Tilley, D.; Ogle, D.; St. John, L.; Shaw, N. 2009. Plant guide: Royal penstemon (Penstemon speciosus Douglas ex Lindl.). Aberdeen, ID: U.S. Department of Agriculture, Natural Resources Conservation Service, Aberdeen Plant Materials Center. 3 p.

Tilley, D.; Ogle, D.; St. John, L.; Shaw, N. 2009. Plant guide: Royal penstemon (Penstemon speciosus Douglas ex Lindl.). Aberdeen, ID: U.S. Department of Agriculture, Natural Resources Conservation Service, Aberdeen Plant Materials Center. 3 p.