Chinese or Japanese Roof Iris
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 groundcover. The flowers appear in late April and early May.
This charming little plant became known as the Japanese roof iris because that is where it was first observed by a Russian scientist, Carl Maximowicz (1827-1891). He spent three and a half years botanizing in Japan in the early 1860s and introduced numerous Japanese plants to Europe through his base in St. Petersburg.
In China, apparently the original home of the roof iris where it has been grown since at least seventh century, the plant grows on the ground like any sensible iris. But in Japan, it was found growing on the ridges of their thatched roofs.
Apparently this tradition started in Japan because of a decree by a Japanese emperor during a period of wartime when it became illegal to waste land growing flowers. All available land had to be used for rice or vegetables.
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.
IThe 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.