Blue Vervain, Verbena hastata, was a bit of a surprise for me. I first mistook it for a bit of spindly Purple Loosetrife, but a closer look sent me digging.
Here's some very cool and informative quotes from Wild Flowers Worth Knowing:
"Seeds below, a circle of insignificant purple-blue flowers in the centre, and buds at the top of the vervain's slender spires do not produce a striking effect, yet this common plant certainly does not lack beauty. John Burroughs, ever ready to say a kindly, appreciative word for any weed, speaks of its drooping, knotted threads, that "make a pretty etching upon the winter snow." Bees, the vervain's benefactors, are usually seen clinging to the blooming spikes, and apparently asleep on them. Borrowing the name of Simpler's Joy from its European sister, the flower has also appropriated much of the tradition and folk-lore centred about that plant which herb-gatherers, or simplers, truly delighted to see, since none was once more salable."
vervain—found growing on Mount Calvary, and therefore possessing every sort of miraculous power, according to the logic of simple peasant folk—the Druids had counted it among their sacred plants. "When the dog-star arose from unsunned spots" the priests gathered it. Did not Shakespeare's witches learn some of their uncanny rites from these reverend men of old? One is impressed with the striking similarity of many customs recorded of both. Two of the most frequently used ingredients in witches' cauldrons were the vervain and the rue. "The former probably derived its notoriety from the fact of its being sacred to Thor, an honor which marked it out, like other lightning plants, as peculiarly adapted for occult uses," says Mr. Thiselton Dyer in his "Folk-lore of Plants." "Although vervain, therefore, as the enchanter's plant, was gathered by witches to do mischief in their incantations, yet, as Aubrey says, it 'hinders witches from their will,' a circumstance to which Drayton further refers when he speaks of the vervain as "gainst witchcraft much avayling.'" Now we understand why the children of Shakespeare's time hung vervain and dill with a horseshoe over the door."
Wild Flowers Worth Knowing. New York, 1917. 185-6. Web. Google Book Search. 26 Jul 2009.