Shrivel-edged, unhinged petal, her first-about-to-fall (7)
This first reference to mortality in the poem is pretty oblique (that is to say, subtle) – we have just the barest hint of something withering ("shrivel-edged"). The petal in question, though, is representative of the beginning of a process that will quickly take all the petals of the flower. Each petal in turn will also be "about-to-fall," and so the observation of this first petal brings the idea of eventual death to the forefront of the speaker's consciousness.
Every breath imperils her. Her crucible is falling apart with its own fierceness. (13-14)
You'd think that the sentence "every breath imperils her" would imply sickness of some sort, as if the flower were really frail. Turns out that it's more complicated than that. This flower, as you've probably figured out by now, is not withering away quietly. Instead, the "crucible" that the petals form is going to fall apart (each petal falling off) from sheer force – as if the energy contained in the color of the flower was simply too much to sustain for any length of time. The flower's riotous beauty and bigness are to be its imminent downfall.
Soon she'll throw off her skirts Withering into vestal afterlife
More impending death – and yet another imagistic way of talking about petals falling off. This time, it's the removal of clothing (hey, risqué!) that portends (is an omen of) death, and this time it's also all of the petals, not just one. Notice also that this line is yet another example of how contradictory this poem is about its subject. In the first part we have a woman undressing, and in the second part we have an "afterlife" that is modified by the word "vestal" which has come to mean virginal/chaste/pure.
Her maternal nectars into her own Coffin—(cradle of her offspring) (19-20)
This is another "baseball bat" of a symbol – if coffins don't represent death, then what does? These lines, however, save themselves from being cliché by placing the coffin directly next to a "cradle," thereby putting into very close relation the ideas of death and birth. In the poppy's case, motherhood is going to kill her, quite literally. Thus, the coffin is also the cradle – to get really morbid about it, the offspring of the poppy emerge quite literally from the mother's dead body. Yikes.
Then we shall say: "She wore herself in her hair, in her day, (21-22)
This quote is a reference to the future, after the death of the poppy, and is all about how we remember the dead. In this case, it's still all in the petals – those big orange, eye-popping petals that throughout the poem have been symbolic of both life and imminent death. Fitting, then, that the first thing the speaker thinks we ought to eulogize about is the flower's "hair," i.e., those very petals.