Sure, if that's the chosen benchmark. I don't think all colonization happened on a single day though. Did other people move plants around before then? Are native plants defined differently in USA than other countries? Does it matter if the plant was moved by humans, or some humans and not others? What if a plant has expanded its range as a result of animal activity? Or weather? Evolution? How far from a spot determined on the chosen benchmark day does a plant need to move to be determined that it is no longer in native territory?
