In obsolete terms the difference between flower and shrub

is that flower is flour while shrub is to lop; to prune.

As nouns the difference between flower and shrub

is that flower is a colorful, conspicuous structure associated with angiosperms, frequently scented and attracting various insects, and which may or may not be used for sexual reproduction while shrub is a woody plant smaller than a tree, and usually with several stems from the same base.

As verbs the difference between flower and shrub

is that flower is to put forth blooms while shrub is to lop; to prune.