Symbolism, Imagery, Allegory
Hang on, Shmoopers, because we are about to ruin The Wizard of Oz for you. It's no accident that the Wicked Witch chose magical poppies to put Dorothy and the gang to sleep: The Any time poppies show up in literature, you better be prepared for death.
Why? A lot of it goes back to World War I, when it became a tradition to plant or wear poppies in remembrance of the dead. As history goes, poppies tend to grow in land that's been uprooted by destruction, such as the damage of war. As a result, poppies frequently grow up around the bodies of the dead, which is kind of creepy when you think about it. Thus began the tradition of poppies as a memorial to the fallen, and their general connection to death.
Okay, we know what you're thinking: This history lesson was fun and all, but we haven't mentioned Jellicoe Road once. Don't worry, we got you covered. What flowers do the five plant on the road to mark the spot where their families were killed? If you said "poppies," you get a gold star. When Jude replants the poppies after they get trampled, we can see friendship blooming in the face of death and destruction, just like poppies do.