Single serves the same role here that any other adjective would do, whether we are talking about a single rose, a solitary rose, a red rose or any other rose.
Introducing an adjective to qualify the rose does not change the need for the article a (or the)
The role of single is to emphasise that there are no other roses, not to replace the article.
It makes the difference between statements such as:
a pupil raised her hand
a single pupil raised her hand.
The adjective changes the nuance. The emphasis moves from the raising of the hand to the fact that only a single pupil does so. The same is true for saying a rose stood in the vase and a single rose stood in the vase.
The same rule applies regardless of whether you are talking about a rose, an ideal or any other noun.