anthesis
noun
an·the·sis an-ˈthē-səs 
: the action or period of opening of a flower
Word History
Etymology
borrowed from New Latin, borrowed from Greek ánthēsis "blooming," from anthē-, variant stem of antheîn "to blossom, bloom" (verbal derivative of ánthos "flower") + -sis -sis — more at antho-
First Known Use
circa 1823, in the meaning defined above