Closely related to the idea of transience and temporality that's also a theme in "Big Poppy," death plays a huge part in creating the world in which this poem operates – a world in which mortality is always in your face, as it were. The poppy, from the outset, is on the verge of dying, even as she's in the peak of her blooming. The poem constantly looks forward towards the time when the poppy's flowering will be over, and will be replaced with a sinister seed pod full of a potential drug (poppies are used to make opiates). The poem doesn't necessarily put the two into direct confrontation (you know, good vs. evil or something), but it does privilege the fiery petals over the flower without its orange "clothing."
The poem places birth and death in extremely close proximity, making them co-dependent, but also making birth the destroyer of beauty.
In "Big Poppy," mortality is almost celebrated, as a kind of necessary means by which we come to appreciate vitality.