Studies were conducted to determine the effect of interference between Pennsylvania smartweed and cotton on plant growth and productivity. Pennsylvania smartweed remained shorter than cotton until at least 80 d after cotton planting. However, Pennsylvania smartweed produced considerable dry biomass by cotton harvest. Pennsylvania smartweed biomass per plant was not affected by weed density when grown with cotton. When grown alone, Pennsylvania smartweed produced 1,640 and 2,060 g dry biomass plant⁻¹ depending on the year. This biomass was over four times greater than the biomass produced by plants grown with cotton. Cotton lint yield decreased between 1.3 and 1.1 kg ha⁻¹ with each gram increase in weed dry biomass per meter of row. The relationship between Pennsylvania smartweed density and cotton percent yield loss was described by the hyperbolic function. The estimated coefficients a (maximum yield loss as density approaches infinity) and i (yield loss per unit density as density approaches zero) were 102 ± 23 and 51 ± 12, respectively, in 1998 and 53 ± 1 and 98 ± 5, respectively, in 2000. Pennsylvania smartweed achene production was also described by the hyperbolic function. Estimated achene production at 1 plant m⁻¹ cotton row was 18,000 and 26,000 achenes m⁻² in 1998 and 2000, respectively.

Nomenclature: Pennsylvania smartweed, Polygonum pensylvanicum var. laevigatum Fern. POLPY; cotton, Gossypium hirsutum L. ‘Stoneville BXN 47’.