You might want to examine how nature is used to reflect the emotional and psychological desolation of the knight as he is left "Alone and palely loitering." Note the way that the action of the poem is placed in a specific context that indicates a certain season:
"The sedge has withered from the lake,
And no birds sing."
And then again in the second stanza:
"The squirrel's granary is full,
And the harvest's done."
Note how we are at the very end of Fall and Winter is beginning. The squirrel has done all of its work to prepare for the cold, dark months ahead, and likewise the harvest is all over. All the sedge has "withdered" from the lake, and the birds are silent. This of course reflects the winter that is in the knight's soul after his experience with the strange woman that has bewitched him. Note in particular the detail of the sedge "withering" and how this is repeated at the beginning and the end of the poem, indicating how the man himself has "withered" thanks to the enchantment of this woman. Thus nature is used to reflect the desolation of the knight.