invasive

I have been interested in Vermont wildflowers for many years. Recently I have been mowing the fields in the back, and came across patches of the flower seen above. I had never seen it before. I looked in my two wildflower books, and didn't see it there. It was a mystery until a few days ago when I saw the following in Allyn's garden.

The two photos show the plant at different stages of growth, but it is definitely the same plant. Allyn thought it was tarragon, but I'm not so sure. The leaves are very similar to ragweed. Hope that I can eventually find out what it is.