Weeds are plants, which grow where they are not wanted (Sharma, R. and Shekara, C. et al.). This applies for agricultural, horticultural and pasture land where weeds grow together with cultivated plants. It applies as well to irrigation and drainage canals, water ways, ponds and lakes, where aquatic plants are growing in the water causing many negative effects. Aquatic weeds are those unwanted plants which grow and complete their life cycle in water and cause harm to aquatic environment directly and to related eco-environment relatively. Though they are not competing directly with crops, they can damage the irrigation system and reduce the water availability and thus contribute to a reduction of growth and yield of cultivated crops in irrigated areas. Aquatic weed management can result in time-consuming and costly control activities which reduce the economic benefit of the yield considerably (ICID).
Negative effects of weeds in water systems:
- Village ponds, tanks and reservoirs get infested with floating and submerged weeds which results in reducing the capacity of the water storage and therefore effecting efficient irrigation. The plants cause tremendous loss of water from water bodies like lakes and dams through evapotranspiration.
- Aquatic weeds have been found to severely reduce the flow capacity of drainage and irrigation canals thereby reducing the availability of water to the farmer’s field. Impediment in flow of water may result in localised floods in neighboring areas.
- Aquatic weeds can even damage irrigation and drainage systems, dams, pumps and turbines in different types of power stations, siphons, valves, bridge piers etc., they can negatively influence power generation and increase the cost of maintenance of power stations.
- The decomposition of huge amounts of biological mass creates condition where CO2 and carbon monoxide are produced and released to the atmosphere.
- The plants can reduce fish production and complicate fish harvesting.
- Dense growth of aquatic weeds may provide ideal habitat for the development of mosquitoes causing malaria, encephality filarasis.
- Dense mats of floating or deep rooted submerged weeds prevent the movement of boats (e. g.; Water hyacinth, Alligator weed) and at times even large ships.
Beneficial effects of aquatic weed species can occur as well:
- Aquatic weeds are useful in paper, pulp and fiber industry.
- Water hyacinth and water lettuce are used for composting; other species serve as green manure. In China, water hyacinth has been largely used for feeding pigs (Amarasinghe, L. and Labrada, R.).
- Weeds may provide food and habitat for fish, other animals living in the water and ducks.
- Theoretically, aquatic weeds could be used for biogas production which is however still difficult to manage (collection of the great biomass of plants).
- Many aquatic plants may play temporarily a beneficial role in reducing agricultural, domestic and industrial pollution.
- Aquatic weeds may be useful by providing continuous supply of phytoplankton and help fish production.
Aquatic weeds in different regions
The world’s total irrigated area was 249.5 m ha in 1997 (FAO 2000, quoted by ICID) which is 17.2% of total arable land. It is this land which provides sustained and assured productivity of crops and employs high input returns. Irrigation depends on the water availability in storages, tanks, lakes, rivers, canals etc. However, these are often infested by aquatic plants which cause manifold problems. The presence of excessive aquatic vegetation influences the management of water in natural waterways, man-made canals and reservoirs which amounts to millions of kilometres / square kilometers of such water bodies around the world. Irrigation canals, drainage ditches, ponds, rivers and lakes can be affected by plants which may block dams and waterways and impede irrigation as well as drainage.
India has 1.2 m ha under irrigation canals and more than 4 m ha under water in reservoirs, ponds and tanks and the largest canal network in the world where the velocity of flowing water is reduced by about 30 - 40 percent due to the presence of aquatic weeds.
The worldwide distribution of aquatic weed species is described by ICID. A selection of effected countries follows below.
Africa: Egypt, Kenya, Tanzania, Uganda, Zimbabwe.
Asia: Bangladesh, India, Indonesia, Malaysia, Philippines, Thailand, China
Latin America: Argentina, Brazil, Central America, Mexico.
Others: Australia, Europe, North America.
Different types and species of aquatic weeds
In many cases, other species than the mentioned ones of the same genus do exist; e. g. Salvinia auriculata, S. molesta, S. natans. Therefore, the identification of the occurring species is necessary in each particular situation before taking any action.
There are some types of water weeds (Anonymous, Anonymous/AQUAPLANT, ICID):
Free floating aquatic plants
These float on the water surface and are not attached to the bottom and may form large floating mats. Examples are small plants like Duckweed (Lemna minor), Giant duckweed (Spirodela polyrrhiza), Water velvet (Azolla imbricata) and Giant salvinia or Water fern (Salvinia molesta), Water lettuce (Pistia stratiotes). The Water hyacinth (Eichhornia crassipes) is a big plant with a great decorative flower. The invasive plant grows in all types of freshwaters and forms large mats that jam rivers and lakes and can seriously interfere with navigation. Anchored Water hyacinth (Eichhornia azurea) is also found throughout Africa. Eichhornia crassipes, Salvinia molesta, Pistia stratoites are some of the most problematic weeds in the tropical and subtropical regions of the world.
Submersed aquatic plants
Those plants root in the sediment and usually grow entirely underwater like Hydrilla (Hydrilla verticillata), Bladderwort (Utricularia flexuosa), Waterthread pondweed (Potamogeton diversifolius), Curly-Leaf pondweed (P. crispus) is a troublesome plant forming dense surface mats, Southern naiad (Majas guadalipensis), Parrot-Feather (Myriophyllum brasiliense), Eel grass (Vallisneria americana), Brazilian elodea (Egeria densa) and plants of the genus Elodea, e. g. the Canadian water weed (Elodea canadensis).
Emersed (or emergent) plants
They are rooted in the bottom, but stems, foliage and flowers extend above the water surface like Cattail (Typha spp.), Besharam (Ipomea carnea), Common reed (Phragmites communis), American lotus (Nelumbo lutea), Water lily (Nymphaea odorata), Pickerel weed (Pontederia cordata), Arrowhead (Sagittaria spp.), Purple loosestrife (Lythrum salicaria) can lead to serious problems in irrigation canals and wetlands.
Algae in shallow water submerged
In some cases, Algae can be considered as weeds. Unlike other aquatic plants, they do not produce flowers or seeds. Possibly important algae are: Blue green algae (Anabaena spp.), Musk grass (Chara zeylanica), Stone wort (Nitella hyalina), Slimy green algae (Spirogyra spp.).
Amphibious species of marshes or swampy areas
There are situations where vast areas of land remain inundated with water for long periods of time, and may only dry out in severe drought conditions (marshes or swampy areas). They support a different type of vegetation which may include weeds that are capable of growing under both flooded and saturated conditions. Those species are: Alligator weed (Alternanthera philoxeroides), Pepper west (Marsilea minuta), Wild mud plantain (Meteranthera limosa), Paragrass (Panicum perpurascens) and Water paspalum (Paspalum fluitans).
Water weeds obtain their nutrients from water rather than soil. They can rapidly spread and increase their distribution area. This is often facilitated by a lack of natural predators, abundance of space and abundant nutrients. The infestation caused by land and water plants is often due to human activity; e. g. the water hyacinth is native to South America and was introduced to Africa, China and elsewhere and is nowadays one of the most noxious water plants. The distribution area of water plants can be monitored by aerial photos and satellite images, and in some cases their spread has been documented through a series of photos over years.
Water hyacinth (Eichhornia crassipes) remains the world’s most important aquatic weed. It is spreading at an alarming rate in Africa and Papua New Guinea and is a major problem in the Indian Subcontinent and South-East Asia. Irrigation water often carries the seeds of aquatic weeds such as Eichhornia crassipes, In India, irrigation canals appear to be a potential source for spreading water hyacinth (Sushil Kumar and Bhan, 1994, quoted by ICID).
The distribution area of water plants can be monitored by aerial photos and satellite images, and in some cases its spread has been documented through a series of photos over years.
Large scale infestation of water hyacinth (Eichhornia crassipes) in drainage system in Haryana, India. ICID (fig. 18, p. 32)
Large scale infestation of Ipomea carnea in water tanks in Maharashtra,India. (ICID, fig. 17, p. 31)
Integrated management of water weeds
Considering the losses caused, it is essential to keep aquatic weeds under control in water bodies, flow water systems, ponds and tanks so that these systems can be utilized to best of their efficiency. Appropriate management of water from source to its utilization is necessary to sustain the normal function of life. The management plan is dependent upon correctly identifying the problem weed(s), (secondary) use of water, environment protection, feasibility of logistics and operational costs.
Long-term weed control is a challenge and can only be achieved by using a combination of recommended methods. Complete removal (eradication) of water plants is practically impossible under normal conditions. The accumulated weight of biomass can be too great or small plants and their seeds cannot be collected everywhere. The situation must be continually managed to reduce the mats. In broader sense, weed “control” means keeping the weed infestation at a level where they do not cause economic damage. The integrated management combines preventive and control measures of existing infestation (ICID; Shelton, J. L. and Murphy, T. R.). For India, Suresh, C. J. recommends an ample management approach comprising surveys, research, awareness creation among the population, establishment of a coordination committee etc.
Quarantine regulations try to avoid the introduction or spread of a weed in a certain region or limit the weed occurrence to a small area where it can possibly be eradicated. Irrigation water often carries the seeds of aquatic weeds such as Eichhornia crassipes, Pistia stratiotes, and Salvinia molesta. It is important to control weeds near and in reservoirs and irrigation canals to prevent them from shedding seeds into the water (ICID).
Physical or mechanical methods
Mechanical control of aquatic weeds primarily consists of removing the weeds physically from the water body. This could be done manually by hand, using hand tools or machine power, or by using a mechanical weed cutter to cut floating and submersed weeds. In some cases, the harvested weeds are collected and water is squeezed from them to hasten dehydration and desiccation.
Dredging (digging out) is done in large water bodies, canals and drains. It is a common method of cleaning ditches but slow, time consuming and is a costly operation. The machine could operate from the ground or from a boat in water.
The introduction of bio-agents will become one of the major methods of controlling aquatic weeds, especially the floating ones. Researchers are envisaging to establish an integrated approach to aquatic weed control using a mix of mechanical and biocidal techniques to control aquatic weeds under specific situations (ICID).
Fish: Fish is used in biological control of aquatic vegetation; e.g. the grass carp (Ctenopharyngodon idella). However the possibility of introducing grass carps and any other animals like snails and insects has to be carefully examined to exclude that the aquatic ecosystem is disturbed and biodiversity reduced.
Snails: Promising results have also been obtained utilizing snails (Pomade canaliculata) against an aquatic weed (Anachaares alensa) in Brazil. Good results have also been observed against other aquatic weeds (Ceratophyllum demersum, Najas guadalupensis and Potamogeton illinoensis) which were controlled completely.
Beetles: Successful biological control can significantly reduce the cover of Water hyacinth in 3 to 10 years after establishment of an agent and has achieved excellent control in number of countries. (Julien et.al., 1996, quoted by ICID). The use of a hyacinth-eating beetle (Neochetina bruchi) for controlling Water hyacinth was investigated in Karnataka (India) in 1984.
Competitive displacement: The approach of replacing harmful vegetation by relatively less harmful and beneficial vegetation is sometimes successful: the growth of Azolla in rice fields effectively controls the growth of other weeds.
Control of aquatic weeds by herbicides is generally easier, quick and usually cheaper, when compared to mechanical methods. The use of herbicides has the disadvantage of being in water as residue and more especially in areas where there is no control on water use by the population. A herbicide should have certain specifications for its use in different types of aquatic environments. Before applying herbicides, the laws and restrictions of the country, appropriate spraying equipment, protective clothing, safety measures and qualification of handling person as well as good agricultural practice have to be taken into consideration (Schuler). The herbicide should be environmentally safe for humans, fish and other aquatic fauna. A single herbicide that controls weeds as well as being safe for all possible uses of the treated water is yet to come (ICID).
Cultural and physiological methods
Drying or water management (decrease of water level) is generally practiced in flowing water systems like irrigation canals, drainage ditches. During the process the water is removed. Control is achieved by either dehydration of the vegetation or by exposure to low temperatures.
Amarasinghe, Lucky and Labrada, Ricardo: The problem of water weeds in China.
Anonymous (Internet 2014): AQUAPLANTA. Pond manager diagnostics tool. Texas A&M AgriLife extension. http://aquaplant.tamu.edu/plant-identification/
Anonymous (Internet 2014): Common Aquatic weeds. Aquarius systems.
Anonymous (2013): Weed Alert: Anchored water hyacinth. New South Wales.
ICID – CIID, International Commission on Irrigation and Drainage (2002): Aquatic weeds and their management, 71 pp. http://www.icid.org/weed_report.pdf
Remark: the publication has valuable information about integrated weed management but should not be considered with regard to recommendations concerning the use of herbicides.
Schroeder, Jill, Murray, Leigh et al. (2012): Identification and Detection of Weeds on Irrigation Canals: Survey of the Vegetation and Soils of the Leasburg Canal System, 2002-2006
Research Report 777. http://aces.nmsu.edu/pubs/research/water/RR-777.pdf
Sharma, Rajeev and Shekara, Chandra et al. (edits.) (2013): Farmer’s Handbook of Basic Agriculture. A holistic perspective of scientific agriculture. Desai Fruits & Vegetables Pvt. Ltd., Navsari, Gujarat, India, 146 pp. Remark: the book has valuable information about integrated weed management but should not be considered with regard to recommendations concerning the use of herbicides.
Shelton, James L. and Murphy, Tim R. (1989): Aquatic Weed Management Control Methods. SRAC Publication No. 360, 2 pp. http://www2.ca.uky.edu/wkrec/360aqplant.pdf
Suresh, C. Jain: Aquatic weeds and their management In India. J. N. Agricultural University Campus Indore, Madhya Pradesh, India, 3 pp. http://www.apms.org/japm/vol13/v13p6.pdf