Great Ragweed, a native species, has been added to the USWildflowers database (08/29/2010.)  Scientific name is Ambrosia trifida.  This plant also is known by the common names Buffalo Weed, Horseweed, Giant Ragweed, and Tall Ambrosia.  Photo below was taken in Walker County, Georgia on August 28, 2010.

Great Ragweed, Buffalo Weed, Tall Ambrosia - Ambrosia trifida