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Herbicide Use

Herbicides are efficient and effective tools for vegetation management and weed control. However, WSDOT recognizes there may be potential impacts to health and the environment, and minimizes herbicide use wherever possible. We uses herbicides two ways:

  • to maintain a vegetation-free strip at the edge of the pavement where necessary
  • to selectively control and eliminate undesirable plants

Historically, about 60 percent of the herbicides used by the agency were for annual maintenance of a vegetation-free strip at the edge of pavement. WSDOT is currently studying alternatives to this practice to determine the most appropriate and cost effective methods for varying roadsides around the state.

For herbicides used to control weeds and other unwanted plants, We follow a process that helps ensure herbicides are used appropriately and only when necessary in combination with other effective control measures. The ultimate goal in any treatment is to replace unwanted vegetation with appropriate native plants. In many cases herbicides are an effective tool for initial control of a problem. When combined with other control measures, herbicide use can be minimized or eliminated over time.

WSDOT has been tracking its herbicide use trends in detail since 2002. Herbicide use is reported annually by pounds of active ingredient applied. This information is available to the public. To obtain a copy please e-mail

Aquatic Herbicide Application

Occasionally it is necessary for WSDOT to apply herbicides for control of noxious weeds or other undesirable vegetation growing in water. WSDOT avoids this type of application whenever possible. Seasonally submerged sites such as retention/detention ponds and river banks dry out during the summer months, during this time weeds can be treated without the presence of water.

Herbicide applications made in or over standing water are regulated under the National Pollution Discharge Elimination System (NPDES) through the Washington State Department of Ecology (WSDOE). WSDOT maintains statewide permits for all aquatic herbicide applications made on state highway property. These permits are administered through the Washington State Department of Agriculture (WSDA). They specify public notification procedures and types of herbicides and surfactants allowed. The permits cover applications made by WSDOT Maintenance and WSDOT contractors working on state highway projects. All applications must be made by persons holding an aquatic certification on their pesticide applicator’s license. WSDOT is required to file annual reports on applications made under the permits.

Applications made for control of emergent noxious weeds are regulated under the Noxious Weed Permit. This permit covers the indirect discharge of herbicides, adjuvants and marker dyes into estuaries, marine areas, wetlands, lake shorelines, rivers, streams and other wet areas. An indirect discharge occurs when there may be incidental overspray or dripping of a chemical from the treated plants into waters of the state.

Applications made for control of nuisance emergent vegetation not listed on the state noxious weed list, but considered undesirable for highway operations (drainage) or for the establishment of desirable vegetation in wetland mitigation sites are regulated under the Aquatic Plant and Algae Management General Permit.

Minimizing Risk from Herbicide Use

Two important factors in herbicide selection and application are potential risks to human health and the environment. It is our policy that any herbicide used on state highway right of way be screened through a scientific risk assessment specific to application rates and methods used by the agency. If certain herbicides are found to have a potential for higher toxicity to human health or the environment, their use on state highway roadsides may be limited, phased out, or immediately eliminated.

In 1993, we completed a toxicological analysis and risk assessment for herbicides used in the roadside vegetation management program. In 2003 and 2005, a consultant updated the information for the agency based on the current use and any new scientific findings. This updated analysis now considers all herbicides and applications methods currently employed by the agency.

Analysis of risk was based on very conservative assumptions of any potential exposures from WSDOT roadside applications. Findings from these assessments indicate that for most herbicides in most situations, WSDOT’s use of herbicides pose a low to very low potential risk to human and environmental health. In cases where the potential risks were calculated to be above low, we have placed limitations on use to further minimize potential for exposure. The table Herbicides Approved for use on WSDOT Rights of Way (pdf 55kb) outlines the use for individual herbicide including precautions and restrictions were they apply. Fact sheets summarizing the toxicology and potential risks for the individual herbicides used by WSDOT can be viewed below.

Contact Ray Willard for additional  herbicide use analysis and risk assessment documentation. 

Introduction - 2003/2005 Supplement to the 1993 Vegetation Management EIS, Appendix B.

Chapter 1 - Herbicide Active Ingredient Properties (pdf 56kb)
Chapter 2 - Wildlife Risk Assessment (pdf 592kb)
Chapter 3 - Aquatic Risk Assessment (pdf 520kb)
Chapter 4 - Human Health Risk Assessment (pdf 608kb)
Chapter 5 - Toxicological Data Summaries (pdf 372kb)

WSDOT Policy for Approval and Use of Any New Herbicide Products

Any new herbicides or formulations of existing herbicides that become available and have potential for use in roadside vegetation management will be screened, evaluated and approved based on the following procedures:

All new pesticide products that may be used for roadside vegetation management by the WSDOT will be formally evaluated for environmental and human health impacts prior to addition to the statewide contract and use on WSDOT rights of way. No pesticide products will be used on WSDOT right-of-way without approval through the process as described below.

Submit request for review/evaluation to the Headquarters Maintenance Office .

If warranted, an internal evaluation will be conducted by the Headquarters Maintenance Office to determine if a formal risk assessment is needed.

If a formal risk assessment is determined necessary, the product will be referred to an independent consultant for analysis and report on potential risk associated with the product.

The Headquarters Maintenance Office will review the formal risk assessment and a determination regarding use will be made. If the product is approved for use, the product will be added to the state contract. If the product poses unacceptable risk it will not be included on the contract and the results of findings will be documented.