The poppy is another symbol that is meant to represent death in a more subtle way.
The opium poppy is the flower from where the opium is extracted, so when it is represented inside a cemetery, the goal is to indicate the eternal sleep.
Poppies have been used for a long time as a symbol of sleep, peace, and death: sleep because the opium extracted from them is a sedative, and death because of the common blood-red colour of the red poppy in particular. In Greek and Roman myths, poppies were used as offerings to the dead.