Penstemon rydbergii vs Rydberg's penstemon

Penstemon rydbergii and Rydberg's penstemon both are nouns.

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

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

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

Penstemon rydbergii (noun)	Rydberg's penstemon (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