Palmer amaranth is the most common and troublesome weed in North Carolina sweetpotato. Field studies were conducted in Clinton, NC, in 2016 and 2017 to determine the critical timing of Palmer amaranth removal in ‘Covington’ sweetpotato. Palmer amaranth was grown with sweetpotato from transplanting to 2, 3, 4, 5, 6, 7, 8, and 9 wk after transplanting (WAP) and maintained weed-free for the remainder of the season. Palmer amaranth height and shoot dry biomass increased as Palmer amaranth removal was delayed. Season-long competition by Palmer amaranth interference reduced marketable yields by 85% and 95% in 2016 and 2017, respectively. Sweetpotato yield loss displayed a strong inverse linear relationship with Palmer amaranth height. A 0.6% and 0.4% decrease in yield was observed for every centimeter of Palmer amaranth growth in 2016 and 2017, respectively. The critical timing for Palmer amaranth removal, based on 5% loss of marketable yield, was determined by fitting a log-logistic model to the relative yield data and was determined to be 2 WAP. These results show that Palmer amaranth is highly competitive with sweetpotato and should be managed as early as possible in the season. The requirement of an early critical timing of weed removal to prevent yield loss emphasizes the importance of early-season scouting and Palmer amaranth removal in sweetpotato fields. Any delay in removal can result in substantial yield reductions and fewer premium quality roots.

Studies were conducted in a stale field production system in 2012 and 2013 to determine the effect of herbicide timing on Palmer amaranth control and ‘Covington’ sweetpotato yield and quality. Treatments consisted of flumioxazin at 72, 90, or 109 g ai ha−1 applied 45 d before transplanting (DBT) or 1 DBT, or sequentially the same rate at 45 DBT followed by (fb) 1 DBT; flumioxazin 109 g ha−1 applied 1 DBT fb S-metolachlor (800 g ai ha−1) at 0, 6 (± 1), or 10 d after treatment (DAT); flumioxazin at 72, 90, or 109 g ha−1 plus clomazone (630 g ai ha−1) applied 45 DBT fb S-metolachlor (800 g ha−1) applied 10 DAT; and fomesafen alone at 280 g ai ha−1 applied 45 DBT. Nontreated weed-free and weedy controls were included for comparison. Flumioxazin application time had a significant effect on Palmer amaranth control and sweetpotato yields, and the effect of flumioxazin rate was not significant. Treatments consisting of sequential application of flumioxazin 45 DBT fb 1 DBT or flumioxazin plus clomazone 45 DBT fb S-metolachlor 10 DAT provided the maximum Palmer amaranth control and sweetpotato yields (jumbo, No. 1, jumbo plus No. 1, marketable) among all treatments. Delayed flumioxazin application timings until 1 DBT allowed Palmer amaranth emergence on stale seedbeds and resulted only in 65, 62, 48, and 17% control at 14, 32, 68, and 109 DAT, respectively. POST transplant S-metolachlor applications following flumioxazin 1 DBT did not improve Palmer amaranth control, because the majority of Palmer amaranth emerged prior to S-metolachlor application. A control program consisting of flumioxazin 109 g ha−1 plus clomazone 630 g ha−1 at 45 DBT fb S-metolachlor 800 g ha−1 at 0 to 10 DAT provides an effective herbicide program for Palmer amaranth control in stale seedbed production systems in North Carolina sweetpotato.