Verbena bonariensis

Verbena bonariensis, thepurpletop vervain,clustertop vervain,Argentinian vervain,tall verbenaorpretty verbena, is a member of theverbenafamilycultivated as a flowering annual orherbaceousperennial plant. In United States horticulture, it is also known by the ambiguous namespurpletop(also used for the grassTridens flavus) andSouth American vervain(which can mean any of the numerousspeciesin thegenusVerbenaoccurring in that continent). For the misapplication "Brazilian verbena" seebelow.
It isnativetotropicalSouth America where it grows throughout most of the warm regions, from Colombia and Brazil to Argentina and Chile.
Verbena bonariensisis a tall and slender-stemmed perennial. It can grow to 6 ft (180 cm) tall and can spread to 3 ft (90 cm) wide. At maturity, it will develop a woody base. Fragrant lavender to rose-purple flowers are in tight clusters located on terminal and axillary stems, blooming from mid-summer until fall 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.
Verbenais derived fromLatin, meaning ‘sacred bough’, in reference to the leafy twigs of vervaine (Verbena officinalis) which were historically carried by priests, used in wreaths for druidic rituals, and for medicine. Named by Virgil andPliny the Elder. The common name, vervaine, comes from theCelticname, ‘ferfain’.
Itsspecific epithetbonariensismeans ‘fromBuenos Aires,Argentina’. ‘Buenos’ means ‘good’ and ‘aires’ means ‘air’ (‘good air’).
Verbena bonariensisis a member of the South American vervains, which arepolyploidand have more than 14chromosomes. Among these, it is part of a lineage which might also includeVerbena intermediaand seems well distant fromVerbena litoralisorVerbena montevidensisfor example.
Sometimes, the nameVerbena brasiliensis, Brazilian verbena or Brazilian vervain, is found for this species.However, this is the result of a mix-up withV. brasiliensis, the "true" Brazilian verbena, which has been erroneously referred to asV. bonariaensisby several botanists.
There are two namedsubspecies:
V. bonariensisis cultivated as anornamental plantfor traditional anddrought tolerantand 'pollinator-host' gardens and parks.
It is a perennial, hardy inUSDA Zones7-11. It can be grown as an annual in areas where it is not winter hardy and will bloom in the first year when grown from seed. Its long internodes give it a sparse appearance but allow it to intermingle and coexist with other plants. The flowers  which appear in mid- to late summer, are very attractive to butterflies, and provide nectar for native bees and many beneficial garden insects.
This species grows best in a well-drained soil. It prefers full sun to partial shade and needs regular moisture. It has a reputation of rarely being attacked by insect pests, but may be susceptible topowdery mildew.V. bonariensisis commonly grown from seed which germinates readily without pre-treatment, but can also bepropagatedfrom herbaceous stemcuttings.
It has gained theRoyal Horticultural Society'sAward of Garden Merit.
V. bonariensisself-seeds readily. This ability has raised concerns that it may become aninvasive speciesandnoxious weedin favorablehabitats.It has naturalized in a number of southern United States.
Presently, the plant is on theinvasive specieswatchlist forWashington state,naturalized in tropical and southern Africa, temperate Asia, Australia, New Zealand, the United States (includingHawaii), theWest Indies,Macaronesiaand theMascarene Islands. According toPacific Island Ecosystems at Risk, it is considered a weed inFiji,New Guineaand otherSouth Pacific islands.