Carp and Weed

Aquatic plants form the basis of the food chain, directly or indirectly feeding all of the animals in the system. Plants provide breeding and nesting sites and cover for fishes, waterfowl, and mammals. Plants also supply oxygen through photosynthesis, stabilize the pond or lake bottom, and prevent shoreline erosion. The amount of nutrients in water and sediment and the availability of sunlight for photosynthesis regulate the growth of aquatic vegetation. Once weeds become established, they can clog water intakes and distribution systems used for irrigation, public water supplies, and hydroelectric generating plants. They interfere with aquaculture, impede boating access and navigation and make swimming, boating, skiing, and sport fishing difficult and dangerous, if not impossible. Sometimes swimmers become entangled in dense growths of aquatic weeds and drown. Weeds increase sedimentation in flood control reservoirs. They crowd out important native aquatic plants and decrease the diversity in the lake. 

How does weed grow? Does it always grow upwards or does it usually spread along the lake bed in clumps?

Carp need aquatic Plants & Weed

There are three types of aquatic weeds. 

1. Submersed plants grow underwater and are generally rooted in the bottom sediment. If they have flowers, they may extend above the surface of the water. This group includes many introduced or exotic species that grow rapidly. These include hydrilla, Brazilian elodea, slender pondweed, curly-leaf pondweed, and slender naiad. Eurasian Watermilfoil (Myriophyllum spicatum) grows mostly around the shoreline, especially in channels and shallow areas. It forms thick underwater stands of tangled stems and vast mats of vegetation at the water's surface, especially in nutrient-rich lakes. Underground runners and plant fragments can generate new plants. 

2. Floating plants either float freely on the surface, obtaining their nutrients directly from the water or they have leaves that float on the surface and are rooted in the bottom sediment. They can form dense mats on water surfaces, reducing light and oxygen levels in the water. Examples include duckweed, watermeal, and water-lily. Azolla (Azolla filiculoides) is a short, branched, floating stem, bearing roots which hang down in the water. It prefers shaded conditions to full exposure to sunlight. The plant is highly productive with the ability to double its weight in 7 days. Water hyacinth (Eichhornia crassipes) has not spread prolifically in the UK because of the colder climate. Once established in favourable habitats, however, it can form dense floating mats. 

3. Emergent plants have most or all of their vegetative and flowering parts above the surface of the water or on the shoreline. They are also referred to as shoreline and wetland plants because they do not need water for support. Many can survive submerged in water or in saturated soils. They include cattails, alligatorweed, torpedograss, smartweed, purple loosestrife, arrowhead and pickerelweed.

Are growth rates affected by temperature change and the depth of water?

Aquatic weeds are a problem during the warmer months of the year when water temperatures rise above 43°F. Reedmace (Typha latifolia) requires moist or wet substrates, warm temperatures of 77°-86°F, low oxygen concentrations, and long day/short night exposures to grow. It grows rapidly, producing excessively long vegetative stems that crowd out native wetland vegetation. It can completely take over a site and, by impeding water flow, convert fast-flowing aerobic streams into partially anaerobic swamps. Reed sweet-grass is typically found in the bank of slow-flowing rivers. 

Submersed aquatic perennial weeds grow rapidly, forming dense mats and crowding out native vegetation. They are found in wetlands, and slow-flowing or standing freshwater, including ponds, lakes, reservoirs, canals, ditches. Crassula helmsii, also known as Australian Swamp Stonecrop and New Zealand pygmyweed, can grow in a variety of different water types, including acid, alkaline and even in semi-saline, down to depths of 3 meters. It does not die back during winter. Slender pondweed (Potamogeton pusillus) has narrow, ribbon-like leaves and grows in water up to 16 feet deep. Hydrilla (Hydrilla verticillata) has horizontal and heavily branched stems near the water surface. In clear water, it can grow in depths of more than 40 feet. 

Does weed provide oxygen or take anything out, does it provide food and/or shelter? Does weed decay and cause problems during other times of the year?

Weeds inhibit water circulation and reduce the levels of dissolved oxygen. They cause the release or build up of nutrients, particularly phosphorus and nitrogen, and inevitably lead to excessive growth of algae or other aquatic weeds. They reduce the number of invertebrates per square meter by half, depriving the fish of food. When water weeds decay, oxygen levels fall. 

What kills weed?

Brazilian elodea (Egeria densa) was originally imported from South America for the aquarium trade. It has few natural predators to keep its growth in check. Once introduced, Brazilian elodea reproduces by the spread of plant fragments. It forms dense mats that choke out native aquatic plants. Sterile (triploid) grass carp find Brazilian elodea very palatable and older grass carp will eat it in preference to other plants.

Canadian Pondweed (Elodea Canadensis) is one of the most familiar non-native aquatic weeds to be found in UK waters. Since the 1950s, it has become less problematic and is now considered preferable to the more invasive Nuttall’s pondweed (Elodea nuttallii) and curly waterweed (Lagarosiphon major). The latter thrives in alkaline waters where it has a greater ability to photosynthesise. Pondweed can grow up to 24 inches in length. Sterile triploid grass carp can be introduced into ponds to destroy the pondweed by eating off the weed tips. Normally, 5-10 grass carp per surface acre can control weed growth in ponds. In heavily weeded areas, 15-20 carp per surface acre are required. The carp population must be monitored and migration barriers erected to prevent them from escaping into other water sources. The drawback to carp control is that it can take up to 18 months to eliminate the weeds. 

The most common method used to control algae is periodic treatment with a copper-based algaecide. Another method of control is the use of a dye containing a blend of blue and yellow dyes to limit sunlight penetration into the water and prevent photosynthesis by the algae and weed growth. The introduction of a safe and harmless blend of bacteria will breakdown pond sludge and reduce excess nutrients from the water. This will help clear the water, reduce odors and digest unwanted organic compounds. Aquatic plants, such as pickerelweed, arrowhead and bulrush, planted along the pond margins will absorb excess nutrients and eliminate excessive algae. 


Does weed need special circumstances for rapid growth like water clarity? Does clearer water mean more weed growth? Does weed need sunlight?

Water clarity is essential for the growth of aquatic plants, but not necessarily so for aquatic weeds and algae. Nutrients and suspended solids gradually cause a loss in water clarity. When phosphorus levels are low, there are no planktonic algae present and the water is clear to the bottom. When the water temperature climbs, phosphorus rises from the sediment and its concentration may quadruple. This facilitates dense algae blooms and a reduction in water clarity. The phosphorus will increase as oxygen is depleted in the bottom water. This causes fish to suffocate and aquatic plants to die. Changes in water temperature or increased rain will redistribute the phosphorus, making it less lethal. The problem, however, is that aquatic weeds will displace the native plants, further reducing water clarity.

Will weed cause water colour changes?

Filamentous algae, long filaments of single-celled algae, can form dense mats over a large part of the pond's surface. Planktonic algae are free-swimming single algae cells that can give the water a green colour and/or impart taste or odors to the water. When a huge amount of either filamentous or planktonic algae is present, it is called an algae bloom. This is harmful because it depletes the oxygen content of the water and suffocates the fish. All can bring about severe habitat reduction, stunt sport fish growth, and reduce the total number of harvestable fish. 

Will weed create a difference in temperatures to the main water body?

Without sufficient light, water temperatures will decrease. Curled pondweed (Potamogeton crispus) grows in dense beds in fresh and slightly brackish waters. It is tolerant of low light and low water temperatures, allowing it grow sooner and outcompete native plants in the spring. The weed invades both shallow and deep water areas. The dense surface mats increase phosphorus concentrations in the water causing an increase in algae blooms. The weed dies back in mid-summer. 

Carp Love Weed. Find weed & you'll Find Carp!

Does weed photosynthesize and result in releasing oxygen in surrounding area?

Watermeal (Wolffia spp.) grows so quickly that it can completely cover a pond in just a few weeks. The complete shading of the water severely limits photosynthesis by submerged plants and algae, depriving the pond of a major source of oxygen. Oxygen consuming respiration by aquatic plants and animals and decay processes continue unabated, with the end result being lethal low-oxygen levels after several weeks. As a result, many fish suffocate. 

Different insects and mites grow on the weed, do carp feed on them?

Plants provide a refuge for small animals against predation by bigger ones. Zooplankton and invertebrates use the plants to escape predation, as do small fish. Stands of aquatic plants have a much richer invertebrate population than unvegetated stands, both in terms of the diversity of species and total numbers of bugs. Aquatic plants are usually coated with periphyton (attached algae), which is an important food source for invertebrates. Carp feed on worms, insect larvae and fresh vegetable matter. These are only partly digested and serve as roughage to keep the intestine active and healthy. Carp will also ingest silt and other indigestible detritus to bulk out their diet.

Are there any natural food sources carp may find apart from weed which may cause them to move out of weed at certain times of the year?

The direction of the wind has a direct impact on the feeding habits of carp. Anglers know that nutrients in the water change direction to match that of the surface winds. Carp follow the nutrients because they can taste and smell items in the water. They have a highly sensitive olfactory system that allows them to sense dissolved substances in the water and determine whether or not it is a good food source. In addition, the lining of the carp’s mouth contains chemically sensitive cells that allow it to determine whether that food is good or bad. As the wind driven water hits a shoreline, the under-current stirs up food from the bottom, attracting the fish. When the food becomes scarce, the carp will move further from the bank and gradually disperse across the lake. A change in wind direction will encourage the fish to move into the windward shore again.

Do carp use weed at any time during reproduction?

Weeds are an essential part of the carp reproduction process. Carp spawn in the spring and early summer in weedy, grassy, shallow areas of lakes and watercourses, usually about 2-3 feet deep. The small, sticky eggs become attached to submerged weeds, grasses or other substrate. For the first 2 to 4 days, fry ingest their yolk sac, which has all the necessary nutrients for survival. When they have completely absorbed it, the fry will feed on microplankton and small invertibrates. They usually remain in shallow water with dense plant growth to protect them from predators and being swept away by the current until they are larger. 

Will carp prefer to hide in weed during weather changes, due to it being warmer or during times of high disturbance around lake?

Most anglers know that weather affects the quality of fishing. Poor weather conditions, such as clouds and rain, increase the probability of catching a carp. Rain in warm weather cools the water and increases oxygen levels. This freshens up the water and makes the carp more sensitive to the smell of the bait. Cold weather appears to have the reverse effect. Carp become less excited and more difficult to catch. In clear water under sunny conditions, the carp’s ability to see is excellent, depending on its depth in the water. Cloudy conditions, on the other hand, reduce the amount of light in the water and have the side effect of hiding the angler’s presence. 

Does fishing in weed have many benefits to the carp angler?

Carp can lose advantage of sight in weed as they cannot see angler, plus they will lower their guard slightly as they tend to feel safer in weed. Although some anglers don’t like fishing in weed, there are techniques that can be used successfully to catch the elusive carp. It is important to try to get the bait as near the bottom as possible. The hook point should be masked to prevent the hook catching on the weed as it goes through. Using a bolt rig will ensure that the hook and bait pass through the weed and land on the clear bottom under the vegetation.


Animal Feed Resources Information System. No date. Azolla spp.

URL:   (Accessed April 8, 2007).

Boone, J.L. 1987. Control of Phragmites communis: Results of Burning, Cutting, and Covering with Plastic in a North Carolina Salt Marsh.

URL:   (Accessed April 8, 2007).

Delaware Division of Fish and Wildlife. Policy on the Use of Grass Carp for Aquatic Weed Control.

URL:   (Accessed April 6, 2007).

Huckel, Jon. 2002. Invasive Non-native Species in the UK.

URL:   (Accessed April 8, 2007).

Invasive Species Specialist Group. 2006. Glyceria maxima (aquatic plant).

URL:   (Accessed April 8, 2007).

----------------------------------------. 2006. Potamogeton crispus (aquatic plant).

URL:   (Accessed April 9, 2007).

Lake Access Project. 1998. Aquatic Plants: Why Those Weeds are Important to Your Lake.

URL:   (Accessed April 8, 2007).

Lynch, Jr. William E. No date. Duckweed and Watermeal: Prevention and Control.

URL:    (Accessed April 8, 2007).

Maddocks, Kevin. No date. Carp Fishing in Weedy Waters.

URL:   (Accessed April 6, 2007).

Mississippi State University. No date. Weeds in Ponds: Grass Carp: Stocking.

URL:   (Accessed April 6, 2007).

Rook, Earl J.S. 2004. Typha latifolia: Common Cattail.

URL:   (Accessed April 8, 2007).

Royal Horticultural Society. 2007. Advice: Aquatic Weeds.

URL:   (Accessed April 8, 2007).

The Lake Doctors, Inc. No date. Weed Types: Submerged Plants.

URL:    (Accessed April 6, 2007).

Washington State Government. No date. Non-native Freshwater Plants: Brazilian elodea.

URL:   (Accessed April 8, 2007).

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