At midnight, local time, the flowers will change.
September will feature dianthus.
Also note, the missions will change up, with spider lilies and dianthus taking the place of hibiscus and sunflowers on missions.
Note: they state that this impacts the next mission you do, not the next stage. I’m not sure if anyone reading this does multiple passes through the challenges, but if you do, this may require planning.
I was in a spot: I accidentally left my petals running when I got home from a walk, and used up all my yellow jamaica, despite needing another 400 to finish a mission. For the last day I’ve been running around trying to bloom some, and just half an hour ago I finally got one (planting yellow flowers + white jamaica) and it gave me just barely enough to finish the mission. I thought I was gonna end up stuck, but now I’m on to the next quarter-stage with plenty of sunflowers in the pipe.