flowering crab vs Southern crab apple

flowering crab and Southern crab apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
flowering crab	Yes	No	No	No
Southern crab apple	Yes	No	No	No

As nouns, flowering crab and Southern crab apple are synonyms defined as:

flowering crab and Southern crab apple: small tree or shrub of southeastern United States; cultivated as an ornamental for its rose-colored blossoms

Other synonyms of flowering crab include Malus angustifolia.

flowering crab (noun)	Southern crab apple (noun)
small tree or shrub of southeastern United States; cultivated as an ornamental for its rose-colored blossoms	small tree or shrub of southeastern United States; cultivated as an ornamental for its rose-colored blossoms
derived from the Iowa crab and cultivated for its large double pink blossoms