This is Lobelia niihauensis. It is a dry-cliff dweller endemic to the Hawaiian islands of Ni'ihau, Kaua'i, and O'ahu. This plant is growing at the Nationnal Tropical Botanical Garden in Limahuli, Kaua'i. This plant is attractive even when not in flower. The leaves on non-flowering branches are arranged in tight rosettes (like Yucca). Lobelia niihauensis is an endangered species.