bitter pea vs shrub

bitter pea and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitter pea	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of bitter pea; that is, shrub is a word with a broader meaning than bitter pea:

bitter pea: any of several spiny shrubs of the genus Daviesia having yellow flowers and triangular seeds; Australia
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of bitter pea include bush.

bitter pea (noun)	shrub (noun)
any of several spiny shrubs of the genus Daviesia having yellow flowers and triangular seeds; Australia	a low woody perennial plant usually having several major stems