Managing Nuisance Aquatic Plants
Management of nuisance aquatic vegetation in public water is regulated under the State Aquatic Vegetation Plan. Under the plan, a treatment proposal must be filed with the Texas Parks and Wildlife Department and the controlling authority for the lake or stream in question. Treatment proposals are not required for private water, but treatment with a restricted or limited-use herbicide requires certification from the Texas Department of Agriculture.
Control options fall into four basic categories: mechanical, environmental, biological, and chemical (herbicides). Each method has advantages and disadvantages. Factors to consider include effectiveness, cost, availability, ease of application, potential environmental consequences, and whether special permits are required.
Draglines, cutters, rakes, booms, mechanical harvesters and bottom barriers are common tools for vegetation management and control. Mechanical controls don’t require the introduction of chemicals to the environment, and some people prefer them for that reason. On the other hand, mechanical methods tend to be labor-intensive and costly.
Large floating species such as water hyacinth can be removed by harvesting or shredding. Marginal plants can often be controlled by cutting, especially if cutting starts early in the growing season. Cutting machines may also be used on submergent and emergent vegetation, but regrowth may occur, making it necessary to cut several times in a single growing season. With species like hydrilla, which can grow from fragments, it’s important to remove cuttings from the water; otherwise, fragments may grow in areas that were previously uninfested. In small areas, control may be achieved by pulling up young plants in the early spring.
Bottom barriers can inhibit the growth of submerged and emergent plant species. Semi-permeable material should be used in order to avoid a buildup of gases underneath the barrier that can lift it off the bottom.
Reshaping the shoreline to eliminate long gradual slopes and reduce the amount of shallow water is one way to reduce shoreline vegetation. Shallow water is especially conducive to plant growth. It warms up first in spring, and sunlight reaches all the way to the bottom, inviting young plants to grow. Lowering the water to allow excess vegetation to dry out or freeze in winter can also be effective. This technique is used in some large, public reservoirs.
Shading is another way to slow plant growth. Plants can’t make food without adequate sunlight. Commercially available dyes can be added to the water to inhibit light penetration. This technique should be used early in spring: dyes are less effective when plants are already growing close to the surface.
Fertilizing the pond can also limit light penetration. It stimulates the growth of phytoplankton, which intercept the light rays. Fertilizing has the added advantage of enhancing fish growth. The microscopic algae provide food for many invertebrates, which are eaten in turn by fish. Timing is important, however. Added nutrients can stimulate growth in undesirable plants if the water is allowed to clear, or if plants have grown close to the surface before fertilizer is applied. Therefore, fertilizing should be viewed as an ongoing process. Begin in the spring when water temperatures reach 60ºF. Continue as needed to keep underwater visibility at 18 inches or less through the growing season. Stop fertilizing in the fall when the water temperature drops below 70ºF. Granular or liquid fertilizers with a high phosphorus content can be used; a 10-34-0 formulation is suggested. Liquid fertilizers are easier to apply and may be up to four times more effective than granular varieties.
Stocking sterile triploid grass carp or white amur (Ctenopharyngodon idella) is a popular method of biological control. Young grass carp are voracious vegetarians. Under certain conditions, they have been known to eat 50% to 300% of their body weight per day. Feeding rates drop in older fish, but remain substantial at 25% per day or more. The grass carp’s preferred food, hydrilla (Hydrilla verticillata) is one of Texas’ most problematic submerged plant species. However, these fish will eat nearly anything green, and should be used with the understanding that they could potentially consume all the vegetation in a pond or lake. Because grass carp are included on the state list of harmful or potentially harmful exotic species, a permit from TPWD is required for stocking.
Insects have been used to manage some plant species. For example, alligatorweed (Alternanthera philoxeroides), can often be controlled by the alligatorweed flea beetle (Agasicles hygrophila), which affects only this species. Insects are also available for control of hydrilla, water hyacinth, waterlettuce, and giant salvinia. Effectiveness varies.
Farmers have long used herbicides to control weeds in their fields, and there are herbicides that work on aquatic weeds, too. Copper sulfate has been in use since 1904. Chelated copper compounds are often preferred today, as they are slightly less toxic to fish and approved for use in drinking water. Organic herbicides became available for aquatic weed control in the 1940s with 2,4-D (2,4-dichlorophenoxyacetic acid), followed by diquat, endothall, glyphosate, and fluridone. Each chemical is sold under several brand names. Only those brands registered and approved by the United States Environmental Protection Agency (U.S. EPA) for aquatic use may be legally used to control aquatic vegetation.
Herbicide is seldom a permanent solution. Many plants have seeds or tubers that are not killed by the chemical and live to sprout another day.
Most herbicides work best when plants are actively growing. They should be applied in the spring after water temperatures reach 60 to 70ºF. However, precautions must be taken in warmer months to prevent oxygen depletion. As treated plants decay, the level of dissolved oxygen in the water decreases. If it drops too low, a fish kill can result. To minimize this effect, severe infestations covering more than half the water body should be treated in stages. Treat about 1/4 of the water body at one time, and wait 10 to 14 days before treating the next section. It is best not to apply herbicides on cloudy days, when oxygen depletion is more likely.
Adjuvants and surfactants are commercially available compounds that help chemicals disperse more evenly and/or provide better leaf penetration. These additives should be used when recommended on the herbicide label.
Product specimen labels should be read carefully for water use restrictions,
application rates, health and safety precautions, and applicability to the
plant species being treated. Improper use of pesticides may endanger people,
livestock, and fish and wildlife resources.