Field trials were conducted over two spring seasons (2005 and 2006) to investigate the interference between American black nightshade and watermelon in polyethylene-mulched and nonmulched (bare ground) trials. Competition studies were performed with watermelon at 1 plant/m in-row and American black nightshade grown at 0, 2, 4, 6, and 8 plants/m² in-row. Watermelon yield reductions started to plateau at only 2 American black nightshade plants/m². Yield reduction was 80, 89, 96, and 98% and 54, 67, 81, and 85% at 2, 4, 6, and 8 American black nightshade plants/m² in mulched watermelon production in 2005 and 2006, respectively. When watermelon was grown on bare ground, yield reduction was 100% at 2 American black nightshade plants/m² in 2005 and 68, 81, 89, and 93% at 2, 4, 6, and 8 American black nightshade plants/m² in 2006, respectively. Watermelon fruit numbers were reduced due to American black nightshade interference, but no significant differences in individual fruit weight were observed.

Nomenclature: American black nightshade, Solanum americanum Mill., SOLAM, watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai. cv. ‘Mardi Gras’