I do not know what this plant is. I love it but it's too big where it is so...
Thank you for the question. Your beautiful plant is called Joe Pye Weed. Information from University of Iowa Extension states: Although Joe Pye Weed or Eupatorium purpureum is the native species, many Joe Pye weed plants sold by the nursery industry are probably E. maculatum. It has more attractive foliage and the flower heads contain nearly twice as many flowers. 'Gateway' is popular cultivar for home gardens because it is somewhat shorter and produces large mauve-pink flowers at the top of reddish-purple stems.
Read the entire publication for complete information http://www.extension.iastate.edu/newsrel/2003/aug03/aug0308.html
Joe Pye Weed provides seeds for birds, pollen for insects, and winter interest poking up through the snow so many people cut the plant back to the ground in the spring. I would recommend dividing it in the spring while it is still dormant and transplant to a moist sun or part sun location as a backdrop to smaller plants.Thank you for contacting Extension and enjoy your plant this fall and winter!