Lysimachia ciliata; Primulaceae (primrose family); some authorities place it in the Myrsinaceae (myrsine family)

Of the 22 species of Lysimachia in the continental US and Canada, this one is by far the most common, found everywhere except California, Arizona, Nevada, Louisiana, and the arctic.  Eight native Lysimachias, some of which are threatened or endangered, can be found in Maryland, as well as four alien species.  Two of these, garden yellow loosestrife and creeping jenny, are considered noxious.

The plants stand one to four feel tall, with flowers in the upper leaf axils.  Since the flowers are nodding, the picture above is what you’ll see of them as you walk along; you have to gently move the plant up and back to expose the flowers’ faces: