How do roses ward off vampires?

* Roses naturally have thorns and a toxic-wax coating that makes their scratches slightly painful. This is as true for vampires as it is for real people. * Roses are the symbol of true love (at least the red ones are) and the association may have an effect on some vampires by inducing guilt. * Some people (and consequently vampires) are allergic to plant pollen. And even though you're one of the undead your Allergies still follow you. A vampire can over-come an allergic condition but it takes time and effort.

As you can see the potential of a roses to ward off a vampire isn't all that potent. There are other things that have more power.