Standing apart from, yet dominated by, the jar is the wilderness. The speaker doesn't seem to think much of this wilderness, calling it "slovenly" (3), and portraying it as completely under the control of the gray bareness of the jar (6). Yet, in the end, we start to think that this wilderness, no matter how tarnished it is by the presence of the jar, is still free. Unlike the jar, the wilderness does give of bird and bush—it is, well, wild and green. It grows and breeds.
Line 3: This speaker doesn't sound so fond of the wilderness upon first glance of this line. It's being described as slovenly: messy, dirty, unkempt. Yet we almost want to root for the wilderness to struggle against the boring jar's command.
Line 5: Again, we see the wilderness being controlled by the jar—rising up to the hill. We've got to think about the word "rose" on both the figurative and literal level. Literally, it could be raising in elevation. Figuratively, it could be raising in status, to reach the man-made jar.
Line 6: Though the wilderness is being described as no longer wild, the word "sprawled" still sounds wild, immense, to us.
Line 11: Here, the jar is being compared to two aspects of the wilderness: bird and bush. These two things are full of colors and procreation—birds breed, bushes grow. The wilderness may finally be getting back at this jar, retaining a power of its own.