What is the theme or the central idea of the poem, Birches?

One important theme in the poem, Birches, is that of the imagination versus the real world. Frost uses his poetic imagination to transcend the limits of the real world. He rejects the true reason the birches have been bent over in favor of his own fanciful explanation. On some level, he is claiming that this act of the imagination embodies a larger "truth" and is a worthy task, one that must be made with great care and diligence.

On the other hand, Frost makes it clear that one must remain within the natural world itself and that complete escape into the world of the imagination is impossible and not even desirable. It is this tension within the poem that makes each world both appealing and painful - the real world might be a place of pain, but it is also the place for love; the imaginary world is innocent, but it is also solitary and, by extension, loveless.