The most common figures of speech in this poem are simile, metaphor, and personification.
In line two Roethke writes:
Bulbs broke out of boxes hunting for chinks in the dark
The bulbs to which he refers are flower or plant bulbs. Neither of these are capable of literally hunting, therefore personification is at work.
Metaphor can be seen in these words of line 6:
And what a congress of stinks!
Ironically, Congress comes from all over the United States to congregate in Washington to work together. Roethke is comparing the smell of the cellar, smells from every little nook and cranny of the room, to a congress or congregation of people.
Simile is used most prolificly throughout the poem:
dank as a ditch (line 1)
Hung down long yellow evil necks, like tropical snakes (line 5)
Roots ripe as old bait (line 7)
Each of these similes are used to help the reader better relate to the speaker's experience. The roots being "ripe as old bait" help a reader understand that the stench is completely unbearable. These particular figures of speech are used to create comparisons to which readers can relate.