Rydberg's penstemon vs Penstemon rydbergii

Rydberg's penstemon and Penstemon rydbergii both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rydberg's penstemon	Yes	No	No	No
Penstemon rydbergii	Yes	No	No	No

As nouns, Rydberg's penstemon and Penstemon rydbergii are synonyms defined as:

Rydberg's penstemon and Penstemon rydbergii: plant with whorls of small dark blue-violet flowers; Washington to Wyoming and south to California and Colorado

Rydberg's penstemon (noun)	Penstemon rydbergii (noun)
plant with whorls of small dark blue-violet flowers; Washington to Wyoming and south to California and Colorado	plant with whorls of small dark blue-violet flowers; Washington to Wyoming and south to California and Colorado