Aquatic plants are not desirable in ponds for a variety of reasons, which include reduced fish production by removal of nutrients, interference with population balance, breeding habitat for mosquitoes, and interference with angling. Aquatic plants can be classified into five basic categories: algae, floating, submergent, emergent, and marginal. Control measures differ for each group, and proper identification of the plant is necessary before the correct control measure can be selected.
Most aquatic plant problems can be prevented in a properly constructed pond by a good fertilization program. Plants need sunlight in order to thrive. If the water depth is a minimum of 2 feet and a satisfactory plankton bloom is maintained by fertilization, sunlight cannot penetrate to the bottom and rooted plants will not grow. For fertilization to be effective, a bloom must be established early before nuisance plants begin to grow. Attempting to fertilize after the plants have become established will only worsen the problem.
In established ponds where the water is less than 2 feet deep, the most practical method of control may be to lower the water during the winter and deepen the shallow areas to a minimum of 2 feet in depth. Deepening the edges should be completed prior to February to allow sufficient time for the pond to refill before the bass begin to spawn. Soil removed from the shallow areas may be used to make earthen piers for better bank access or to form underwater structure in the pond.
Many plants in the marginal or emergent groups may be removed by hand. It is a simple task to pull a few water lilies or cattails from the pond before they have time to grow, reproduce, and cause major problems.
Chemical control with registered herbicides is effective on most aquatic plants. The correct herbicide depends upon proper plant identification. To achieve the desired results, it is important that all label instructions be followed when an aquatic herbicide is used.
Some plants can be controlled by natural or biological methods. One such control agent is the white amur (grass carp). When stocked at the proper rates (see table), these fish can provide long-term control on aquatic plants and do not interfere with the sportfish population. In ponds with established bass populations, the amur should be at least 10-12 inches in length so that they will not be consumed by the bass. A spillway barrier should also be utilized to prevent the amur from escaping during periods of heavy rainfall. White amur will usually control aquatic plants for about 5 years. Older, larger amur become ineffective; therefore, they may have to be restocked periodically.
Control of aquatic plants depends on correct identification followed by the proper control. A fishery biologist will be contacted for plant identification and control measures that are best suited for your pond. Aquatic plants from water gardens or ornamental ponds should never be placed in a fish pond. Many of these plants are not native to Alabama and can have serious environmental impacts on ponds or adjacent natural waters. Most are illegal to place in the public waters of Alabama.