Field and herbarium studies of Penstemon cyananthus Hook. ssp. longiflorus Pennell suggest that this taxon be elevated to species rank.
Clark, Stephen L.
"A new combination in Penstemon (Scrophulariaceae),"
Great Basin Naturalist: Vol. 35
, Article 11.
Available at: https://scholarsarchive.byu.edu/gbn/vol35/iss4/11