Penstemon

1. Corolla ca. 3.3–4.5 cm long; upper leaves (if not all) broadly ovate, ± obtuse to rounded at apex and subcordate-clasping at base, strongly glaucous.

P. grandiflorus

1. Corolla not over 3 cm long; leaves narrower, very acute, often not glaucous.

2. Throat of corolla inflated, distinctly differentiated from the narrower tube; lower (3) lobes of corolla slightly longer than upper lobes; anthers usually at least sparsely bearded; stem glabrous below the inflorescence or rarely locally puberulent.

3. Anthers glabrous; calyx lobes linear-lanceolate.

P. calycosus

3. Anthers with at least a few bristly hairs (rarely glabrous) on the back (near attachment of the filament); calyx lobes lanceolate-ovate.

P. digitalis

2. Throat of corolla not sharply differentiated, only gradually expanded from the tube; lower lobes of corolla distinctly longer than the upper; anthers glabrous; stem ± uniformly puberulent to glandular-hairy from base to near the inflorescence.

4. Stem with numerous hairs 0.5–1 mm long (or even longer); lower lip humped, nearly closing throat of corolla; corolla (20–) 22–28 (–30) mm long, purple-lavender with whitish lips but not streaked with purple lines (nectar guides).

P. hirsutus

4. Stem with few or no hairs as long as 0.5 mm; lower lip not humped, the throat open; corolla (14–) 16–20 (–22) mm long, streaked with nectar guides (usually not visible on dry specimens).

5. Longest (lower) branches of inflorescence usually at least 2 cm long before first pedicels; cauline leaves puberulent on both surfaces (at least on main veins and margins); corolla white.

P. pallidus

5. Longest branches of inflorescence less than 2 cm before first pedicels; cauline leaves glabrous; corolla pale violet.

P. gracilis

All species found in Penstemon

Penstemon calycosusSMOOTH BEARD-TONGUE 
Penstemon digitalisFOXGLOVE BEARD-TONGUE 
Penstemon gracilisSLENDER BEARD-TONGUE 
Penstemon grandiflorusLARGE BEARD-TONGUE 
Penstemon hirsutusHAIRY BEARD-TONGUE 
Penstemon pallidusPALE BEARD-TONGUE 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 14, 2017. http://michiganflora.net/genus.aspx?id=Penstemon.