Purpletop vervain's first foray into gardens was in 1726, when James & William Sharden plucked some seeds from a dried specimen that made the long voyage from Brazil to England. The soaring stems of tall verbena support a sea of violet-purple flowers in rounded heads, adding a hazy see-through effect to your garden. But if you take a closer look, you'll see that the flowers are a gathering of tiny nectar-filled tubes that attract all the butterflies and hummingbirds in the neighborhood. Blooming continuously until late fall freezes, with nary a demand, this is a must-have annual that self-sows for a yearly show.