Aquatic Herbicides - An Aquatic Plant Management Method

Aquatic herbicides

The Western Aquatic Plant Management Society

Description of Method

Aquatic herbicides are chemicals specifically formulated for use in water to kill or control aquatic plants. Herbicides approved for aquatic use by the United States Environmental Protection Agency have been reviewed and considered compatible with the aquatic environment when used according to label directions. However, individual states may also impose additional constraints on their use.

Aquatic herbicides are sprayed directly onto floating or emergent aquatic plants or are applied to the water in either a liquid or pellet form. Systemic herbicides are capable of killing the entire plant. Contact herbicides cause the parts of the plant in contact with the herbicide to die back, leaving the roots alive and capable of regrowth. Non-selective herbicides will generally affect all plants that they come in contact with. Selective herbicides will affect only some plants (often dicots - broad leafed plants like Eurasian watermilfoil will be affected by selective herbicides whereas monocots like Brazilian elodea are not affected).

Because of environmental risks from improper application, aquatic herbicide use in some western states is regulated and has certain restrictions and permit requirements. A 9th Circuit District Court decision in March 2001 has further impacted the way some western states regulate aquatic herbicide applications. Some states now require coverage under a National Pollutant Elimination System Discharge permit (NPDES). Check with your state and local governments before planning any aquatic herbicide applications. Information about each aquatic herbicide is presented below:

Active ingredient glyphosate. This systemic non-selective herbicide is used to control floating-leaved plants like water lilies, and shoreline plants like purple loosestrife and water primrose. It is generally applied as a liquid to the leaves. Glyphosate does not work on underwater plants such as Eurasian watermilfoil. Although glyphosate is a non-selective herbicide, a good applicator can somewhat selectively remove targeted plants by focusing the spray only on the plants to be removed. The plants can take several weeks to die. A repeat application is often necessary to remove plants that were missed during the first application.
Active ingredient imazapyr. This slow-acting herbicide is systemic and non-selective. It was recently registered for use as in the aquatic environment by the EPA. Imazapyr is used to manage emergent plants such as spartina, purple loosestrife, reed canary grass, and phragmites. It does not work on underwater plants such as hydrilla and Eurasian watermilfoil. Because this is a very slow acting, but effective herbicide, applicators must wait for many months to accurately assess the treatment efficacy.

Active ingredient fluridone. Fluridone is a slow-acting systemic herbicide used to control Eurasian watermilfoil and other underwater plants. It may be applied in pelleted form or as a liquid. Fluridone can show good control of submersed plants where there is little water movement and an extended time for the treatment. Its use is most applicable to whole-lake or isolated bay treatments where dilution can be minimized. It is not as effective for spot treatments, although some new slow release granular formulations may provide much better spot treatment than older formulations of this product. It may take six to twelve weeks before the dying plants fall to the sediment and decompose. When used to manage Eurasian watermilfoil, fluridone is applied several times during the summer to maintain a low, but consistent concentration in the water. Although fluridone is considered to be a non-selective herbicide, when used at low concentrations, it can be used to selectively remove Eurasian watermilfoil. Some native aquatic plants, especially pondweeds, are minimally affected by low concentrations of fluridone. Emergent or wetland plants are not killed by the concentrations used to manage Eurasian watermilfoil.

Active ingredient 2,4-D. 2,4-D is a relatively fast acting selective, systemic herbicide. It can be applied as a granular formulation or a liquid and can be effective for spot treatment of Eurasian watermilfoil. When used at the label rate of 100 pounds per acre, 2,4-D has shown to be exceptionally selective to Eurasian watermilfoil, leaving native aquatic species virtually unaffected.
Active ingredient triclopyr. Like 2,4-D, triclopyr is a selective, systemic herbicide that can be used to manage emergent plants such as purple loosestrife and submersed species such as Eurasian watermilfoil. It can also be used to effectively remove Eurasian watermilfoil leaving most native aquatic plants unharmed.

Active ingredient endothall. Endothall is a fast-acting non-selective contact herbicide which destroys the vegetative part of the plant but does not kill the roots. Endothall may be applied in a granular or liquid form. Generally endothall compounds are used primarily for short term (one season) control of a variety of aquatic plants. However, there has been some recent research that indicates that when used in low concentrations, endothall can be used to selectively remove exotic weeds; leaving native species relatively unaffected. Because it is fast acting, endothall can be used to treat smaller areas effectively.

Copper Compounds - copper sulfate and chelated coppers - Copper is a rapidly acting, non-selective contact herbicide or algaecide. Plants like hydrilla and Brazilian elodea are susceptible to copper. Sometimes copper is used with other herbicides to enhance uptake rates. When used as an algaecide, several treatments each season may be needed to control algal blooms. Copper compounds are toxic to fish and must be used with extreme care. Copper does not break down and can accumulate in sediments. Some states requires that sediments be tested for copper concentrations before a permit can be issued for copper application.

Active ingredient diquat. Diquat is a fast acting, non-selective contact herbicide. It is very effective on Brazilian elodea and American waterweed, plants that are not generally effectively treated by endothall products. Turbid or muddy water substantially reduces the effectiveness of diquat because the chemical tightly binds to the soil particles.

Advantages

Aquatic herbicide application can be less expensive than other aquatic plant control methods.
Aquatic herbicides are easily applied around docks and underwater obstructions.
Washington has had some success in eradicating Eurasian watermilfoil, a state listed noxious weed, from some smaller lakes (300 acres or less) using fluridone.

Disadvantages

Some herbicides may have swimming, drinking, and water use restrictions or advisories.
Herbicide use may have unwanted impacts to people who use the water and to the environment.
Non-targeted plants as well as nuisance plants may be controlled or killed by some herbicides.
Depending on the herbicide used, it may take several days to weeks or several treatments during a growing season before the herbicide controls or kills treated plants.
Rapid-acting herbicides like endothall or diquat may cause low oxygen conditions to develop as plants decompose. Low oxygen can cause fish kills.
To be most effective, generally herbicides should be applied to rapidly-growing plants.
Some expertise in using herbicides is necessary in order to be successful and to avoid unwanted impacts.
Many people have strong feelings against using chemicals in water. Find out what your neighbors think about chemical use before deciding to treat your water plants with herbicides.
Some local jurisdictions have policies forbidding or discouraging the use of aquatic herbicides.

Permits

State differ in their permitting requirements. Check with your state and local jurisdictions before using any aquatic herbicides. Some states require that licensed applicators to be used to apply these products. In states requiring coverage under an NPDES permit, permit fees may be charged and monitoring may be required.

Costs

Costs will vary with the size of the treatment, difficulty of site access, and travel time for the applicator. Here are some very approximate costs for one acre of herbicide treatment:

Glyphosate: $250
Imazapyr: $600
Fluridone: $900 to $1,000
2,4-D: $300
Triclopyr: $700
Endothall: $500 to $600
Diquat: $300
Copper compounds: $150 to $200