A weed-looking plant popped up amongst the rock border of our home, on the south side. I let it grow and now it's long spikes with purple flowers. I thought this was special, until I drove to the Linden Hills neighborhood by St. Louis Park, and saw it growing along many yard borders. In fact, it was so prevalent that people had pulled some up and heaped in piles. What is this, and is it noxious?