Herbicide Options for Weed Management in the North Carolina Highway Wildflower Program

The North Carolina Department of Transportation Roadside Environmental Unit plants wildflowers on approximately 3500 roadside acres in North Carolina. Methyl Bromide, a broad-spectrum soil fumigant, has been used to prepare roadside areas for wildflower plantings. Out of concern for the atmospheric ozone layer, a complete ban of methyl bromide production by 2005 was agreed to by 160 nations. The loss of methyl bromide facilitated the need for research to find alternative weed control herbicides. Field trials were conducted at North Carolina Department of Agriculture research stations in the piedmont and coastal plain regions of the state. Research was conducted with multiple preemergence and postemergence herbicides having differing modes of action on twenty-one wildflower species. Periodic visual evaluations were made to determine wildflower tolerance and weed control. This final year’s research concentrated on herbicides that were tolerant to the most number of wildflower species and herbicides that were labeled for use on roadsides and/or wildflower plantings. Additionally, Callisto and Staple were evaluated due to outstanding tolerance with several wildflower species. For the most part, wildflower species were more tolerant to herbicides applied preemergence. However, there was great variability with regards to tolerance among the 21 wildflower species evaluated. For example, many species were tolerant to Prowl H2O 3.8CS with the exception of red corn poppy, which was severely injured. In contrast, red corn poppy was tolerant to Callisto 4SC applied PRE and POST; however, most all other species were severely injured. No herbicide evaluated proved to be as versatile as methyl bromide with regards to tolerance. When planting wildflower species, NC DOT personnel should consider grouping species with similar herbicide tolerances in order to have an effective herbicide program.