Mulching between rows of plastic used for vegetable production can be an effective practice for controlling weeds. An existing round-bale unroller was modified to create an offset bale unroller, allowing round bales of hay to be unrolled between planting rows with a tractor. This modification has made the practice of mulching with round bales of hay or wheat straw more efficient. This offset round-bale unroller was used to apply hay and wheat straw mulch to between-row areas of ‘Crimson Sweet’ watermelon ( Citrullus lanatus ) in 2009 and 2010. Hay and wheat ( Triticum sp.) straw mulches were applied at two thicknesses, corresponding one and two layers of mulch from the round bale, respectively. All of the hay and wheat straw mulch treatments controlled weeds significantly better than the non-treated controls in both years. There was a significant mulch-type by year interaction for weed control, with 1-year-old hay having less weed control in 2010 compared with 2009, whereas other mulches had improved weed control in 2010. One-year-old wheat straw and new hay had the lowest levels of weed biomass present compared with new wheat straw and the no-mulch control. Mulch thickness significantly affected weed control, with mulches applied in two layers having significantly less weed biomass than those applied in one layer. Weed pressure was significantly less in 2010 compared with 2009. The offset bail-unroller that has been developed to apply mulches to between-row areas of plastic-covered beds is a useful tool that can be used to efficiently unroll round bales of a variety of organic mulches for weed control.