Alliteration (including sibilance) occurs frequently throughout "November Cotton Flower." For the most part, it creates a harsh sound that matches the poem's unforgiving, wintry atmosphere. This effect is most prominent in the opening lines:
Boll-weevil’s coming, and the winter’s cold,
Made cotton-stalks look rusty, seasons old,
And cotton, scarce as any southern snow,
Was vanishing [...]
Try saying these lines out loud—it's almost like teeth chattering! The repeated /k/ sounds are hard and unpleasant, while the /s/ sounds, in this context, sound whispery and eerie, like a cold wind blowing across the land. Additional /k/ consonance (e.g., "stalks") and internal /s/ sibilance (e.g., "rusty," "scarce") contribute to this evocative soundscape, too.
Sibilance hits new heights around lines 9-12:
Such was the season when the flower bloomed.
Old folks were startled, and it soon assumed
Significance. Superstition saw
Something it had never seen before:
(Note that "assumed" is alliterative here because that shared /s/ sound lands at the start of a stressed syllable.) This is the moment when the cotton flower appears, supposedly bringing with it a new sense of beauty and hope. In the absence of harsher alliteration (/k/ sounds, etc.), these /s/ sounds have a soothing quality that might reflect the more optimistic mood. But they also subtly echo those bleak opening lines, perhaps suggesting that any hope is tempered by the ongoing drought—along with whatever social conditions (racism, poverty, etc.) the drought may symbolize.