Greenhouse and field experiments were conducted to determine the effects of nitrogen (N) supply and purple nutsedge population densities on the yield of radish. In the greenhouse studies, additive series with purple nutsedge densities of 0, 50, 100, 200 or 350 plants m−2 were established. Nitrogen rates of 0, 110, 220, or 330 kg ha−1 were provided to the potting medium. A significant density by N interaction was found for radish fresh weight. Within a given nutsedge density, radish yield decreased as N rate increased. In field studies, additive series of 0, 50, 100, 150, or 200 nutsedge plants m−2 were established the same day radish was sown. Nitrogen rates were 100 or 200 kg ha−1. Marketable radish yield losses and nutsedge shoot dry weight and height were determined 30 d after seeding the crop. Nutsedge densities and N rates interactively influenced radish root yield. Radish yield loss reached 100% at nutsedge densities of 75 and 125 plants m−2 at 200 and 100 kg N ha−1, respectively. Purple nutsedge produced larger shoot biomass as N increased from 100 to 200 kg ha−1. Results of both greenhouse and field studies showed that as N increased, the negative effect of the weed on the crop was enhanced.