Biology and Ecology of Palmer Amaranth (Amaranthus palmeri)

Palmer amaranth (Amaranthus palmeri S. Wats.) is a troublesome weed of crops in southeastern United States. This research highlights studies on the biology and ecology of Palmer amaranth. Following dispersal in fall, Palmer amaranth seeds require high mean temperatures of 25 to 40 C for germination, which is not likely to occur in South Carolina until the following spring. With dormancy reduction over winter, seeds can germinate at high temperatures (≥25 C) and thermal amplitudes of 15 C during late spring (May) in the presence of light. A majority (\u3e90%) of the non-dormant population in the soil seedbank emerge from early May through mid-July, with two to three peak emergence periods which often follow rainfall events. No difference in emergence was observed between no-tillage and shallow spring tillage situations. Early canopy closure in drill-seeded soybean (18-cm row width) had a suppressive effect on the emergence of Palmer amaranth cohorts following early July. This is attributed to the decrease in photosynthetic active radiation (PAR) and red:far-red (R:FR) ratio experienced by seeds lying on the soil surface beneath the canopy in no-tillage systems. Seed germination during fall (August to November) is phytochrome-regulated, with germination stimulation by R light and inhibition by FR light. Burial of seeds to a 10-cm depth for 3 to 6 months induced dormancy with a R or natural light requirement for germination. Palmer amaranth seeds developing under shade conditions (87% reduction in PAR) showed increased dormancy, a survival mechanism in low-light environment. In addition, seeds maturing in the bottom-third of a mother plant exhibited increased dormancy, partially explaining variability in timing and extent of germination within a single seed population. Besides exhibiting increased seed dormancy, Palmer amaranth showed photosynthetic and morphological acclimation to 87% shading. These characteristics make Palmer amaranth a troublesome weed in crop-production systems. Based on this research, an early-season glyphosate application preferably at the V3 stage of glyphosate-resistant soybean in conjunction with early planting dates (April) with narrow row (18-cm wide) spacing can be a promising strategy to reduce Palmer amaranth interference and seed production and improve soybean yields