The primary objectives of this research were (1) to characterize intraspecific variation in Powell amaranth seed germination and emergence response to nitrogen fertilization, and (2) to evaluate whether germination and emergence characteristics varied between seeds from populations originating on organic vs. conventional vegetable farms. We hypothesized that nonherbicide–based weed management and use of slower-releasing forms of N on organic farms may have selected for seeds with lower dormancy and lower germination sensitivity to N fertilization than seeds from conventional farms. Seeds were collected from five conventional and five organic vegetable farms in central New York State. A second generation of seeds, produced under common greenhouse conditions and stored for at least 3 mo at 5 C was tested for both germination in petri dishes and emergence in the field under multiple rates of ammonium nitrate (NH4NO3). Both seed germination and emergence were greater for seeds originating from organic compared with conventional vegetable farms. However, seed responsiveness to fertilization did not vary significantly by habitat of origin. Reduced rates or split applications of NH4NO3 significantly reduced emergence in the field in 2003 but had no significant effect on emergence in 2004. Large interpopulation variation in germination and emergence patterns suggests that for Powell amaranth and similar weed species, (1) species-level models of emergence may not be very robust across different farms, and (2) the effectiveness of manipulating emergence through soil fertility practices is likely to vary substantially according to farm and year.
Nomenclature: Powell amaranth = green pigweed, Amaranthus powellii S. Wats. AMAPO.