for biological control of the weed in canals, lakes, and ponds.
Aquatic Weed # 3. Potamogeton spp. (pondweeds):
Potamogeton spp. are submersed, rooted herbs with varied leaf shapes. In India, P. nodosus and P. pectinatus are common species of this aquatic weed. They are noxious in our valuable Bhakra-Nangal and Chambal Command canals where they obstruct badly the flow of water, fishing, and aquatic sports. The pondweeds propagate through rhizomes, tubers, and sometimes also by turions.
The physical control of pondweeds is similar to that already given for hydrilla. Of the herbicidal options, 2,4-D granules (BEE), chlorthiamid, acrolein, and aromatic solvents have been found particularly effective against pondweeds. The exposed bottom treatment of canals with dichlobenil (4 kg ha-1) and fenac (10-20 kg ha-1) is permitted.
Aquatic Weed # 4. Salvinia spp. (waterfern, salvinia):
Salvinia is a free-floating water fern. It propagates by offsets and floats with the help of leaf hair which trap air. The plant has three phases in its growth. Its mat phase, which is the most prominent phase, has conduplicately folded leaves, covering continuous large areas of water surface.
In India, S. molesta and S. nutans are noxious species of waterfern. While the former infests waterbodies in Kerala and Orissa, the latter is found weedy in the Kashmir lakes. The fern was introduced in India from Brazil as a specimen for its botanical study, which later got spread to rivers, canals, lakes, and paddy fields in the above states.
Of the physical methods of weed control, netting of waterfern has been practised on small scale by the natives. In Kerala, sometimes its mats are physically diverted towards sea waters where they wither away. But none of these practices yielded results.
The herbicidal control of salvinia is still in infancy. Kerosene oil and PCP were found effective in destroying Salvinia leaf hair, which forced the fern to sink. But large scale application of these chemicals from boats has not met with much success.
In paddy fields, thiram, a fungicide, has been found to rid the crop of the fern. Its effective rates are 7-12 kg ha-1. More recently hexazinone and ametryne have been found very effective against salvinia. Further research is needed with these herbicides for their field feasibility.
The biological control of Salvinia has been found extremely successful in India.
Aquatic Weed # 5. Typha spp. (cattails):
Cattails are robust, grass-like, perennial plants forming up to 3 m high clumps. In India these are particularly problematic in drainage channels and ditches in her Canal Command areas and shallow water table places. Here its biomass production of up to 500 t ha-1 has been frequently recorded. T. angustata (narrowleaf cattail) is the most common species of cattail in the country although T. latifolia and T.elephantina are also found in some places.
Typha spp. propagate by its cylindrical rhizomes and numerous, air-borne seeds. Mowing with swords and sickles is practised in India to harvest cattails and clean the drainage systems. But the weed regrows fast. Superior control of Typha can now be obtained by using certain herbicides.
A mixture of dalapon (15-30 kg ha-1) and Amitrole (6-12 kg ha-1) applied to foliage of the weed at its full spike stage will destroy its shoots as well as the shallow rhizomes. The treated plants lose grip in the soil within 2-3 days whence these can be easily pulled out of the drains. Other herbicides found effective against cattails are glyphosate, diquat, and paraquat.
Aquatic Weed # 6. Pistia stratiotes (waterlettuce):
Like waterhyacinth, waterlettuce is a mat forming floating plant with the difference that its mats are light, being only 3% as heavy as waterhyacinth. Also, it does not bear ornamental flowers.
Waterlettuce is notorious for harbouring vectors of malaria and like fevers. It can be controlled with foliar spray of 2,4-D/MCPA, paraquat, Amitrole-T. Also, anhydrous ammonia disintegrates its mats rapidly.
Aquatic Weed # 7. Nymphaea (waterlily) and Nelumbo spp. (lotus):
These are surface, rooted aquatic plants with large, roundish leaves placed flat on the water surface. Both, waterlily and lotus put forth highly attractive flowers, clearly above the surface of water. These are often sold in the cities for worship. The unripe seeds of lotus are edible and its rhizomes form delicious vegetable.
Despite all these economic uses, waterlily and lotus act as weeds in several confined waterbodies meant for pisciculture, irrigation, and potable uses. Also, the waterlily and lotus infested lakes become unfit for boating and bathing.
2,4-D at 1.5 kg ha-1 has been found very effective in clearing the fish ponds of waterlily and lotus. Addition of a suitable wetting agent to the spray tank will improve weed kill. Silvex and glyphosate are also found effective against these two noxious weeds.
Aquatic Weed # 8. Vallisneria spiralis (eelgrass):
Eelgrass is a submersed, rooted aquatic weed of confined and slow moving waterbodies. It interfers with the healthy aquatic sport and free flow of water. Eelgrass is found susceptible to 2,4-D (20-60 ppm) and CSP (2 ppm). Also, grasscarp is a rapid feeder of this weed.
Aquatic Weed # 9. Ipomoea carnea (gramaphone flower):
I. carnea is a semi-woody shrub of ditches and waterlogged areas. It grows up to 3m tall, with attractive bell-shaped flowers. Introduced as a hedge plant from South America, now this plant has spread as a noxious weed in numerous roadside ditcheds and paddy fields. 2,4-D (2-3 kg ha-1) has proved very effective in controlling this weed.
Aquatic Weed # 10. Lemnids (duckweeds):
The lemnids comprise species of Lemna and Spirodella. These are very minute (a few mm across), free-floating plants providing a green carpet to the surface of stagnant waterbodies like roadside ditches and small ponds. Lemnids propagate through seeds and by rapid frond division.
Their main weedy nature results from their minute size which enables them to easily enter the irrigation pumps, sprayers, siphon tubes, and the sprinklers; all of which are badly choked. Duckweeds are susceptible to 2,4-D, diquat, and endothall (Na and K salts). Grasscarp and mirrorcarp feed heavily upon these plants.
Aquatic Weed # 11. Alternanthera philoxeroides (alligatorweed):
Alligatorweed is an invasive alien, aquatic, perennial weed. It is native to South America, more specifically Argentina. In India it has spread in her varied waterbodies as well as in regularly inundated, high rainfall terrestrial habitats in almost 16 states since its first record from Bihar in 1965.
In some states it has already assumed serious status. Alligatorweed can tolerate brackish waters. Owing to its high regeneration capacity, the alligatorweed has potential to become serious menace and threaten biodiversity in the country.
In some states, the natives have used alligatorweed shoots as green fodder. Three herbicides, 2,4-D ethyl ester (2-2.5 kg/ha), glyphosate (2.5-3 kg ha-1), and metsulfuron methyl (0.016-0.020 kg ha-1), are found very promising in damaging alligatorweed foliage.
Biological control of the weed is an interesting option in restricting the spread of this perennial weed. Thus for insect bioagents found promising against alligatorweed include- Cassida sp. nr. enervis (turtlebeetle) and Agasicles hygrophyla (fleabeetle).