The earliest idea of a three-dimensional flower bed

Quick answer：

The earliest known idea of a three-dimensional flower bed dates back to ancient Egypt and Greece, where elevated flower beds were constructed for aesthetic purposes. These elevated beds, often called 'peristyle' or 'sunken garden,' were designed to showcase a variety of plants and flowers, creating a visually stunning environment. Throughout history, the concept has evolved, with various cultures and societies contributing their own designs and innovations.