maybe Epilobium sp.
This appears to be the first record of a naturalized population in Pennsylvania. This species is native to Japan and is widely cultivated in the southern US. It is growing on a tufa formation, where the year-round seepage moderates the microclimate, which probably permits this species to grow farther north than it would otherwise.
It is considered invasive farther south, but at this point in time it is not likely to be a problem at this latitude. This population consists of approximately 2 dozen plants.
this clump stretches for several meters along the trail.
2 clumps along trail, 6 m apart. A bit to big to pull up.