One thing that I think is also important to note about Janie's hair is that once Jody dies, one of the first things Janie does is to remove the head wrap and let down her hair.
Janie's most noticable physical characteristic is her straight hair, which she takes great pleasure in showing off. Janie's hair is symbolic not only of her power, but it is what makes her different and sets her apart. Characters like Jody recognize the power of her hair; it is why he forces Janie to bundle it up and keep it hidden under a rag. Jody fears the individuality and independence Janie's hair represents.