The sixth 節気 “sekki” division of the traditional Japanese calendar is:
‘grain rain’ (solar term)
The third and final “kō” microseason within this division is:
(ぼたん はな さく)
“botan hana saku”
That makes 牡丹華 the eighteenth microseason of the year!
It typically lasts 30 April ~ 4 May.
Image credit: Gardenia.net.