Using herbicides while maintaining vegetation

Herbicides are useful tools for vegetation management and weed control. However, we recognize there may be potential impacts to health and the environment. Our crews use herbicides only when necessary and use is minimized whenever possible.

We uses herbicides two ways:

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

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 used in combination with other control measures overtime, problem areas may be converted to low-maintenance, desirable native plant communities where herbicide us in no longer needed.

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 our crews to apply herbicides for control of noxious weeds or other undesirable vegetation growing in or along the edges of water. We avoid this type of application whenever possible. Underwater 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). We 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 workers and 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 aquatic weeds are regulated under the 2018 Noxious Weed Permit. This permit covers the indirect discharge or accidental leakage of chemical into Washington waters of herbicides, adjuvants and marker dyes into estuaries, marine areas, wetlands, lake shorelines, rivers, streams and other wet areas. 

Minimizing risk from herbicide use

Two important factors in herbicide selection and use are potential risks to human health and the environment. It is our policy that any herbicide used on state highway right of way is studied based on how we will be using the chemicals. If an herbicide is found to have a negative effect on human health or the environment, its use on state highway roadsides is limited, phased out, or immediately stopped.

In 1993, we completed an initial independent 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 our current usage and any new scientific findings. This updated analysis now includes all herbicides and applications methods currently used by our maintenance crews. 

Analysis of risk was based on the smallest likelihood of any potential exposures from WSDOT roadside applications. Findings from these assessments show that most of the herbicides we use pose a low to very low potential risk to human and environmental health. In cases where the potential risks were found to be anything above low, we placed limits on use to prevent causing harm. 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

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

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.