Louis XVIII couronne la rosière de Mittau (1817)

Jean-Charles Tardieu: Louis XVIII couronne la rosière de Mittau, 1799

Rendering the title of this lovely work into English is complicated by the fact that I can't seem to find a translation of "rosière." Basically it works out to "Louis XVIII crowns the rosière of Mittau," so rosière is either archaic or colloquial.