Yellow Trillium, a native species, has been added to the USWildflowers database (08/23/2012.) Scientific name is Trillium luteum.  Photo below was taken at Reflection Riding in Hamilton County, TN on March 21, 2012. Go to the Yellow Trillium detail page for more photos and information.

Yellow Trillium, Yellow Wakerobin – Trillium luteum