old-maid's bonnet vs subshrub

old-maid's bonnet and subshrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
old-maid's bonnet	Yes	No	No	No
subshrub	Yes	No	No	No

As nouns, subshrub is a hypernym of old-maid's bonnet; that is, subshrub is a word with a broader meaning than old-maid's bonnet:

old-maid's bonnet: stout perennial of eastern and central North America having palmate leaves and showy racemose blue flowers
subshrub: low-growing woody shrub or perennial with woody base

Other hypernyms of old-maid's bonnet include suffrutex.

old-maid's bonnet (noun)	subshrub (noun)
stout perennial of eastern and central North America having palmate leaves and showy racemose blue flowers	low-growing woody shrub or perennial with woody base