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.