aka large bluets, Houstonia purpurea; Rubiaceae (madder family)
According to the USDA plants database, five species of Houstonia can be found in this area. I’ve only ever seen azure bluets (FOTD 4/25), so I was pretty excited when I spotted these plants. I’ve only found one small clump so far, along a section of the canal towpath that rarely fails to surprise me.
The plant stands about a foot and a half tall, though it can be somewhat weak-stemmed (the picture above is actually rotated 90 degrees; the plant was mostly horizontal). The flowers are pretty tiny.