I am leaning toward it being great blue lobelia (Lobelia siphilitica).

Alison

Here is a link that might be useful: Link to great blue lobelia info

The foliage is exactly like my Dame's Rocket, but the bud formation is not. I think the bloom time is too early for the great blue Lobelia. I wonder if it is one of the several cultivars of Dame's Rocket - apparently 3 made their way to North America.

No, not a Lobelia, but I sure like that one too.

Whenever I walk by it you can hear me saying, "Common! Bloom you fool! I need to see you in all your glory!" I'm not usually this impatient...

It does look a bit like my Great Lobelia (which, it is debatable as to whether it really is GL or Adenaphora...I guess they both look pretty similar)
It's not a type of Lychnis, is it? It's funny...it looks almost like Maltese Cross but not quite.
(So, like, I shouldn't have even attempted to answer your question, I'm just taking up space...lol)

Linda

It is not a great blue lobelia. Those ones you have to wait to bloom until August.

Hi again Tiffy. I was out near my Dame's Rocket today and found something which looks similar to your mystery plant. It was growing behind the wood pile, so it is not very green and I found another one behind the shed. I posted an ID question for two of the same things which are still in a container and about to bloom. Is yours growing in a shady spot? Evening Primrose has the same shape leaves, but they are a little fuzzy. This is what mine looks like:
{{gwi:404576}}
Do you think they are the same? They are about 24 inches tall.

You had another post, but it IS a Dame.