Where It All Goes Down
Setting is everything in "Binsey Poplars." It's announced in the title (check out "What's Up With the Title?"), it's the central preoccupation of the speaker (check out "Speaker"), and it's key to the poem's themes (check out…"Themes"). Really, though, we're dealing with a micro-setting and a macro-setting here, so it's worth touching on both.
The micro-setting, the most immediate setting, is the village of Binsey in Oxfordshire, England. Hopkins lived and worked near there (see "In a Nutshell" for more), so he knew the setting and its natural features. He thought he knew them anyway, as one day the disappearance of a familiar stand of poplar trees disturbed him enough to write this poem.
The macro-setting of this poem, though, in a larger sense is Nature itself (or "her" self, as the poem puts it in line 17). It's not just that our speaker is over-the-top in love with some trees. He sees a bigger problem in them being cut down. Specifically, human interference in the natural world effectively stops Nature from being, well, natural. And once that happens, we can never go back.
The poem's setting, then, is our own setting: the natural environment all around us. The speaker wants to let us know how fragile and important it is. So, maybe put down that chainsaw, okay?