Eastern black nightshade (Solanum ptycanthum Dun.) has been recognized by ND and MN growers as one of their worst weed problems. Chateau (flumioxazin) provides excellent control of nightshades but has been shown to also cause injury to potato. To alleviate most of the injury to potato, the label restricts use on potato to only 25 states that includes North Dakota. The label further restricts and limits the herbicide application timing. Furthermore, the label states: “In areas with historically higher amounts of rainfall during the time of preemergence herbicide applications, including the Red River Valley, Minnesota and North Dakota, the requirement for two inches of settled soil is critical to avoid crop injury.†The objective was to determine the effect of Chateau application timing and rate on weed control and potato safety under irrigation. Treatments included the labeled use rate and half that rate at three application timings: two days after planting (DAP) no hilling, after regular hilling, and hilling 2 DAP then applying. Additional treatments included Chateau + Boundary along with a standard of Boundary after regular hilling. Results indicated that all treatments provided similar excellent control of broadleaf annual weeds. Plots treated with 0.5X or 1X Chateau after an early hilling had the greatest total and marketable yields but this was only greater than one other treatment. Plots treated with 1X Chateau after an early hilling or 0.5X Chateau after early hilling followed by 0.5X Boundary after regular hilling had the greatest percentage of marketable yields but this also was only greater than a different single treatment. None of the treatments caused visible potato injury, which was attributed to the lack of rainfall prior to the initiation of irrigation. Future research will vary irrigation applications following the Chateau applications to ensure evaluation of crop safety with the application timings.