Penstemon flavescens is in subgenus Penstemon section Penstemon subsection Proceri. This subsection is complicated in that it has many species that are polyploid, and there are close affinities with subsection Humiles. The Proceri are characterized by an inflorescence of tightly packed flowers arranged in spaced whorls. The technical name for this is verticillaster, but that’s a mouthful for most people. I think of this architecture as flattened pompoms separated by some stem, but that’s a mouthful, too. What’s really cool about P. flavescens is that it is the only species of Penstemon that has a yellow flower. The term ‘flavescens‘ means “becoming yellow.” The flowers open as a pale yellow/creamy white and then turn yellow as they age and senesce.
This species occurs only in the Bitterroot Mountains which are in western Montana and central to eastern Idaho. Paul Blischak and I collected it from 2105 to 2149 m in elevation (6906 to 7050 ft) on both sides of the Idaho/Montana border.
Many individuals have only one whorl of flowers, but others have 2 to 4. In flower, the plants are up to 40 cm (16 inches) in height.
The plants are glabrous throughout. Flowers are 12 – 16 mm long (1/2 – 5/8 inches), moderately expanded, two-ridged on the ventral surface, with golden hairs scattered across the lower interior surface. The staminode is included, has a slightly expanded tip, and is covered in golden hairs. The anthers are reddish to dark purple, open from end to end, but the anther sacs are somewhat narrow, making the anther appear canoe shaped at maturity.
The calyx of the flowers are scarious margined, which looks like ruffles along the edges.
Stem leaves are either sessile or clasping the stem, entire, ovate to lanceolate in shape, and robust.
The basal leaves have a long petiole and are rather oblong in shape.
Penstemon flavescens occurs in spreading mats with new shoots arising from underground rhizomes.
As in all penstemons, the fruits are capsules. These are just starting to ripen.
Penstemon flavescens is found in open woodland in rocky soil. The patches can be modest in size, or number in the thousands.
This particular population was amazing – it covered an entire slope.
Most of the plants are found in between patches of conifer trees.
While photographing this species, I observed several different species of bees and a bombylid fly visiting the flowers. Most were probably good pollinators, but I think there was also a pollen parasite at work. The photo above shows two bumblebees at work. I think they may be two different species.
Lots of bombylid flies were at work, too. They crawl into the opening to sip the nectar. You can see the long tongue in this photo.
A bee line to the flowers! A bumble bee and two Osmia bees heading for the inflorescence.
Here’s a species of sweat bee (Lasioglossum sp.), which is just stealing all the pollen. I watched this bee slurp out the anther sacs, leaving no pollen behind to be spread from one flower to another.
Paul Blischak is the graduate student in my lab who is working on the Proceri group. Penstemon flavescens was the next-to-last species he needs for his phylogenetic survey of the subsection.
As with most species of Penstemon, this one is a new favorite of mine. It’s not used in the garden very much, but I think it is charming and would look great along a border of trees. My first impression of this plant was that it reminded me a bit of Penstemon globosus, except there aren’t as many flowers in the terminal verticillaster, and the flowers are yellow instead of purple.