Iris tectorum (ht 30 cm) comes from China and like the other crested Iris enjoys woodland conditions of semi-shade and humus-rich soil, and can tolerate quite wet conditions. However, it will also grow in full sun and quite dry soil. The plants have fans of ribbed leaves and pretty blue-lilac or white flowers; they multiply fairly quickly to form a clump. This Iris needs to be divided every few years. They do best if given plenty of fertiliser and water, but it will still do well without them. The white form is said to be less vigorous, but I have found it to perform just as well as the blue. Iris tectorum is known colloquially as the roof iris because in Japan it is grown on the ridges of the roofs of thatched houses. Legend has it that this was because during a famine many years ago, only food crops could be grown in the ground so somewhere else had to be found to grow the Iris, the roots of which were made by the women into a face powder. Another theory is that the plants were used on the roof because their roots helped to bind together the thatch and the wet clay used to construct the roofs.
Postscript: My plants did survive OK but never really flowered as well as I would have liked them to. I have moved them on to a colder-climate garden, where they are doing a lot better.