Studies were conducted to evaluate density-dependent effects of Palmer amaranth on weed and peanut growth and peanut yield. Palmer amaranth remained taller than peanut throughout the growing season and decreased peanut canopy diameter, although Palmer amaranth density did not affect peanut height. The rapid increase in Palmer amaranth height at Goldsboro correspondingly reduced the maximum peanut canopy diameter at that location, although the growth trends for peanut canopy diameter were similar for both locations. Palmer amaranth biomass was affected by weed density when grown with peanut. Peanut pod weight decreased linearly 2.89 kg/ha with each gram of increase in Palmer amaranth biomass per meter of crop row. Predicted peanut yield loss from season-long interference of one Palmer amaranth plant per meter of crop row was 28%. Palmer amaranth seed production was also described by the rectangular hyperbola model. At the highest density of 5.2 Palmer amaranth plants/m crop row, 1.2 billion Palmer amaranth seed/ha were produced.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; peanut, Arachis hypogaea L. ‘Perry’.