All vegetation is not bad. A certain amount is needed for good fish growth and protection. In fact fish will benefit from more vegetation than anglers will typically tolerate. Plants produce food for many insects which in turn are eaten by fish. They also provide habitat for many fish food organisms and cover for small fish. Plants produce oxygen, protect the shoreline from wave erosion, and serve as feeding and nesting habitat for wildlife.
Aquatic plants can become so abundant that they interfere with fishing, swimming, and boating. Excessive vegetation can also provide too many hiding places for small bluegills so bass have difficulty controlling their numbers. This often leads to overpopulated bluegills. Periodic die-offs of dense vegetation, which usually occur after periods of cloudy weather, or when the water is muddy after a rain, or at the end of their growing season, can also threaten fish. Oxygen is consumed by bacteria that decompose dead plants. Low oxygen levels stress fish so they do not feed and grow, and often die (summerkills and winterkills). Decayed plant material also produces offensive odors and imparts undesirable flavors to water.
To control aquatic plants, it is important to know what type is causing problems. Aquatic plants can be grouped into four general categories: algae, floating plants, submersed plants, and emersed or marginal plants.
Algae: Algae are small plants which do not have true leaves or flowers. Different types of algae take on different forms. Microscopic, single-celled, free-floating algae are called phytoplankton. This form is used by microscopic animals (zooplankton) as food. Phytoplankton gives water a green to greenishbrown tint, but individual plants cannot be seen. Filamentous algae, commonly called “moss,” consists of masses of long, stringy, slimy or cottony strands which float on top or just under the surface of the water. Chara, commonly called muskgrass or stonewort, is a larger form of algae which grows on the pond bottom and has stem-like and leaf-like structures. It is often confused with flowering aquatic plants. Filamentous algae and Chara are usually considered undesirable.
Floating Plants: This group includes plants which have leaves that float on the surface and roots that hang down in the water without being connected to the bottom. Duckweed (Lemna) and watermeal (Wolffia) are common members of this group.
Submersed Plants: These plants grow under water, are rooted in the bottom, have stems, leaves, and produce seeds. These plants usually consist of a long flexible stem with clumps of narrow leaves along the stem. Some species have leaves that reach the surface which are a different shape than the lower leaves. Common examples of this group which occur in Kansas are pondweeds (Potamogeton), bushy pondweed (Najas), coontail Ceratophyllum), and water milfoil (Myriophyllum), and water buttercup (Ranunculus).
Emersed or Marginal Plants: Emersed or marginal plants are rooted in the pond bottom and have parts extending above the water’s surface. Shoreline plants are also included in this group. These plants usually occur in shallow water, but some species can grow out from shore, forming a thick belt of vegetation. Common examples of this group of plants are cattail ( Typha ), bulrush ( Scirpus ), rush ( Juncus ), cut-grass ( Zizaniopsis ), smartweed ( Polygonum ), creeping water primrose ( Jussiaea ), arrowhead ( Sagittaria ), willow ( Salix ), and cottonwood ( Populus ).
If aquatic plants occupy more than one-third of the pond area, one of four categories of control can be considered. They are: preventative, mechanical, chemical, and biological.
Preventative: Prevention is always the best control method. Plants are common in ponds that have clear water, high fertility and extensive shallow areas. Plant problems can be minimized through pond construction. All shallow mud flats should be eliminated by digging the shore areas to at least 3 feet deep with a 3:1 slope. Existing ponds with extensive shallow areas can be dug deeper during periods of low water.
High fertility can cause a plant problem because nutrients can be channeled into plants. It is desirable to avoid rich sources of nutrients, such as runoff from livestock holding areas or septic tank drainage.
Mechanical or Physical: Vegetation around the shore can be controlled by hand pulling, cutting, or mowing. Hand pulling is effective for controlling cattails, willow trees and cottonwood trees while they are small. As they get larger, chemical control is needed. Most submersed plants can be partially removed by raking or by pulling a chain or cable through the pond between two tractors.
Submersed vegetation can also be controlled by shading with dark plastic screen, similar to screening used for shade in greenhouses. A large piece of screen should be weighted down on the patch of plants. This compresses and shades the plants and they die. After 2 weeks, the screen can be moved to a new area. The advantage of this method is that fishing, swimming and boating can take place over the screen.
All mechanical and physical methods are temporary and normally affect only a portion of the pond’s vegetation. They must also be used frequently during the growing season.
Chemical: It is important to identify the problem plants, since there is no all-purpose chemical for aquatic vegetation control. Different herbicides are effective on different types of plants. Since the status of chemical registration is always changing, specific chemical names will not be listed. Aquatic herbicides are available at most dealers that handle agricultural chemicals. County agricultural agents and district fisheries biologists can give recommendations on which specific chemical to use.
Chemicals are registered for specific uses. Directions on the label should be followed explicitly and precautions should be observed. Many chemicals have restrictions on the use of water for a period of time after application. With some chemicals, fish should not be eaten for a period after application, or livestock should not drink the water for some time. These restrictions will determine which chemicals can be used.
Most chemicals are applied at a certain dosage per acre-foot of water in the affected area. Volume of the area to be treated can be calculated as described previously in the “Muddiness Caused by Soil Type” section or obtained from the Natural Resources Conservation Service if they designed the pond.
Most aquatic herbicides will not harm fish if applied according to directions. They are most effective if applied during April or May as the vegetation begins to grow. If applied after May or if the growth is heavy, only half of the pond should be treated at a time. The second half of the pond should be treated 2 weeks later. If the entire pond is treated at once, bacteria decomposing the dead vegetation could consume all of the dissolved oxygen, resulting in a fish kill.
One chemical treatment per year is usually sufficient, but, in some cases, a partial treatment is needed later in the summer. Chemical control is only temporary and must be repeated almost every year. It is expensive but effective if executed properly.
Certain chemical dyes can be added to the water to shade out the plants. These also are temporary and they impart an unnatural tint to the water for a period of time.
Biological: The most effective form of biological control is use of the herbivorous fish, the grass carp. This fish is a native of the large rivers of China and Siberia. Grass carp are mobile, and often escape through emergency spillways with high water flow. When small, it feeds on small crustaceans and insects. As it gets larger, its diet consists almost entirely of aquatic plants. It prefers some plants more than others but will eat most submersed aquatic vegetation found in Kansas. It has a voracious appetite and grows rapidly. Grass carp should be stocked at a density of 5-15 individuals per acre in ponds with severe vegetation problems. If adult bass are present, the grass carp should be at least 10 inches long when stocked to avoid predation. Ponds with only a narrow belt of vegetation should not be stocked with grass carp because these fish will eliminate habitat bass and bluegills need. More and bigger fish can be caught from a boat in such a pond than would be taken from shore if no vegetation existed.
Since the grass carp is an exotic fish, its use is not recommended by some federal agencies. In addition, the fish is not permitted to be used by the public in some states. In Kansas, no such bans exist and grass carp are available from many commercial fish growers. However, Kansas law requires that only triploid (sterile) grass carp be stocked. District fisheries biologists can provide further information and suggestions.
Fertilization can be used to control aquatic plants, but as stated previously, it can cause oxygen depletion problems and is not recommended in Kansas. Ducks, geese, or swans have also been used to control aquatic plants. They are esthetically pleasing but can be messy.