I am trying to identify a plant that appeared this year in my garden. It grows about 18" tall in a very tight patch that may or may not be connected by the root system underground. It has grown throughout the summer, and now forms a dense patch about 8 feet in diameter. It is so resistant that 2 applications of Roundup barely touched it until the first killing frost. I would very much like to know what it is and especially whether it is an invasive species. I have attached a picture of it from July when it flowered. Located in Upstate NY just south of Syracuse. I've searched extensively but cannot identify it. Please post reply or email me if you can do so. Thanks!

Jeff