Because soil characteristics and weed densities vary within agricultural landscapes, determining which subfield areas are most favorable to weed species may aid in their management. Field and greenhouse studies were conducted to determine whether subfield environments characterized by higher soil organic carbon (SOC), or ridge vs. furrow microsites, affect common sunflower seed germination after winter burial, seedling emergence, or the control afforded by a preemergence herbicide in a ridge-tillage corn production system. Among seeds buried in situ during winter months and germinated in the laboratory, no differences in common sunflower seed germination or mortality were detected between high-SOC (1.8% mean) and low-SOC (1.1% mean) locations. However, seeds buried at 5-cm depth had about 40% laboratory germination compared with about 10% for seeds stratified on the soil surface or under crop residues. In field emergence and survival experiments, the SOC main-plot effect indicated 25% greater seedling survival in high- than in low-SOC locations. In the absence of herbicide, both emergence and survival were ≥ 35% greater in the ridge than in the furrow microsite, and seedling survival was 48% greater in high- vs. low-SOC furrow environments. However, common sunflower seedling survival was similar between herbicide-treated high- and low-SOC ridges. Greenhouse studies indicated a 13 to 24% increase in common sunflower seedling biomass per 1% increase in SOC under three atrazine doses. Altered or additional weed management tactics should be considered for common sunflower in high-SOC environments to offset the greater potential for seedling survival and growth.

Nomenclature: Atrazine; common sunflower, Helianthus annuus L. HELAN; corn, Zea mays L.