Area-of-influence experiments were conducted on high–organic matter soils to assess the effect of common lambsquarters distances from ‘South Bay’ lettuce on yield and quality under two phosphorus (P) regimens. P was applied either banded (125 kg P/ha) or broadcast (250 kg P/ha) before lettuce planting. Lettuce plants were harvested at 0, 25, 50, 75, 100, and 125 cm from each side of the common lambsquarters plant after season-long interference. The regression equations were y = 0.40 0.36/(1 e^{−0.09(x−52.5)}), r² = 0.92, for banded P; and y = 0.40 0.29/(1 e^{−0.07(x−69.3)}), r² = 0.94 for broadcast P. Banded P produced 9% more fresh weight per head than the broadcast P treatment under weed-free conditions. Within the broadcast P treatments, common lambsquarters growing at 125 and 100 cm did not affect fresh weight of harvested lettuce. Values for fresh weight per head declined 18, 34, 38, and 38% when lettuce grew at 75, 50, 25, and 0 cm distance from common lambsquarters, respectively. Fresh weight of lettuce in banded P treatments was reduced only at 50, 25, and 0 cm distance from common lambsquarters plants, representing 31, 44, and 44% less than its weed-free control, respectively. Banding P at 50% (125 kg/ha) of the recommended broadcast rate reduced the area of influence of common lambsquarters with lettuce.

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; lettuce, Lactuca sativa L.

Additional index words: Competition, integrated weed management, interference, nutrients.