Purple top vervain blooms from mid-summer until autumn. Each flower has five small sepals that are fused together at the base into a tube. The beautiful flowers are exceptionally rich in pollen and nectar, therefore attracting bees, butterflies and moths.
Purple top vervain self-seeds readily. This ability has raised concerns that it may become an invasive species and noxious weed in favorable habitats. Presently, in some places the plant is on the invasive species watchlist.