2015 PURPLE TOP Verbena bonariensis One of the best plants are for attracting butterflies. 'Purple Top' Verbena is a long-blooming, tough, drought tolerant plant that is among the easiest perennials to grow. The rosy-lavender flowers are tiny and extremely profuse, held on long and strong 4-5 foot stems that are great for cutting. Re-seeds readily but is not invasive. A winter hardy perennial to zone 7, it can be grown as annual everywhere when started early.