The North Carolina Department of Transportation (NCDOT) has implemented a highway wildflower program that now encompasses 3500 roadside acres throughout the state. Methyl Bromide has become essential in establishing these beds. However, the Environmental Protection Agency (EPA) has classified Methyl Bromide as an ozone-depleting substance, and has banned production in 2005. Research was conducted 1.) to determine if wildflowers could be established and maintained using pre-emergence (PRE) and / or post-emergence (POST) herbicides, 2.) to evaluate the efficacy of these herbicides on native vegetation, and 3.) to evaluate the efficacy of non-chemical weed control options such as cover crop plantings.
Twenty-one PRE and 20 POST herbicides were evaluated on 29 wildflower species in 3 greenhouse and 2 field trials. In general, wildflowers were more tolerant to the PRE herbicides. However, no herbicide came close to Methyl Bromide with respect to safety and tolerance of all wildflower species. Field personnel must be able to select species based on weed presence and the herbicides needed to control those specific weeds without injuring the wildflowers.
Based on NCDOT surveys, the 3 most problematic weeds throughout the state are vetch species, curly dock and Carolina geranium. These weeds were controlled PRE, and also POST, with Cotoran 4L, Sinbar 80WP and Velpar 75DF. However, only limited wildflower species displayed tolerance to these herbicides. None of the 29 species evaluated have tolerances to Velpar 75DF applied POST, so this treatment is not an option. Ox-eyed daisy (mature plantings of 1 year or more) and spurred snapdragon were the only wildflower species with POST tolerance to Sinbar 80WP.
Greenhouse trials were initiated to see if cover crop plantings would control weeds. Rye, wheat and oats were tested on 12 weed species. None of the weeds were completely controlled, but dry weights were reduced in hairy vetch in 2 of 2 trials. Dry weights were reduced in common lambsquarter, curly dock, redroot pigweed, wild mustard, spiny sowthistly and Carolina geranium in 1 of 2 trials. Cover crops should not be considered a substitute for herbicides for weed control in wildflower beds.
The NCDOT Roadside Environmental Unit can utilize these data to develop wildflower herbicide programs for any specific bed throughout the state based on weed presence. For efficient herbicide use, NCDOT field personnel must identify weeds in an accurate and timely fashion. These persons could receive training from NCSU extension faculty members, who conduct weed management workshops several times a year in various North Carolina counties.
These herbicide recommendations will be incorporated into a computerized wildflower decision aid model developed by Dr. Gail Wilkerson at NCSU. A decision aid will allow interested parties to enter a number of variables (weeds, wildflowers, herbicides of interest, etc.) in order to construct a customized weed-management program for a given area. With all of the possible herbicide / wildflower combinations, a decision aid will be an efficient and necessary tool for NCDOT personnel.