Research was conducted in Isabela, Puerto Rico, to determine the tuber productivity of the weed purple nutsedge (PN) and the yield of 'Crimson Sweet' watermelon when grown with or without organic soil bed mulches [hays of millet (Pennisetum glaucum), nutsedge (Cyperus rotundus), sunn hemp (Crotalaria juncea), sorghum (Sorghum bicolor), cowpea ( Vigna unguiculata), cogongrass (Imperata cylindrica), Bahiagrass (Paspalum notatum), and rye (Secale cereale)]. The mulches covered the top of the soil beds, and were set the same day the watermelon was established. Natural populations of PN on the site were approximately 100 viable tubers/m2. PN shoots able to grow through the mulches were left unchecked until the final harvest of the crop. For non-mulched checks, we had a bare soil weed-free treatment and a bare soil season-long-PN-infested treatment. There were significant effects on watermelon yield and PN tuber productivity by mulch material. The tuber productivity of PN production was significantly reduced when watermelon was mulched with Bahiagrass (68% lower), nutsedge (45% lower), cogongrass (36% lower), millet (36% lower), and sorghum (34% lower), as compared to PN-infested checks. When mulching with cogongrass and nutsedge, watermelon yield was significantly higher than with other mulches.