Research was conducted in 2012 and 2013 in Fayetteville, AR to evaluate the impact of Palmer amaranth (Amaranthus palmeri (S.) Wats.) emergence date on plant biomass, height, and seed production as well as the corresponding influence on cotton (Gossypium hirsutum L.) biomass and yield. Palmer amaranth was evaluated in the presence and absence of cotton and when emergence was delayed in cotton (0, 2, 4, 6, 8, and 10 wk). Seed production per plant was reduced by a greater extent in the presence of cotton, compared to the absence of cotton. Palmer amaranth plants emerging 10 wk after cotton were able to produce on average 880 seed per plant, which is a sufficient amount to replenish a soil seedbank. The late-emerging plants competing with cotton were smaller in size than earlier emerging plants. Seed production in the presence of cotton was correlated with Palmer amaranth biomass production (r2 = 0.63). Furthermore, the later-emerging cohorts responded to the presence of cotton by producing less biomass more so than a reduction in plant height with delayed emergence. This research shows that Palmer amaranth cohorts emerging as late as 10 wk after cotton emergence must be removed to prevent weed seed production.
