2017 ASHS Annual Conference

Competition from weeds impacts the growth rate and overall health of container and field grown crops, especially during establishment. Methods to control weeds include cultural, biological, and chemical control. However, it is the latter that generally is most effective and the most economical way to reduce weeds around the base of the crops. Although chemical use may be economical, there is potential for sustained injury due to herbicide application. In 2015 and 2016, dimethenamid-p was applied to container grown Spanish chestnut to determine crop safety. Spanish chestnut trees were potted into 1 gal (3.785 L) containers filled with Fafard Mix 52 amended with Harrell’s 19N-1.7P-6.7K, 5 - 6 month control release fertilizer with minors. Treatments consisted of a nontreated (control) group and three rates of dimethenamid-p applied 1 day after potting (June 16, 2015; May 26, 2016) and 6 weeks after potting (July 29, 2015; July 13, 2016): 0.98 lb ai (1×), 1.97 lb ai (2×), and 3.94 lb ai (4×). To assess herbicide injury, plants were evaluated at 1, 2, and 4 weeks after first and second herbicide application for phytotoxicity symptoms on a 0 to 10 scale (0 representing no injury and 10 representing complete kill) and at the end of the experiment final growth indices [FGI = ((height + width + perpendicular width) ÷ 3)] was recorded. In both years, Spanish chestnut leaves appeared lighter in color, had purple spotting, and curled leaves 1 week after application of dimethenamid-p. Initial injury was more severe on plants treated with the 2× and 4× rates compared with the 1× rate. However, plants receiving the 4× rate were severely injured at the initiation of the trial and plants never fully recovered during the trial. Overall growth (size of the plant) was not affected by dimethenamid-p application.