Tall or Giant Sunflower [Helianthus giganteus] found at Fletcher Park on 22 August 2013. It’s a member of the Aster (Asteraceae) family and Newcomb’s Wildflower Guide, pages 388-389, states it blooms in summer and fall in swamps and damp thickets. The stem is “often purple,” as seen below. USDA Plants Listing: Helianthus giganteus
