During the summer of 2023, a herbicide screening study was conducted on eight wildflower species currently being planted or considered by the N.C. Department of Transportation (NC DOT) Roadside Environmental Unit for inclusion in the wildflower program. The following eight species were investigated: narrow-leaf sunflower (Helianthus angustifolius), black-eyed susans (Rudbeckia hirta), maximilian sunflower (Helianthus maximiliani), sweet william (Dianthus barbatus), clasping coneflower (Dracopis amplexicaulis), bidens (Bidens aristosa), dame’s rocket (Hesperis matronalis), and indian blanket (Gaillardia pulchella). These species will henceforth be referred to by their common names.
NC DOT planted all eight wildflower species on May 10, 2023, at Central Crops Research Station in Clayton, North Carolina and Garrett Seed Farm in Smithfield, North Carolina. At the Clayton site, emergence was considerably variable across all species, likely due to a significant hard-packing rain a week after planting. All wildflowers did not emerge evenly, and data was not taken at this location due to high variability in each plot. At the Smithfield location, five species (black-eyed susan, narrow-leaf sunflower, maximilian sunflower, clasping coneflower, and sweet william) emerged consistently across the trial area. Data was collected and will be presented on these five species.
At planting, a PRE-emergence herbicide trial was established based on products currently registered for use on roadsides and products with potential safety on these species. Herbicides treatments and rates are listed in Table 1. All treatments were applied immediately following planting on May 10, 2023, using a CO2 pressurized backpack sprayer equipped with Teejet flat-fan AIXR 11002 nozzles, calibrated to deliver fifteen gallons per acre at three miles per hour. Treated plots at both locations were eight feet wide by seven feet long. The site was weed free at the time of application. An activating rainfall occurred within the first week of planting. Herbicide treatments were replicated 4 times and randomized in a strip block design, withs species being a fixed variable. Stand counts were taken four weeks after emergence and were taken per two feet of row within each plot.
There are several viable herbicide options for weed control in Cosmos bipinnatus. This wildflower is planted late in the season which limits weed growth and there are more weed control options. By looking at the treatments that did not injure the Cosmos, the majority provide residual weed control only, and will not control emerged weeds. These herbicides could be used in addition to current at planting herbicide recommendations to prevent weeds from emerging during the growing season.
