Roses are a small but potent symbol in Mockingjay. Though we usually think of roses as symbols of love or beauty, here they are usually a reference to the ultra-evil President Snow. At the beginning of the book he has left a white rose in Katniss's house as a kind of message for her. The smell of roses makes her violently ill, probably because she associates them with Snow, who's out to get her. Prior to Snow's execution, Katniss selects a white rose from his garden for him to wear when he dies. She's sending her own message back to him.
Roses are also noteworthy because they end up being a reference to Katniss's sister, Prim. Prim's name is short for "Primrose," another kind of flower. At the end of the book, Peeta brings Katniss primroses as part of the healing process once the war ends.