Hydrangea
Introduced in episode 192, two hydrangeas form Persephone’s hairstyle as she takes over Demeter’s job in the mortal realm. The meaning of this flower are many across time and various cultures including:
apology
unveiling
gratitude
understanding
devotion to a noble cause
All of these meanings ring true for this episode, where Persephone lacks confidence in her ability to take on the role but does so with the support of her found family. She even apologizes at the beginning but by the end is committed to making it work.
A couple of interpretations on the hydrangea by @thysiwon and @theloresofnature below. Interestingly, if a soil is acidic, it turns the hydrangea flower blue, a color (in Victorian society) that symbolized frigidity.