It is interesting to do a Google search on "plant migration". I did it to find the title of a book I read in the 1950s about scientists tracking weeds to map prehistoric migrations of people. One chapter was on sunflowers on the plains east of the Rockies. I haven't found the name of the book yet, but I'm fascinated by the articles I am finding, many about colonization of areas after the ice ages. It represents such a different attitude to say "plant migrations" rather than "alien invasions". Diane Whitehead