Snow-on-the-mountain and ghost weed are the common names of euphorbia marginata. It is native from the plains of the Dakotas to Texas. The flowers bloom July to October.
GalleriesFlowers and Shrubs: S
L. H. Bailey Standard Cyclopedia of Horticulture (New York, New York: The MacMillan Company, 1917) volume 2, page 1168