Wingleaf Primrose-willow, a native species, has been added to the USWildflowers database (09/16/2013.) Scientific name is Ludwigia decurrens. Photo below was taken at Amnicola Marsh in Hamilton County, TN on Aug 17, 2013.  Go to the Wingleaf Primrose-willow detail page for more photos and information.

Wingleaf Primrose-willow – Ludwigia decurrens