Iris tectorum (blue)

The horticulturist Scott Ogden, in Garden Bulbs for the South, sets the scene: the Japanese roof iris, Iris tectorum, is famous in the native country as a flower for planting on sod roofs, just as houseleeks are used on the cottage roofs of France. In gardens the silky green fans of leaves form large patches, a fine subject for the foreground of a shady border, and in April the ruffled, orchid-like blooms appear among the handsome leaves. This is the common form, in which the flowers open a rich mottled blue, veined and freckled toward the center, with white crests.

The common name and the Latin tectorum, meaning of roofs, both come from a genuine and rather poignant piece of history. Iris tectorum is native to China, where the species has been grown since at least the seventh century, and reached Japan long ago. During a period of famine and war, so the story goes, an imperial decree reserved all arable land for food crops and made growing flowers on the ground illegal, so Japanese households moved the iris up onto the thatched ridgelines of their roofs, where the plant grew and bloomed out of reach of the law.

The roofs were not only a hiding place. The real value lay underground, in the thick, thumb-sized rhizomes, ground into a fine white powder used as a face cosmetic, the pale makeup of the geisha among the uses, and worked into hair dye and corn plasters besides. In China the same rhizome carries a long medicinal history under the name chuan she gan, taken in traditional practice as a decoction to soothe sore throat, ease cough, and clear what the old texts call heat. Modern laboratories have since studied the flavonoids and other compounds the rhizome holds, though the tradition ran centuries ahead of the chemistry.

In the garden the roof iris is grown for foliage as much as flower. Set the broad, ribbed, silky green fans at the foreground of a shaded border, along a path, or at the top of a low wall where the fans can spill, and let the mottled blue, white-crested blooms open among the leaves in mid to late spring. Pair with ferns, hostas, and other shade companions in fertile, well-drained soil, give a little afternoon shade where summers are fierce, and divide every few years to keep the clump flowering. Deer tend to leave the fans alone.