It's the season when the silk tree flowers bloom.
white leadtree; Leucaena leucocephala
silk tree; mimosa tree
Ateji spelling for the silk tree; the reading ねむ is a gikun/jukujikun. This kanji form is uncommon and mainly encountered in botanical or literary contexts.
Kana spelling is the usual way to write this word in modern Japanese.
The reading ねむ is a gikun (meaning-based reading) for the ateji 合歓. The origin of the word is uncertain, but it may be related to the tree's leaves closing at night, resembling sleep (眠る).