Verbena brasiliensis Veil. (Verbenaceae), an alien perennial herb native to South America, has spread rapidly in riparian habitats of Japan. To provide a scientific basis for assessing the impact of this plant on riparian ecosystems, we examined its phenology, growth and reproductive traits, seed germination, and its relationships to other plant species on a floodplain of the Ohtagawa River, Hiroshima Prefecture, Japan. This species has spread vegetatively by sprouting new shoots and forming adventitious roots from old, inclined stems. Stems that had inclined as a result of flooding were able to rapidly regenerate new shoots and flower. The seeds were found to require some light for germination, and the seed bank contained numerous viable seeds. After the autumn floods, several seedlings were observed under mature V. brasiliensis. In the laboratory, the powdered leaves of this species suppressed the growth of lettuce seedlings, suggesting allelopathy. Our data suggest that V. brasiliensis has a high potential for spreading and dominating riparian habitats. Further field research and an early strategy for successful control are needed.