Penstemon lentus and Penstemon osterhoutii are very similar plants, so similar that in his 2001 edition of Colorado Flora: Western Slope, Weber combined them under the name of P. osterhoutii. However, both species are listed in Weber's 2011 edition of Colorado Flora: Western Slope bringing his book in-line with the taxonomy of other botanical authorities who recognize the two species.

P. lentus is found only in the Four Corners area of Utah, Colorado, New Mexico, and Arizona.   P. osterhoutii is found only in northwest Colorado. (See maps below.)

Harrington's 1964 Manual of the Plants of Colorado gives the following characteristics to separate the two species:

P. lentus: Inflorescence more or less secund, peduncles elongate, flowers not congested in fascicles, 20-30 centimeters tall, stems few to several, flowers are dark blue, to light purple to bluish-purple.
P. osterhoutii: Inflorescence not secund, peduncles short, flowers in congested fascicles, 30-70 centimeters tall, stems solitary to few, flowers are light purple to violet-blue.

The two species are, according to Harrington and other authorities, almost identical in all other characteristics.  Unfortunately, the key characteristics that Harrington provides for P. lentus are not agreed on by the authors of Intermountain Flora or A Utah Flora or by Craig Freeman (who will be writing the Penstemon section of the Flora of North America).  Freeman provides the following key characteristics to distinguish between the otherwise nearly identical species:

P. lentus: Corolla tube very lightly or not lined internally, glabrate within, 5-6 mm wide at throat. Staminode 8-9 mm long, .6-1 mm wide.   Inflorescence usually secund, sometimes cylindric. Peduncles to 28 mm long.

P. osterhoutii:  Corolla tube lined internally and onto the lobes with red-purple nectar guides, sparsely white-hairy within, 7-10 mm wide at throat. Staminode 11-13 mm long, 2-3 mm wide. Inflorescence cylindric.  Peduncles to 18 mm long.

In both species, the stem leaves are sessile, clasping, and opposite.  The common stem of a cluster of flowers (the "peduncle") arises from the axil of these leaves on either side of the main plant stem (see photograph at right).  The peduncle can be a distinguishing characteristic of the two species:

1) The peduncles of P. lentus (shown at right) are as long as 28 mm.  P. osterhoutii peduncles are no more than about 18 mm.

2) The peduncles of P. lentus sometimes?, often?, lean to the same side of the main plant stem, making the plant "secund".  The lower pair of peduncles has more of a tendency to be secund than the 3-7 pairs above, although often some of these may be secund. 

P. lentus does not appear to be secund at first glance because the swirl of its many flowers obscures the peduncles (as is the case, unfortunately, in my photographs below).  When you next encounter P. lentus, push the flowers aside and look at the peduncles; you will see the secund tendency.

The photograph of a dried P. lentus shows the peduncles leaning to one side of the stem after they emerged from the leaf axils on either side of the stout main stem.  

P. lentus and P. osterhoutii are found in Sagebrush, Pinyon-Juniper, Gamble Oak, and Ponderosa Pine communities in sandy or gravelly soils from about 4,800-7,500 feet.