(Verbena hastata) A native of the United States and Canada, Blue Vervain is also called Swamp Verbena because of its preference for wet areas (it can even tolerate periodic flooding). Plants can reach five feet when fully grown and produce lots of tiny light blue flowers on spikes that are loved by bees and butterflies. Birds will feed on seeds.3-5 feet. Seeds should be planted in warm conditions. Perennial. 50+ seeds.