Pictures aren't very good, but I hate to pass on the opportunity to record a new find. Meet the Spotted St. Johnswort (Hypericum punctatum). I saw only this one little stand of plants, but they closely match the pictures and descriptions I found at the MissouriPlants site.
I did not notice the little black spots on the leaves, which are what give this version of St. Johnswort its name. One thing in favor of this ID, though is that Common St. Johnswort has dark dots on the petals. This one doesn't:
The Spotted St. Johnswort is a native plant.