Thalia dealbata

About

Thalia dealbata, the powdery alligator-flag, hardy canna, or powdery thalia, is an aquatic plant in the family Marantaceae, native to swamps, ponds and other wetlands in the southern and central United States. Its range includes much of Coastal Plains and the lower Mississippi Valley. The plant has been grown as an aquatic ornamental because of the pretty violet flowers, and in cultivation has been proved hardy as far north as Philadelphia (Pennsylvania) and Vancouver.

Summary from Wikipedia