Purpletop verbena is an erect, clump-forming perennial with stiff, widely branched stems. It can reach 3-6 ft (0.9-1.8 m) in height with an open, airy spread of 1-3 ft (0.3-0.9 m). The scabrous (sandpapery) stems and branches grow in an upright pattern and are square in cross section. Most of the leaves are clustered in a mounded rosette at the base of the plant. The relatively scarce stem leaves are opposite, 3-5 in (7-13 cm) long and clasping (i.e. the leaves have no petioles and their bases wrap around the stem). The flowers are purple, a quarter-inch across, and borne in rounded clusters about 2-3 in (5-7.6 cm) across. Botanists call this type of inflorescence a cyme: a flower cluster in which the center flower opens first, and later-opening flowers are on the ends of lateral branches that arise from below the first flower. Purpletop verbena displays its showy flowers all summer long, until the first frost of autumn. The airy, see-through habit of purpletop verbena makes it a good choice for the front or middle of a mixed border. It doesn't cast much of a shadow and you can see other plants behind and under it. Weave a line of purpletop verbena through a bed or border of other butterfly flowers. It's best planted in columns or masses because it is so thin it will be overlooked all by itself. In mild climates, purpletop verbena will self sow rather freely. However, it is easily kept under control.
