I can't remember what this is, I planted this a couple of years ago. It grows to about 1.5/2 m tall and has conical flowers, that are pale blue. This year the plant was hacked down to ground level by my wife, she's not known for her gardening prowess. I've noticed that it's come back up and starting to flower. The pictures show the flowers not completely set, but the flower head is about thumb size, the leaves are serrated. Any help would be appreciated.