St John's wort vs subshrub

St John's wort and subshrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
St John's wort	Yes	No	No	No
subshrub	Yes	No	No	No

As nouns, subshrub is a hypernym of St John's wort; that is, subshrub is a word with a broader meaning than St John's wort:

St John's wort: any of numerous plants of the genus Hypericum having yellow flowers and transparently dotted leaves; traditionally gathered on St John's eve to ward off evil
subshrub: low-growing woody shrub or perennial with woody base

Other hypernyms of St John's wort include suffrutex.

St John's wort (noun)	subshrub (noun)
any of numerous plants of the genus Hypericum having yellow flowers and transparently dotted leaves; traditionally gathered on St John's eve to ward off evil	low-growing woody shrub or perennial with woody base