Penstemon ambiguus has two varieties. This one is P. ambiguus var. laevissimus. The shape of the flower is very unusual for Penstemon in that it is tubular-salverform – having nearly radial symmetry as compared to extremely bilateral for nearly all other species. Early Penstemon specialists hypothesized that it must be pollinated by moths or butterflies (Pennell 1935) because of the resemblance of the flower to Phlox or other lepidopteran-pollinated genera. However, Straw (1963) discovered that this species is actually pollinated by small bee flies of the genera Oligodranes and Mythicomyia (Bombyliidae). The flies land on the extended lower lip, enter the corolla, turn upside down at the anthers, and then crawl down the curved tube to reach the nectar. The flies then have to back out to the opening of the corolla to exit. Straw also proposed that the reproductive mode was likely to be self-pollination because the flies visit many of the hundreds of flowers on the same plant.
The species is wide-spread in the arid west of North America, ranging north to south from Colorado and Kansas to the state of Chihuahua, Mexico, and east to west from eastern Nevada to western Texas. I’ve seen populations growing mostly in sand dunes or deep sand in the Mohave and Chihuahuan deserts, but also on sandy slopes in the northern part of its range. The elevation for this species ranges from 762–1950 m (2500–6400 feet).
The habit for P. ambiguus is also unusual for the genus, being a shrub nearly a meter high by a meter wide.
It’s definitely one of my favorite species, and I’ve visited it several times across its range including Chihuahua (Mexico), Arizona, Nevada, New Mexico, and Utah. Here are some more photos of this lovely species.
Pennel FW (1935) The Scrophulariaceae of eastern temperate North America. Monograph 1. The Academy of Natural Sciences of Philadelphia.
Straw RM (1963) Bee-fly pollination of Penstemon ambiguus. Ecology 44, 818-819.