Below are some pictures of waterhemp, both were very tall (12-15") and both were treated with a quart of Roundup. They were also found about 3 feet apart in the same field. The surviving 12-15 inch tall waterhemp looks very healthy and coming back another quart of Roundup will most likely do nothing to impede its existence. In fact, effective chemical control options are limited for the surviving individual in this year. In future years it will be necessary to start with a strong preemergence herbicide program and then follow with alternative modes of action for postemergence control.
Given the fact that waterhemp can continue to emerge late into the season, it would be wisest to use postemegence herbicide options with some residual control. In this particular field more flushes of weeds are coming, including some waterhemp along with some grass. A different mode of action will need to be used, and as for the tall survivor, it is advisable to get out and prevent this individual for flowering (hand pulling). It is the only way to keep this problem from persisting for years to come and to reverse this course.