Nodding Wild Onion, a native species, has been added to the USWildflowers database (09/06/2013.) Scientific name is Allium cernuum.  Also known as Nodding Onion.  Photo below was taken in Hamilton County, TN on Aug 17, 2013.  Go to the Nodding Wild Onion detail page for more photos and information.

Nodding Wild Onion – Allium cernuum