I think that you could justify either understanding of the poem. "Root Cellar" is an imagery ridden poem which attacks the senses in many different ways. The visual imagery brings about a very detailed and distinct picture for the reader. At the same time, a reader, familiar with the smells associated with a cellar described as such, can smell the mildew and rot described in the poem.
Another interpretation of the poem could reference life itself. The cellar represents an object in life (a lie, a bad relationship, a choice gone bad) and depict that the atmosphere with which the object dwells is not one that can sustain a proper life or being. Therefore, the cellar could show that a "bad thing" will continue to grow until removed from the poisonous atmosphere.
One last justification of the poem, and the meaning of the cellar, could speak to the fact that no matter how diseased or dark a place is, life is possible. WHile the poem speaks to the negativeness of the setting, it shoes the hope that one can embrace in regards to getting out of the dark.