Smooth Horsetail, a native species, has been added to the USWildflowers database (06/26/2011.)  Scientific name is Equisetum laevigatum. Photo below was taken along the Hull’s Gulch National Recreation Trail in Ada County, ID on June 21, 2011.  Go to the Smooth Horsetail detail page  for more photos and information.

Smooth Horsetail, Smooth Scouring Rush – Equisetum laevigatum