This line comes very early on in the poem, and already the speaker is imagining that the poppy isn't just swaying in the breeze, but is actually swaying towards August. What does it mean to sway towards August? Well, August is typically how we think of the end of summer, no? After that, things start cooling down, and plants begin their fall transformations. So, the first instance of this poppy's movement is already towards her own transformation at the end of the season. This early line sets up the poppy's transience for the rest of the poem.
Up the royal carpet of a down-hung, Shrivel-edged, unhinged petal, her first-about-to-fall. (6-7)
This image is really specific – we're talking here about one single petal that's flopped down a bit, letting the bee crawl up it into the flower. The focus on this particular petal is more important than just a runway for the bug, though. It's also symbolic of the extremely temporary nature of the poppy's flowering. Since we know that it's not actually the end of summer (the "towards August" bit), this quote gives us a specific image of the poppy, having just flowered, already beginning to wither a bit.
Already her dark pod is cooking its drug. (12)
Impending transformation here – even though the poppy is still in bloom, the speaker knows what's coming, and what's coming is a seed pod full of poppy seeds, which are used to make opium. This is the first directly dark notion of transformation we've had in the poem so far, and it makes us think perhaps that the poppy's production of opium is a little scary to the speaker, like he almost can't believe that something so pretty and passive could produce something so aggressively intoxicating.
Bleeding inwardly Her maternal nectars into her own Coffin – (cradle of her offspring). (18-20)
Like the quote above, these lines move towards the end of the summer, when the poppy's petals are gone and the plant is doing other, more sinister work. The poem is gory about it, too – instead of a word like "transforming," we have "bleeding," and the process of becoming mother is linked entirely to the process of dying, all of which happens very quickly in this poem.
"She wore herself in her hair, in her day, and we could see nothing but her huge flop of petal, (22-23)
This first part of the eulogy for the poppy seems to be reflecting upon a kind of transience, especially with the "in her day" bit in there – the poppy did in fact have a day, but that day was only twenty-odd lines long. (We know, we know, the poppy flowers for a time period, lines are not a time period, but in this poem creates a world! So the poppy, for purposes of this kind of analysis, only exists in the poem. For 26 lines.) During that extremely brief period, the poppy was a riot of color and sensuality, so much so that those who looked upon her "could see nothing" but her.