Smells are very important in the novel, and they often evoke the past. Wisteria is a very Southern vine, and its smell is associated in the novel with Miss Rosa, who is obsessed with the past. When the smell of wisteria wafts through a scene, it's often accompanied by thoughts of the past. The smell even follows Quentin all the way to his dorm room at Harvard. Like the past, the scent is unshakable. (Keep your eye out for this one, too: wisteria shows up in other Faulkner novels as well, like The Sound and the Fury).