Mystery plant

Last year I potted a number of those roots you buy and quite a few never came up at all.  I had put the pots to the side all winter in case any of thing sprang into life and this is the only one which has come up.  Trouble is - I haven't a clue what it is - if it IS one of the roots or if it's a weed.  I had planted 3 echinacea roots, some globe thistle things and something else which sadly I can't remember.

Does anyone have a clue what this is?  It only started showing up in March and is now a foot tall.