Verbena hastata and Verbena urticifolia; Verbenaceae (verbena family)

Blue vervain, also known as swamp vervain, likes wet soils and full sun, and can be found all across the US in two varieties.  Some authorities consider it weedy in the West.  It grows up to five feet tall, with flower spikes up to half a foot long; the individual flowers are about 1/4″ across.

White vervain grows somewhat larger (up to six feet), with longer flower spikes (up to two feet), but much smaller flowers (1/8″ across).  It prefers slightly drier soils and more shade, though I have seen it growing mere feet away from blue vervain.  There are two varieties of white vervain, too, but neither is found in the West.  Some authorities consider it potentially weedy or invasive.  It is listed as possibly extirpated in Maine.