Symbolism, Imagery, Allegory
A Rose-Shaped Birthmark By Any Other Name
Like the symbol of "Birds" (go check that one out if you haven't already), the symbol of flowers shows up in the dangerous-yet-alluring characters of Rochelle and Sula.
When Nel meets Rochelle, she notices that she smells like gardenias. Sula has a birthmark shaped like a rose, and "The Rose Tattoo" is the source of the novel's epigraph—check out "What's Up With The Epigraph?" for more on that. These particular flowers are beautiful and fragrant; their perfume is often described as intoxicating.
And so are these two women.
Rochelle can be seen as intoxicates the young Nel; Sula intoxicates the many men around her. We'd be amiss if we didn't mention the sexual connotations of these flowers here: both gardenias and roses are seen as symbols of boldness and passion. A daisy may be dainty, but a rose and/or gardenia exudes full-blown sensuality.
But the thing about flowers is, once they're picked, they don't live for very long. Just as the flower's beauty is fleeting, so too is the presence of both of these women in the novel.