All species of Penstemon are strictly endemic to North America, making it the largest group of flowering plants restricted to the continent. Penstemon azureus, like the majority of Penstemon species, is native to western North America; it is found only in southwestern Oregon and northern California.
Azure penstemon has been the subject of some recent research to determine its evolutionary origins. As plants of Penstemon azureus contain six times the base number of chromosomes (6x), it has been speculated that the species is the result of the hybridization of two closely related species of Penstemon, one containing four times the base number of chromosomes (4x) and one containing the “typical” two (2x) — in other words, it is a polyploid, and more specifically an allopolyploid (even more exact, an allohexaploid). Testing the hypothesis of allopolyploidy in the origin of Penstemon azureus (Plantaginaceae) is a talk being presented at Botany 2012. The researcher, Travis Lawrence, makes note in his presentation’s abstract that Penstemon azureus is one of only four species in Penstemon section Saccanthera with either 4x, 6x, or 8x the base number of chromosomes; the other twenty species have 2x. Through examination of nuclear DNA in particular, Lawrence was able to narrow the likely progenitors of Penstemon azureus to three species: Penstemon heterophyllus (some individuals have 2x, others have 4x), Penstemon laetus (2x), and Penstemon parvulus (4x). Additional research is necessary to better determine the likely progenitors.
This photograph was taken at the Rough and Ready Botanical Wayside near O’Brien, Oregon. Two potential progenitors also are known to occur here: Penstemon laetus and Penstemon parvulus. Penstemon identification can be difficult, particularly when closely-related species are sympatric (or overlapping in distribution). Fortunately, I was able to determine this species with the key in the recently-released second edition of The Jepson Manual. Penstemon laetus was discarded as a possibility as it has a glandular inflorescence; I can see no glands when I look at the full-size rendering of the image (all parts of the inflorescence are glabrous (smooth or hairless)). One of the key differences between Penstemon parvulus and Penstemon azureus is length of corolla (14-20mm vs. 20-35mm), and I am quite confident these fall within the larger range.
Interested in penstemons? You might like to consider the American Penstemon Society; they’ve helped fund a small research project on penstemon hybridization and evaluation for home garden use here at UBC Botanical Garden, for which we’re grateful.