This plant has quite a long history in cultivation, dating back to the early 1700’s. It is a native of Argentina and Brazil and derives its species name from the city of Buenos Aires. Verbena belongs to a plant family with more than 250 species of annual or perennial flowering plants. Some verbena varieties are also natives of Europe. Verbena flowers vary distinctly in color with the most common being mauve, red, apricot, white and pink. V. bonariensis is commonly known as purple top and sometimes as Argentinean vervain, South American vervain, purple top vervain, or purple top verbena.
Verbena bonariensis is a tall and slender-stemmed perennial. It can grow to 4 ft (120 cm) tall and can spread to 3 ft (90 cm) wide. At maturity, it will develop a woody base. It produces tall, spiky stems that rise well above the plant. At the tip of each stem, the plant produces clusters of tiny violet-colored flowers. The clusters bloom up to 2 inches across and self-seed. The flowers appear in midsummer and continue until first frost.The stem is square with very long internodes. Leaves are ovate to ovate-lanceolate with a toothed margin and grow up to 4 in (10 cm) long. Fruits separate into 4 individual nutlets.
So far, the non-native Purpletop Vervain has rarely escaped from cultivation and become naturalized in Illinois. When wild plants do occur, they rarely persist for very long. Purpletop Vervain was introduced into the United States from Argentina and Brazil as an ornamental garden plant. In Illinois, it has been encountered as a naturalized plant in a weedy area along a pond among cattails (Coles County), and it has been found growing wild at a construction site near the Urbana Free Library by the webmaster (Champaign County). Outside of the state, Purpletop Vervain has been found in such habitats as fields, roadsides, weedy meadows, and other disturbed areas. It is often cultivated in flower gardens.
Because these flowers attract the Monarch, Red Admiral, Checkerspots, and other butterflies, it is a good choice for the butterfly garden. While this species is regarded as invasive in some areas of the United States, it appears to prefer highly disturbed areas in Illinois, rather than high quality natural habitats. Sometimes Purpletop Vervain is incorrectly referred to as ‘Brazilian Vervain,’ which corresponds to another South American species, Verbena brasiliensis. Brazilian Vervain has escaped from cultivation in southeastern United States, but there are no records of this species naturalizing in Illinois. Unlike Purpletop Vervain, Brazilian Vervain produces distinct cylindrical spikes of flowers within each floral cyme of the inflorescence.
Source: illinoiswildflowers – Wiki