This is an interesting question, because the poem has no obvious setting. It is not a narrative with an external location; rather, it is a hypothetical train of thought. Therefore, the literal setting of the poem would be the mind or brain of the speaker, though that is likely not the answer you are seeking.

We can tell throughout the poem that a person is wondering and pondering this poem to themselves, from the beginning question, "What does he who plants a tree?" (line 1) to the ending conclusions of the question—that "all the growth of all our land" (line 25) is what a man plants when he plants a tree. As we can see, the poem contains no events that take place in a tangible location; rather, the poem itself is the event, unfolding in the mind of the speaker. The asking and answering of the poem's question both occur internally.