Field trials were conducted in the spring of 2007 and 2008 to investigate the critical period of interference between American black nightshade and triploid watermelon. To determine the critical period, the maximum period of competition and minimum weed-free period were examined. American black nightshade (2 plants m⁻²) was established into watermelon plots at watermelon transplanting and removed at 0, 1, 2, 3, 4, and 5 wk after transplanting to determine the maximum period of competition. American black nightshade (2 plants m⁻²) was established into watermelon plots at 0, 1, 2, 3, 4, and 5 wk after transplanting and remained until watermelon harvest to determine the minimum weed-free period. To avoid yield loss from exceeding 10% of a crop grown weed-free, the maximum period of competition and minimum weed-free period were found to be 3.9 and 3.6 weeks after transplanting, respectively. Therefore, if American black nightshade is controlled at any time during the critical period of 3.6 to 3.9 wk after transplanting, yield loss should not exceed 10% of a crop grown weed-free.

Nomenclature: American black nightshade, Solanum americanum Mill. SOLAM; watermelon, Citrullus lanatus (Thunb.) Matsumura and Nakai cv. ‘Super Crisp’.