Aquatic Plant Management - Aquatic Herbicides

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 (EPA) have been reviewed and are considered compatible with the aquatic environment when used according to label directions. However, some individual states, including Washington, 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 able to regrow. Non-selective, broad spectrum 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 (Myriophyllum spicatum) will be affected by selective herbicides whereas monocots like Brazilian elodea (Egeria densa) may not be affected).  Most aquatic plants are monocots.

Because of environmental risks from improper application, aquatic herbicide application in Washington state waters is regulated and has the following restrictions:

Ecology issued a "lake" NPDES general permit March 1, 2006 to cover the management of in-lake noxious weeds and native aquatic plants and algae. The Washington Department of Agriculture also has a general NPDES permit for the management of noxious weeds growing in wet areas such as freshwater wetlands, rivers, and estuaries.

More information about permits for the application of aquatic pesticides.

For in-lake projects applicators and/or the state or local government sponsoring the project must obtain coverage under Ecology's Aquatic Plant and Algae Management NPDES permit before applying herbicides. For non-lake projects involving the treatment of noxious weeds in wet areas and estuaries, government sponsors and other entities can "contract" with the Washington Department of Agriculture under their NPDES permit for noxious weed control.

Find out how to go about contracting with Agriculture to manage state-listed noxious weeds.

Aquatic Herbicides

Ecology currently issues permits for seven aquatic herbicides and two algaecides (as of 2006 treatment season) for aquatic plant treatment for lakes, rivers, and streams. Plant control in irrigation canals is covered under another permit. The chemicals that are permitted for use in 2006 under the Aquatic Plant and Algae Control Permit and the Noxious Weed Permit are:

Glyphosate

Trade names for aquatic products with glyphosate as the active ingredient include Rodeo®, AquaMaster®, and AquaPro®. This systemic broad spectrum herbicide is used to control floating-leaved plants like water lilies and shoreline plants like purple loosestrife. 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 broad spectrum, non-selective herbicide, a good applicator can somewhat selectively remove targeted plants by focusing the spray only on the plants to be removed. Plants can take several weeks to die and a repeat application is often necessary to remove plants that were missed during the first application.

Fluridone

Trade names for fluridone products include Sonar® and Whitecap®. Fluridone is a slow-acting systemic herbicide used to control Eurasian watermilfoil and other underwater plants. It may be applied as a pellet 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 effective for spot treatments of areas less than five acres. It is slow-acting and may take six to twelve weeks before the dying plants fall to the sediment and decompose. When used to manage Eurasian watermilfoil in Washington, fluridone is applied several times during the spring/summer to maintain a low, but consistent concentration in the water. Granular formulations of fluridone are proving to be effective when treating areas of higher water exchange or when applicators need to maintain low levels over long time periods. Although fluridone is considered to be a broad spectrum herbicide, when used at very 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.

2,4-D

There are two formulations of 2,4-D approved for aquatic use. The granular formulation contains the low-volatile butoxy-ethyl-ester formulation of 2,4-D (Trade names include AquaKleen® and Navigate®). The liquid formulation contains the dimethylamine salt of 2,4-D (Trade names include DMA*4IVM). 2,4-D is a relatively fast-acting, systemic, selective herbicide used for the control of Eurasian watermilfoil and other broad-leaved species. Both the granular and liquid formulations can be effective for spot treatment of Eurasian watermilfoil. 2,4-D has been shown to be selective to Eurasian watermilfoil when used at the labeled rate, leaving native aquatic species relatively unaffected. (Read Ecology’s risk assessment). By court-order the butoxy-ethyl-ester formulation of 2,4-D can not be used in waters with threatened and endangered salmon-bearing waters in the Pacific Northwest.

Endothall

A trade name for the dipotassium salt of endothall is Aquathol®. Endothall is a fast-acting non-selective contact herbicide which destroys the vegetative part of the plant but generally does not kill the roots. Endothall may be applied in a granular or liquid form. Typically 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 some native species unaffected. Because it is fast acting, endothall can be used to treat smaller areas effectively. Endothall is not effective in controlling Canadian waterweed (Elodea canadensis) or Brazilian elodea. (Read Ecology’s risk assessment for endothall)

Diquat

A trade name for diquat is Reward®. Diquat is a fast-acting non-selective contact herbicide which destroys the vegetative part of the plant but does not kill the roots. It is applied as a liquid. Typically diquat is used primarily for short term (one season) control of a variety of submersed aquatic plants. It is very fast-acting and is suitable for spot treatment. However, turbid water or dense algal blooms can interfere with its effectiveness. Diquat was allowed for use in Washington in 2003 and Ecology collected information about its efficacy against Brazilian elodea in 2003. A littoral zone treatment in Battle Ground Lake in Clark County Washington in 2003 resulted in nearly complete removal of Brazilian elodea in that water body. (Read Ecology’s risk assessment for diquat). Read the journal article about the Battle Ground Lake study.

Triclopyr-TEA

A trade name for triclopyr is Renovate3®. There are two formulations of triclopyr. It is the TEA formation of triclopyr that is registered for use in aquatic or riparian environments. Triclopyr, applied as a liquid, is a relatively fast-acting, systemic, selective herbicide used for the control of Eurasian watermilfoil and other broad-leaved species such as purple loosestrife. Triclopyr can be effective for spot treatment of Eurasian watermilfoil and is relatively selective to Eurasian watermilfoil when used at the labeled rate. Many native aquatic species are unaffected by triclopyr. Triclopyr is very useful for purple loosestrife control since native grasses and sedges are unaffected by this herbicide. When applied directly to water, Ecology has imposed a 12-hour swimming restriction to minimize eye irritation. Triclopyr received its aquatic registration from EPA in 2003 and was allowed for use in Washington in 2004. (Read Ecology's EIS for triclopyr)

Imazapyr

A trade name for imazapyr is Habitat®. This systemic broad spectrum, slow-acting herbicide, applied as a liquid, is used to control emergent plants like spartina, reed canarygrass, and phragmites and floating-leaved plants like water lilies. Imazapyr does not work on underwater plants such as Eurasian watermilfoil. Although imazapyr is a broad spectrum, non-selective herbicide, a good applicator can somewhat selectively remove targeted plants by focusing the spray only on the plants to be removed. Imazapyr was allowed for use in Washington in 2004. (Read Agriculture's risk assessment for imazapyr)

Adjuvants

There are a number of adjuvants (surfactants, stickers, sinking agents) allowed for use under the NPDES permits (follow this link for a list of products). Please consult the permit webpage and the latest permits for a list of the allowed products.

Algaecides

Endothall - Amine Salt

A trade name for the amine formulation of endothall is Hydrothol 191®. Hydrothol 191® is a rapidly acting non-selective contact herbicide or algaecide. In Washington Hydrothol 191® may only be used at very low concentrations for filamentous algae control or cyanobacteria control (blue-green algae) in selected waterbodies. Several treatments each season may be needed to control algae/cyanobacteria. Hydrothol 191® has a high acute toxicity to fish and must be used with extreme care. Because of fish impacts, Ecology does not allow concentrations higher than 0.2mg a.e./L of Hydrothol 191®. Unlike copper compounds that are also used for algae control, Hydrothol 191® does not accumulate in sediments and breaks down rapidly. There are water use restrictions associated with the use of Hydrothol 191® in Washington. (Read Ecology’s risk assessment)

Peroxygen-based Granular Algaecide

Trade names include GreenClean® and Pak27®. These are peroxygen-based granular algaecides used for the prevention and control of algae in ponds, streams, irrigation systems, ornamental pools, and fountains. Areas being treated with these products must be closed to recreational activities during and for two-hours after treatment. (Read a human and environmental  risk assessment).

Copper Compounds

Copper compounds are no longer allowed for aquatic use in Washington state waters except under the NPDES permit for Irrigation Districts. (Read Ecology's risk assessment).

Advantages

Disadvantages

Permits

An NPDES permit is needed to apply any aquatic pesticide (including herbicides) to waters of the state. Some herbicide residue monitoring may be required and there is a permit fee for coverage under the Aquatic Plant and Algae Management permit. For aquatic plant or algae management in lakes (includes both noxious weeds and native plants and algae) apply to Ecology for coverage.

For noxious weed control in wet areas, estuaries, and shorelines only apply to the Washington Department of Agriculture to contract with them under their NPDES Aquatic Noxious Weed Control permit each treatment season. There is no fee to become a contractor under Agriculture's permit. However, the targeted weeds must be on the Washington’s Noxious Weed List or on Agriculture’s quarantine lists.

Costs of Herbicide Treatment

These costs are estimates and will vary from site to site depending on treatment rates, water depths, amount of notification needed, difficulty of access to the site, and other factors. Approximate costs for one acre herbicide treatment: Glyphosate: $300, Fluridone: $900 to $1,000, Endothall: $650, 2,4-D: $300-600, Diquat: $300 to $400

Read about an innovative approach to using aquatic herbicides.

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