Studies were conducted in 2008 and 2009 to determine the effect of S-metolachlor rate and application time on sweetpotato cultivar injury and storage root shape under conditions of excessive moisture at the time of application. S-metolachlor at 1.1, 2.2, or 3.4 kg ai ha⁻¹ was applied immediately after transplanting or 2 wk after transplanting (WATP) to ‘Beauregard’, ‘Covington’, ‘DM02-180’, ‘Hatteras’, and ‘Murasaki-29’ sweetpotato. One and three d after S-metolachlor application plots received 1.9 cm rainfall or irrigation. S-metolachlor applied immediately after transplanting resulted in increased sweetpotato stunting 4 and 12 WATP, decreased no. 1 and marketable sweetpotato yields, and decreased storage root length to width ratio compared with the nontreated check. Sweetpotato stunting, no. 1 and marketable yields, and storage root length to width ratio in treatments receiving S-metolachlor 2 WATP were similar to the nontreated check. In 2008, Covington and Hattaras stunting 12 WATP was greater at 2.2 and 3.4 kg ha⁻¹ (11 to 16%) than 1.1 kg ha⁻¹ (1 to 2%). In 2009, S-metolachlor at 3.4 kg ha⁻¹ was more injurious 4 WATP than 2.2 kg ha⁻¹ and 1.1 kg ha⁻¹. While cultivar by treatment interactions did exist, injury, yield, and storage root length to width ratio trends were similar among all cultivars used in this study.

Nomenclature: S-metolachlor; sweetpotato, Ipomoea batatas L. Lam. ‘Beauregard’, ‘Covington’, ‘DM02-180’, ‘Hatteras’, and ‘Murasaki-29’.