I feel there isn't enough to go on Lin! However, your suggestion does look good.
The plant looks more like a woody plant than herbaceous to me. Are you sure there are none there now? Maybe you did cut them back, or pull them out?
The location for Populus angustifolia looks to be close to where you are.
http://www.public.iastate.edu/...
http://www.pollenlibrary.com/S...
http://luirig.altervista.org/s...
As there was a group of them, that suggests there is a tree nearby which dropped seed. What trees are in the vicinity?
There is Salix amygdaloides, Peachleaf Willow, no wonder the leave reminded me of peach leaves!
http://www.arborday.org/trees/...
http://luirig.altervista.org/s...
http://www.cod.edu/people/facu...
Is it growing near water?
http://www.fnanaturesearch.org...
