Need i.d. for purple flowered tall perennial,

I plant mostly seeds native to the eastern u.s., I believe this to be a native unless it wandered into my garden by itself.

It is over six feet tall, grows in full sun in well drained average soil. It blooms from summer until fall, the flowers turn into spiky burs and grow all up and down the stem. It is growing next to black eyed susans. Young leaves are narrow, older leaves heart shaped and ridged. I have searched and search through many a guide to no avail, It doesn't match any of my seed packets. I thought it could be an Ironweed but the flowers don't match

Any ideas or help would be greatly appreciated

photo:

http://farm5.static.flickr.com/4047/4211329618_7d3edd315d_b.jpg

http://farm5.static.flickr.com/4007/4210600167_91210ed2f6_b.jpg