Current Status of Vegetation Management in Roadside Ditches and Stormwater Management Facilities

Abstract

This research report is an initial assessment of current vegetation maintenance practices in bioswales, wetponds, and roadside ditches. Through the use of interview surveys and a literature review, this report has compiled empirical evidence to evaluate the effects of different maintenance practices, particularly mowing, on the pollutant-removal capabilities of these facilities. Of particular focus is the continuing need for improved maintenance practices and a recognition of several important unmet research needs in this area. The results of the survey documented a significant lack of information on the types of mowing practices or vegetation that provide the greatest improvement to the quality of the water leaving these facilities. The current best management practices (BMP’s) for vegetation maintenance and mowing, specified in agency-developed design manuals, have been established through general observation and are based on the assumption that greater grass densities remove more pollutants. However, some of the limited published research conflicts with these assumptions for certain pollutants of concern. Current vegetation management practices are being implemented by local governments in the Puget Sound lowlands to the maximum extent that jurisdictional budgets will allow. Yet these practices are frequently not in accord with design standards. The primary shortfall is in the lack of removal of grass clippings after mowing. The water-quality consequences of this failing are completely unknown. Future research in several areas could significantly improve current vegetation management programs, particularly in (1) how to maximize stormwater treatment throughout the storage and conveyance system, and (2) how to minimize agency maintenance costs by identifying unnecessary or ineffective actions. Optimizing the pollutant removal capabilities of bioswales, wetponds, and roadside ditches is essential to make efficient use of the existing drainage system for water-quality improvement. Such optimization is also likely to achieve a significant improvement in overall watershed conditions. Data to guide agencies in these areas, however, are simply not available at the present time.