"Big Poppy" hinges on temporality, in a lot of ways. What do we mean by that? Well, a lot of what gets the speaker so riled up about this flower is not only that it's very pretty, but also that it's very temporary – as soon as it flowers, it's already in the process of transforming into something much darker. This theme is very much related to the theme of Mortality. They have kind of a causal relationship – the transformation, in this case from orange flower to dark green/black seed pod, is indicative (it tells us) of the plant's impending death.
Much of the force of beauty in "Big Poppy" lies precisely in the temporary nature of the flower itself.
The flower's impending transformation into a drug for human consumption renders it dangerous and alluring all at the same time.