This is loosestrife, a perennial native to Eurasia. There is a small patch of this plant, a relict of one of the old gardens at Terra Nova, growing near the base of the elephantine beech. Without looking, I almost parked my bike on it. This species can be invasive, especially if planted at the edge of a pond or other damp place, but is not as disasterously invasive as its partial namesake, purple loosestrife,
which also occurs at Terra Nova and is spreading out beyond the dyke. The plague in this picture was eventually eradicated by building a subdivision on top of it. That'll teach it.