Area of influence experiments were established on high-organic matter soils to assess the effect of varying common lambsquarters distances on lettuce marketable yield and quality under two phosphorus (P) regimes. Banded (125 Kg/ha) or broadcast (250 Kg/ha) Ρ was applied before lettuce planting. A single 2-week-old common lambsquarters plant was transplanted in the center of each lettuce bed. Lettuce plants were arranged in pairs from the distance to the lambsquarters plant of a given treatment. A weed-free control was also established. Lettuce plants were harvested in pairs at distances of 0, 25, 50, 75, 100 and 125 cm to each side of the lambsquarters plant after season-long interference. Average head fresh weight was collected. Results indicate that lambsquarters severely reduced fresh weight and quality of lettuce plants as spacing decreased. In all cases, banded Ρ diminished the impact of lambsquarters on lettuce. Lettuce pairs grown at 75 cm or more from lambsquarters were apparently not affected by weed interference when Ρ was banded, whereas no effect was present at 100 cm or more when Ρ was broadcast. These data indicates that banding Ρ gives an additional advantage to lettuce within a given distance, reducing the area of influence of lambsquarters.