Help Needed.

Can anyone identify this plant? We were given a cutting when we moved into our house eleven years ago. It flowers mid-to-late August and is obviously hardy perrennial.

The lady who gave it to us said that she had got a cutting from a friend in Canada - so do any of you Canadian Bloggers know what it's called?  We've never seen it anywhere else on our travels in the UK.

Any suggestions, names, natural habitat etc. greatly appreciated.