The setting for most of this poem is fairly static – since the entire piece is a meditation on a plant, and plants don't typically get up and walk away from you, we're in once physical place for the duration of the poem. Conveniently, Hughes sets us up with the "where" right away – the second line of the poem is "At the trim garden's edge," and so we're in a kind of controlled pastoral scene. While we're not out in the woods, we're outside, in a "natural" setting.
The end of the poem, however, moves into the future – the setting doesn't change when the poem tells us that "soon she'll throw off her skirts / Withering into vestal afterlife" (16-17), as theoretically that's going to happen in the same exact place that the poppy exists in the poem when it's alive.
The only place where the setting becomes ambiguous is towards the very end, when the poem begins to talk about how "we" are going to talk about the poppy after it's dead (see lines 22-26). We might not be in the garden anymore at that point. We could be practically anywhere, remembering this incredible, short, botanical life. So, in that sense, the setting of the poem moves rapidly outward at the end, encompassing a myriad of potential locations for this kind of memory to happen. We've gone from the garden into the world, then. In death, the poppy travels far.