Who keeps screaming “Poppies! Poppies!”?
While not exactly "screaming" the Wicked Witch of the West does utter those words in the Wizard of Oz:
Poppies.... Poppies. Poppies will put them to sleep.
Additionally, poppies represent sleep, death, and resurrection - and thus transition to a "new phase."
This seems to be a theme throughout this song - the transition from childhood to adulthood, forgetting the simple times of youth and having to face the complications of adult life.