Vegetation Identification
When using chemical methods, it is critical to identify aquatic vegetation accurately. Refer to Additional Information, page 42, for literature on aquatic vegetation identification. Aquatic vegetation is a frustrating problem for many pond owners. Excessive water weeds affect both fish and fishing as well as render the pond useless as a water reserve for irrigation, fire fighting, drilling oil wells and swimming. Dense vegetation ties up nutrients which otherwise might be used for fish growth, and it can cause low oxygen levels in ponds. It is difficult for bass to feed on small fish in dense weeds, frequently resulting in overcrowded sunfish populations. In aquaculture, water weeds hamper seining as well as harbor hosts for fish parasites such as snails.
Aquatic vegetation provides food and habitat for ducks and other wildlife as well as refuge for small fish. Water plants help prevent erosion and also tie up nutrients that support the growth of microscopic green algae. This resulting increase in water clarity contributes to the growth of rooted aquatic plants.
The ideal abundance of aquatic vegetation (10-20 percent coverage) is difficult to achieve. In bass-bluegill ponds, some weeds are desirable for fish habitat. For swimming ponds, weeds keep the water clear but hamper swimming. Complete eradication of vegetation is best in aquacultural ponds. Unfortunately, methods of control do not always produce the desired abundance of aquatic vegetation.
Vegetation problems are caused by combinations of several factors. Aquatic vegetation thrives best where the water is shallow. Deep, steep-sided ponds rarely have weed problems. Nutrient runoff from feed-lots, septic systems, golf courses and residential lawns can cause weeds to proliferate. Clear-water ponds often have aquatic vegetation problems.
Vegetation Control
Aquatic plants can be controlled in several ways. The best method is prevention. Ponds constructed with limited shallow areas and steep-sided sloops (3:1) down to at least four feet, usually do not develop rooted aquatic plants. It may be practical to excavate the sides of existing ponds to help prevent weeds. Allowing cattle complete access to ponds usually increases bank erosion resulting in increased vegetation. If cattle must water in a pond, fencing to provide limited access will usually prevent overabundant vegetation. Complete elimination of vegetation may cause the pond to become muddy.
Mechanical Control
Physically removing the weeds from a pond is only a short-term, extremely labor intensive form of control. This can be done by raking, dragging a chain or cable through the pond or by pulling the weeds by hand. These methods are not effective for larger ponds. Mechanical weed harvesters are available but are not common and usually are not cost effective.
Fertilization
Fertilization of a pond to control weeds can be effective, but it can also contribute to the problem. Fertilizing a pond in the spring can result in a bloom of microscopic algae that will shade out any developing vegetation. A fertilization program requires intensive monitoring to prevent undesirable results. Refer to Pond Fertilization, page 36, for more details.
Herbicides
Aquatic herbicides can be safe and effective in killing aquatic plants if selected and applied properly. Only herbicides for aquatic use should be used. To select the appropriate aquatic herbicide, the problem plants must first be identified. The herbicide will list those plants for which it is effective. Read and understand the label before buying any aquatic herbicide. Apply it only as instructed on the label. Herbicides are generally expensive and require repeated application. Specific recommendations for chemical control are listed in the ODWC brochure Controlling Pond Vegetation, and in publications mentioned under Additional Information, page 42. In hot weather it is best to kill vegetation in no more than a quarter of the pond at a time because decomposing vegetation will lower pond oxygen levels and can cause fish kills. Seek professional advice before applying herbicides, especially in hot weather. An oxygen test also may be useful.
Water Dyes
Water dyes are available that will shade out developing submerged vegetation. They give the water a green or blue color. These dyes are inert and quite safe. Dyes probably are not cost effective for larger ponds or ponds with a high water flow.
Fall-Winter Drawdown
If a pond has a drain structure that can facilitate a drawdown of four to eight feet, a late fall-early winter drawdown may kill shoreline vegetation through freezing and drying. This may not be effective for some hardy plants such as cattails, however. After a drawdown, the pond should be allowed to refill prior to the spawning season. This method can be effective although some types of aquatic vegetation may increase after a drawdown. Seek professional advice specific to your situation before attempting this control method.
Grass Carp
Triploid Grass carp are legal in Oklahoma and require no permit. The grass carp is a different species from the common carp and can be distinguished by its cylindrical body shape, mouth at the tip of its head, lack of barbells (whiskers), and a lack of saw-toothed spines on its dorsal and anal fins. These fish are voracious plant eaters, consuming up to three times their weight a day in plants. The state record grass carp is almost 69 pounds. However, in most ponds grass carp probably won’t exceed 20 to 30 pounds.
Grass carp also are sold in hybrid and triploid forms. The hybrid has proven ineffective for controlling vegetation. The triploid is functionally sterile and is the only grass carp permitted in many states. This is why it is only legal to stock the Triploid Grass carp in Oklahoma, by using triploid grass carp, the pond owner is ensuring that the escape of fish into receiving streams will not result in reproduction in the wild.
Screens and other barriers are required to prevent the escape of grass carp; even a trickle of water is enough to allow escape. The cost of a screen is minimal when considering the cost of continually restocking to replace escaped fish (see Pond Construction, page 5, and Additional Information, page 42, for barrier construction information).
Grass carp will control most common pond weeds, although their feeding preferences change as they grow. Because larger grass carp do not eat filamentous algae, to control this plant small grass carp must be stocked.
In waters without predators (bass), stock grass carp no smaller than four inches. In waters with bass 10 to 12 inches long, small grass carp are a preferred forage. Reportedly they don’t avoid predation by bass and are easily eliminated. Grass carp larger than eight inches are worth the added expense in such situations.
Stock grass carp in lightly infested ponds at a rate of five to 15 per acre. In ponds larger than three acres, stock at 10 to 15 per acre. In heavily weeded ponds, stock 15 to 30 per acre. To control filamentous algae, stock 30 to 60 per acre using smaller grass carp. To eradicate vegetation in aquaculture ponds, stock as many as 40 to 80 per acre.
In most cases, control with grass carp occurs in the second year after stocking when the fish reach three to five pounds. An increased stocking rate will provide quicker results. Stocking grass carp after using an aquatic herbicide also should be effective. Restock grass carp as vegetation reappears in five to seven years or earlier.
Once the desired control is achieved, some grass carp may need to be removed. They can be caught on hook-and-line using dough balls or catfish chow; they also can be shot with a bow and arrow. Grass carp make fish table fare with their firm, white flesh and excellent flavor. However, proper preparation includes removing or otherwise handling small bones in the flesh.
Overstocking grass carp may lead to complete eradication of vegetation. This usually increases pond turbidity and lowers fishing quality. Be patient after stocking grass carp as it often takes one to two years before major vegetation reduction is noticed. For a list of grass carp suppliers, please visit the Oklahoma Department of Agriculture, Food & Forestry's aquaculture page at: https://ag.ok.gov/