The roof iris is native to China, but was first discovered in the 1860s, growing in Japan on the roofs, hence the common name. Instead of standing stiffly erect, the leaves arch over, making them useful as a graceful ground cover. The flowers appear in late April and early May.
The main reason for growing the plant was not for its flowers, but for a white powder that was made by grinding the roots. The makeup used to create the white faces of the Geisha girls was made from the rhizomes. So, the plants moved from the garden to the roofs where it remained until being "discovered" by science.
The Japanese roof iris is unique amongst irises because it grows about as well in the shade as in the sun. Like all irises, it should be planted with the rhizomes just at the surface of the soil.