AQUATIC WEEDS AND THEIR CONTROL

INTRODUCTION
Aquatic weed are those unwanted plants which grow in water and complete at least a part of their life cycle in water. Many aquatic plants are desirable since they may play temporarily a beneficial role in reducing agricultural, domestic and industrial pollution. Letting a crop of plants grow in a lake or pond and then killing it over a period of time, and consequently releasing nutrients back into the water, may help in fish production. However, many aquatic plans are considered weeds when they deprive human beings of all facets of efficient use of water and cause harmful effects, some of which are discussed below.

Submersed, immersed, emerged and marginal weeds in and along irrigation canals, ditches, and drainage channels impede water flow, increase evaporation, cause damage to canals and structures, and clog gates, siphons, valves, sprinkler heads bridge piers, pumps, etc.

Floating and deep-rooted submersed weeds interfere with navigation. Some of the tougher and densely growing weeds, e.g. water hyacinth and alligator weed become impenetrable and prevent boats and even steamers moving through. Submersed and floating aquatic weeds in farm ponds, village tanks and water reservoirs reduce their utility for water storage and irrigation.

Aquatic weed growth also prevents or impairs the use of inland waters for fishing. The weed assimilates large quantities of nutrients from water, thus reducing their availability for desirable planktonic algae. They cause oxygen deficiency and prevent gaseous exchange with atmosphere, resulting in an adverse effect on fish production. Excessive growth of these weeds may provide excessive cover, resulting in an overpopulation of small fish and interference with fish harvesting.
Aquatic weeds provide a suitable habitat for development of mosquitoes in impounded waters, causing malaria, filariasis and encephalitis. These weeds serve as the primary vector for the disease-causing organisms.
Aquatic weeds reduce the recreational values of lakes, tanks, streams, etc. as the water is made turbid or dirty with an undesirable odour

TYPES OF AQUATIC WEED

There are two types of aquatic plants: algae and hydrophytes.

ALGAE-Algae normally inhabit the surface of fresh and saline waters exposed to sunlight. While some kinds of algae are found in solid and on terrestrial surfaces exposed to air, the majority aquatic and adapted to live in ponds, lakes, reservoirs, steams, swimming pools and oceans.

Freshwater algae are of two types: planktonic and filamentous. planktonic algae, called phytoplankton, include the truly aquatic single-celled algae and the simplest filamentous or colonial forms. A heavy growth of algae may colour the water shades of green, yellow, red and black. Hey may also form water blooms or scums. They convert solar energy into food, remove CO2 from water during photosynthesis (in day time) and produce oxygen as a by product. During the night or in cloudy water, they release co2 in the water through respiration as consume O2.

Certain planktonic algae are beneficial as they can maintain biotic balance in natural aquatic environment because of their ability to produce oxygen and maintain an aerobic condition. They are the original sources of food for most fish and aquatic animals. Although planktonic algae are beneficial, their overabundance may be undesirable for many domestic and commercial water uses.
Excessive phytoplanktonic blooms often result in zooplanktonic (the microscopic animal forms) development that may deplete result in zooplanktonic the water of oxygen and lead to over fertilization or eutrophication and destruction of fish and other aquatic wild life. Dense growth of planktonic algae will shade bottom muds sufficiently to prevent germination of seeds and growth of many species of rooted submersed weeds, thus affecting the stability of the habitat

Generally, planktonic algae do not interfere with the use of surface waters or irrigation purposes. But some of them, the blue-green algae and green algae produce odours and scums that make unfit for swimming. Several of the blue-algae produce toxic substances that kill fish, birds and domestic animals.

Another group of algae called filametous algae (nanoplankton) consists of single celled joined end to end which may form single thread, branched filaments, nets, or erect stem like whorled branches or forked leaf like forms. They don’t have roots, stems or leaf as do higher plants. The important genera of the filamentous algae are: Chara, Nitella, Spirogyra, Hydrodictyon, Cladophora, Pithophora.

HYDROPHYTES-The hydrophytes, which represent more than 100 families, are vascular plants. They grow wholly or partially submersed in either fresh or saline water or in plaustrine areas. They are structurally different from mesophytes and xerophytes that grow in moisture-deficient situations. The protecting and conducting tissues of hydrophytes are less developed. They have extensive provision for aeration and buoyancy, particularly in the leaf mesophyll, ground tissue of the petiole and the cortex of the stem and root. Buoyancy is provided by aerenchyma or by air chambers. The air chambers may be either schiogenous or lysigenous, or both. Hydrophytes weeds can be grouped as submersed, emersed, marginal and floating weeds.

1- SUBMERSED WEEDS
Submersed weeds are mostly vascular plants the produce all or most of their vegetative growth beneath the water surface. Most submerged vascular weeds are seed plants and have true roots, stems and leaves. Abundance and density of these weeds is primarily dependent on depth and turbidity of water and physical characteristics of the bottom. A maximum depth of 3.5-4 m in clear waters is the limit for most of the submersed plants. They are capable of absorbing nutrients and herbicides through the leaves and stems as well as roots. They compete for nutrients with planktonic algae and decrease their production and a corresponding decrease in fish production.
The submersed weeds belong to the following genera: Potamogeton, Elodea, Myriophyllum, Ceratophyllum, Utricularia, Ranunculus, Heteratheral Alisma, Zannichellia, Lemna, etc.

2- EMERSED WEEDS
Emersed weeds are those plants rooted in the bottom muds with serial stems and leaves at or above the water surface. They grow in situations where the water level ranges from just below ground level to about half the maximum height of the plant. They differ in leaf shape, size and pint of attachment. Some of the weeds of this group have broad leaves, 5-50cm in diameter, and others have ling narrow leaves like grasses, less than 3-15cm or more in width; the latter are commonly called reeds. The leaves of emersed weeds do not rise and fall with water level as in the case of attached floating weeds. Some of the emersed weeds belong to the genera Nuphar, Nelumbo, Jussiaea, Myriophllum, etc.

3-MARGINAL WEEDS
Most marginal weeds are emersed weeds that can grow on saturated soil above the water surface; they grow from moist shoreline areas into water up to 60-90cm in depth. Marginal weed vary in size, shape and habitat species of this group are the most widely distributed rooted aquatic plant. Plants of this group are broad leaves herbs, shrubs, trees and some grasses. The important genera to which they belong are : Phragmitis, typha, Polygonum, Alternanthera, Populus, Tamarix, Cephalanthus, Juncus etc.

4-FLOATING WEEDS
Many water plants have leave that float on the water surface either singly or in rosettes .They have true root and leaves .Some are free floating and others rooted in bottom mud have floating leaves that rise or fall with the water level .They reproduce very rapidly under favorable conditions and are among the most troublesome of aquatic weed .Floating weed belong to the genera Eichhornia, Pistia, Salvinia , Lemna , Nymphaea and brasenia.

MAJOR AQUATIC WEEDS OF INDIA

Common Name Botanical Name
Water hyacinth Eichhornia crassipes
Cattails Typha angustata
Pond weed Potamogeton spp.
Hydrilla Hydrilla verticillata
Water lettuce Psitia stratiotes
Salvinia Salvinia molesta
Swamp morning glory Ipomea aquatic
Alligator weed Alternanthera spp.
Arrow head Sagittaria spp.
Spatter dock or Yellow lily Nuphar spp.
Pickerel weed Ponterdenia cordata
Reed weed Pharagmites communis
Swamp morning glory Ipomea aquatica

METHODS OF CONTROL OF AQUATIC WEEDS

Mechanical & Manual Methods
These methods employ physical forces to remove weeds:

1. Dredging- It is most common way of cleaning weeds in ponds and ditches.A dragline dredge may be equipped with a bucket or with a weed fork or other special tools. The bucket dreadge will also remove mud along with weeds while the weed fork will leave the mud. This method is labour expensive and slow.
2. Draining - Draining is an offseason weed management normally in drainage ditches by which weeds are cut manually or mechanically or spraying with a total weed killer or bottom ploughed to kill vegetative structures and root stock

3. Drying- Simple and inexpensive. The tops of under water weeds are exposed to sun by draining the water from ditches and ponds are allowed to dry. This method is effective in areas where the ponds are seasonal in use and remain dry during summers and rainy seasons. Drying is ineffective against emersed weeds and some of floating species.

4. Burning- It is used to control weeds in the banks above the water line. For obtaining the best results first searing the green vegetation and secondly after 10 -12 days with complete burning. In searing a hot flame is passed over the vegetation at such a rate that the plants wilt but are not charred. Mowing followed by burning the dried weeds may increase the effectiveness of mowing.

5. Chaining- This is relatively inexpensive method which is widely used. A heavy chain is attached between two tractors or teams on the opposite banks of ditch. As they move the chain drags over the weeds and breaks them off. Chaining is generally done only when the ditch is severely clogged. Chaining is primarily to ditches of uniform width and accessible from both sides with tractors. It is effective for cattails, tules, bur reed, arrow head and other emersed weeds.

The major disadvantages of chaining are its too laborious has to be repeated at regular intervals reverse chaining cannot be done.

6. Cutting- A mechanical weed cutter is used to cut the submersed weeds at 1 to 1.5 m deep in water. It consists of a sharp cutter bar operated hydraulically from a boat. The harvested weeds float to the water surface and removed manually or by sieve buckets.

Disadvantages of Mechanical Control Methods- They do not provide effective and economical weed control because of which repetitive operations are required. The weed fragment remaining proves as a source of new infestation.


HERBICIDES USED FOR CONTROLLING AQUATIC WEEDS

Asulam-Docks and bracken on banks.
Dalapon-Effective against grasses and cattails when applied on foliage. Draining of water before application is advisable. It is used in irrigation and drainage channels, lakes, ponds & ditch banks. Applied @ 15-20 Kg/ha for surface area. It is normally harmless to fish. However treated water is unsuitable for potable & irrigation purposes.

Dichlobenil-Effective on Elodea, watermilfoil, chara and potamageton species and is applied before weed starts growing. This is applied at the rate of 5 to 10 Kg/ha to the exposed bottom after draining water. This pre emergence application inhibits regeneration from roots and rhizomes. Water is let in after a month of treatment.

Diquat & Paraquat-These are generally used for control of floating weeds like water hyacinth and water lettuce with foliar application at 1 to 2 Kg/ha. There application in muddy water is ineffective. Efficiency is increased by combining with copper sulphate and triethanolamine.

Aqualin-The compound acrolein or acrylaldehyde has been named as aqualin. It is an active chemical attacks plant cells and kills them. It has lachrymatory effect on man and has to be sold by licensed operators.

Endothall-Aquatic weeds including algae without harming fish or aquatic life. It can be used for both submerged as well as emersed aquatic weeds. Granular formulation is Aquathall.

Copper sulphate-It is effective against many kinds of algae including chara and other species that causes scum. It is either applied as crystals or by placing crystals in a bag or towed behind a boat until the chemical is dissolved. This is applied at the rate of 0.5-1 ppm W. A concentration upto 2.5 ppm W is considered safe for human consumption while above 1 ppm W is considered unsafe for fish, but can be used for irrigation purpose.

Silvex-Effective for control of surface and emersed weeds like alligator weed, water lily, arrow head etc. prevalent in standing waters. Treated water is unsuitable for any purpose.

Method of application
Application is done by pumping the liquid into the water and allowing it to move as a blanket over and through the aquatic weeds. The herbicide may be introduced over a time period ranging from 45 min to 5 hrs. Temperature of water is important consideration. At 150C the dosage must be twice that at 28 0C. Hence the dosage must be adjusted with the plant population and temperature. In fast flowing streams, contact is not so thorough as in slow and hence, dosage must be increased when flow is greater than 10 cumec.

For proper control of weeds of wider water bodies power sprayers are mounted on motor boats. The spray range in this case will be 10 to 20 mts. With a discharge rate of 20 to 200 lts per min. For injection in stable water a hard pipe with several small holes is fitted. In flowing water the herbicide is injected from the shore itself. The herbicide is carried down the stream. During spraying the boats should maintain a speed of 2-5 Km/hr.

Biological control-

Name of Bioagent Weed controlled
Congo tilapia & Jawa tilapia Algae (chara & Nitella) & saw weeds (Najas)

Chinese grass carp or white amur
(Ctenopharyngodon idella) Aquatic plants

Common carp It is mud bottom feeder and it controls submersed aquatic weeds due to its uprooting plants and breaking up mats of algae in its search for food.

Marasmiellus inoderma Thread blight in water hyacinth

Flea beetles (Agasiches hydrophilla) Water hyacinth and Salvinia