Which country does the kingdom of roses refer to?

The kingdom of roses refers to Bulgaria.

Bulgaria's Balkan foothills have mild climate, abundant rainfall and fertile soil.

The whole rose is in full bloom here. Among the green branches and leaves, all kinds of roses are swaying in the wind, emitting intoxicating fragrance. This is the largest rose garden in the world. Rose oil, the price of which is half higher than that of gold, brings considerable economic income to this "rose kingdom" every year.