tall sunflower vs helianthus

tall sunflower and helianthus both are nouns.

Word	Noun	Adjective	Verb	Adverb
tall sunflower	Yes	No	No	No
helianthus	Yes	No	No	No

As nouns, helianthus is a hypernym of tall sunflower; that is, helianthus is a word with a broader meaning than tall sunflower:

tall sunflower: very tall American perennial of central and the eastern United States to Canada having edible tuberous roots
helianthus: any plant of the genus Helianthus having large flower heads with dark disk florets and showy yellow rays

Other hypernyms of tall sunflower include sunflower.

tall sunflower (noun)	helianthus (noun)
very tall American perennial of central and the eastern United States to Canada having edible tuberous roots	any plant of the genus Helianthus having large flower heads with dark disk florets and showy yellow rays