Roses need not be literally spoken of to have power. Think of roses and their various roles in human life. Roses are often present at weddings, a promise of love despite hardship, the combination of pain and beauty.
However, roses because of their strong odor, are also used in funeral homes to cover the stench of decay. In Faulkner's day as well, older ladies, those of the late 1800s, favored rose water parfumes as a means of hiding bodily odor (no deodorants then, you know.) Emily tries hard to be something she is not...young, engaging, marriageable.
The covering of stench could be applied to a variety of characters: the town in its neglect of one of its own, Emily in covering the death of her lover, the dead flowers symbolic of Homer's apparently neglected promise.
Additionally, it has been traditional for brides and lovers to press and preserve roses. The rose of the title then, may symbolize Emily's stagnant dreams for a life with Homer.
Or, one could view the rose as the narrator's offer of friendship, extended too late. Like a clipped rose, life itself is short and once it is gone, the rose can never be restored to its previous glory.