Field and greenhouse experiments were conducted in 2007 and 2008 to evaluate weed competitiveness and herbicidal sensitivity of grafted tomatoes. Three weed interference levels were established in the field by imposing different weeding pressures. Results indicated that grafting of tomato seedlings did not increase the ability of plants to suppress weeds over self rooted plants. Grafted tomatoes had more vigorous growth and fruit yield compared to self rooted tomatoes across all weed levels. Weeds were found to have more prominent adverse effects on tomato productivity, but had less adverse effects on plant growth. Greenhouse experiments were conducted to detect herbicidal sensitivity of grafted and self rooted tomatoes towards metribuzin and sethoxydim herbicides. The experiments revealed that grafted tomatoes showed a relatively higher herbicidal sensitivity than self rooted seedlings. Grafted plants probably were less able to metabolize and detoxify high herbicide rates whenever variations in plant height or dry matter accumulations were detected in the experiments. Researchers and producers should be aware of this newly observed sensitivity when designing herbicide application programs for weed management in grafted crops.