The rose garden represents many things, here is an excerpt from the enotes:

The fact that Death appears in the rose garden underscores the place of death in the order of the natural world. He not only encounters the Gardener there but the Master as well, emphasizing that death’s dominion is nature itself. A rose garden is a place of great beauty, but that beauty is seasonal. When the season changes, the roses wither and die. So, too, with human beings.