This plant’s common name is White snakeroot. Its scientific name is Ageratina altissima. Here’s another photo of a single plant. If you look closely, you can see that the lower leaves have stalks while the upper leaves do not.
Now here’s an intriguing piece. When I studied White snakeroot, I was struck by its flower which immediately reminded me of Boneset (Eupatorium perfoliatum). Here are two photos of Boneset . . . the first shows the upper part of the plant with its leaves surrounding the stem and with the developing flower clusters:
You can see the similarities between the White snakeroot and the Boneset flowers. When I first learned the scientific name for White snakeroot, it was Eupatorium rugosum. And Boneset was (and remains) Eupatorium perfoliatum. With the on-going study and re-classification of plants, White snakeroot was moved from the Eupatorium genus to the Ageratina genus and was given a new species name (altissima).