Sabi Planter is made through a process called metal spinning. Due to the nature of the process, the spun pieces that come out of the mold have a hole in the center. Taking advantage of the properties of the manufacturing process, Jung has created a set of planters in different materials: steel, copper, and brass.
As your plants grow, the Sabi Planter will age as well, developing its unique color and patina due to the natural behavior of metal. There's no need to worry about harming your plants as rust is not harmful to the plant life, though growing vegetables for consumption is not recommended as a precaution.