Beginner Gardening: Plant ID, 1 by lovemyiris

lovemyiris wrote:
Can anyone help me identify this plant? It came up volunteer, or else was dormant several years before deciding to grow as I've not seen it in the garden before. It's approximately 3-1/2 feet tall and is just beginning to get flower buds. The stems are smooth with some ridges (similar to celery, but not as deep). I'm in Zone 4/5, if that's of any help.

There is a lavender bachelor button growing behind it, so don't be confused by the different foliage and color there.

Thanks!