This is a botania thing. Passive flowers (those which don't have a cost to keep running - Daybloom, Nightshade and Hydroangea) will wither away after a period of time. In Regrowth I've set this time to be equivalent to 5 in game days.i have a bug with botania.
When i place a Hydroangeas, a Daybloom or a Nightshade after a few minutes it become a bush.
This dont happend with my endoflames or Kekimurus flowers.