Field studies were conducted in 2007 and 2008 at Clinton and Faison, NC, to
evaluate the influence of Palmer amaranth density on ‘Beauregard’ and
‘Covington’ sweetpotato yield and quality and to quantify the influence of
Palmer amaranth on light interception. Palmer amaranth was established at 0,
0.5, 1.1, 1.6, 3.3, and 6.5 plants m−1 within the sweetpotato row
and densities were maintained season-long. Jumbo, number (no.) 1, and
marketable sweetpotato yield losses were fit to a rectangular hyperbola
model, and predicted yield loss ranged from 56 to 94%, 30 to 85%, and 36 to
81%, respectively for Palmer amaranth densities of 0.5 to 6.5 plants
m−1. Percentage of jumbo, no. 1, and marketable sweetpotato
yield loss displayed a positive linear relationship with Palmer amaranth
light interception as early as 6 to 7 wk after planting (R2 = 0.99, 0.86, and 0.93, respectively). Predicted Palmer
amaranth light interception 6 to 7, 10, and 13 to 14 wk after planting
ranged from 47 to 68%, 46 to 82%, and 42 to 71%, respectively for Palmer
amaranth densities of 0.5 to 6.5 plants m−1. Palmer amaranth
height increased from 177 to 197 cm at densities of 0.5 to 4.1 plants
m−1 and decreased from 197 to 188 cm at densities of 4.1 to
6.5 plants m−1; plant width (69 to 145 cm) and shoot dry biomass
plant−1 (0.2 to 1.1 kg) decreased linearly as density
increased.