I stand by my earlier statement — that's a Hypericum
species. It looks like an immature plant, possibly, but nonetheless it is one.
For comparison, here's some H. ellipticum growing along the edges of a pond down the road from m house:
Yours is probably not this species, as I don't recall its range extending as far as western TN, but it's a close relative. Here's a photo
of the same species in bloom — note the foliage shape in comparison to your plant.
HeyPK, I'm loving that wild Ludwigia glandulosa. Gorgeous color — I'm totally jealous of you guys in the south and your awesome local plant species.