buckler mustard vs shrub

buckler mustard and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
buckler mustard	Yes	No	No	No
shrub	Yes	No	No	No

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

buckler mustard: plant of southeastern Europe having yellow flowers like those of mustard and pods with open valves resembling bucklers
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of buckler mustard include bush.

buckler mustard (noun)	shrub (noun)
plant of southeastern Europe having yellow flowers like those of mustard and pods with open valves resembling bucklers	a low woody perennial plant usually having several major stems