Thought I'd share this, so often when growing penstemon species they can turn out to be the eastern North American P. hirsutus. Pictures is what was donated to NARGS Seed Exchange 2009-2010 as Penstemon arkansanus, but it turns out to be a regular light bluish-lavender for of P. hirsutus.
Here are a couple links that show the white flowered (although can be pinksih too) Penstemon arkansanus. The white flowers have pinkish "guidelines" in the throat, the leaves of P. arkansanus are different than P. hirsutus too.
Penstemon hirsutus (from NARGS 09-10 seed labeled as P. arkansanus:
...foliage detail:
links to see true Penstemon arkansanus, a species from Texas northeast to Illinois:
http://www.southeasternflora.com/view_flora.asp?plantid=1592#
http://www.wildflower.org/gallery/result.php?id_image=9710