1620s, "a bursting into flower, act of blossoming out," from French efflorescence, from Latin efflorescentem (nominative efflorescens), present participle of efflorescere "to bloom, flourish, blossom," from assimilated form of ex "out" (see ex-) + florescere "to blossom," from flos "flower" (from PIE root *bhel- (3) "to thrive, bloom"). Sense in chemistry is from 1660s.
