Once Geralt brings Triss to the elven ruins in the forest outside Flotsam, a new rose bush bearing purple roses appears. These are roses of remembrance.
Triss tells the witcher the legend of Eldan and Cymoril which states that the roses will wilt unless they are nourished with blood or if they are sold. If one gives the flower to someone they love, they will live forever. Geralt then has the option to give the rose to Triss, or not.
Rose of Rememberance returns as a quest item in the third game, regardless if it was given to Triss or not. However, with the passage of time, it's wilted, like any other picked flower. It can be looted from Triss' bedside table at her ransacked house in Novigrad.