Which country does the "Rose Kingdom" refer to?

The Rose Kingdom refers to Bulgaria.

The Balkan foothills in Bulgaria have a mild climate, abundant rainfall, and fertile soil.

A whole field of roses are blooming here. Among the green branches and leaves, roses of various colors sway in the wind, exuding intoxicating fragrance. This is the largest rose garden in the world. Rose oil, which is half as expensive as gold, will bring considerable economic income to this "Rose Kingdom" every year.