Iris tectorum Seeds | Roof Iris
LATIN NAME: Iris tectorum
COMMON NAME: Roof Iris
Plant Profile
The roof iris is a rhizomatous, clump-forming iris from China and Myanmar, with broad, ribbed, glossy green leaves in a fan, thirty to forty-five centimetres tall. In late April and May it carries two or three flowers on each stem, flat and wide open, in lavender-blue to violet, with a white and yellow crest on the falls and a pattern of darker veins.
A white form, f. alba, is common in gardens. The plant spreads slowly by rhizomes into a dense clump, and the leaves stay green through most of the winter in mild climates. Its common name comes from its use in Japan and China, where it was grown on the ridges of thatched roofs, apparently in the belief that the rhizomes held the thatch together and deterred pests.
Science & Biotechnology
Iris tectorum is placed in the subgenus Limniris, the beardless irises, in a section of its own, and its flat, crested flowers are unlike those of the bearded irises despite the crest, which is a ridge of tissue rather than a beard of hairs.
The species is unusual in its chromosome number, which differs from that of most of the beardless group and has been used to argue for its isolated position. Recent molecular work places it close to Iris speculatrix and Iris wattii, both of which also have crested falls and broad leaves. The plant’s association with roofs is documented in Japanese garden writing from the Edo period, though the practical reason is uncertain: the rhizome may have helped bind the thatch, or the plant may simply have been a way of using the roof space.
Uses & Benefits
In gardens the roof iris is used at the edge of a path, on a bank, at the front of a border and in light shade under trees, where its broad leaves make a bold clump and its spring flowers are a strong patch of colour. It is more tolerant of shade and of damp than most irises, though it needs drainage in winter.
The flowers are good for cutting, and the seed pods, which are large and angular, are used in dried arrangements. In Chinese medicine the rhizome is used as a purgative and diuretic, but the fresh rhizome is toxic and the dried one is a professional preparation; the same caution applies to all irises, whose rhizomes contain iridin. The plant has no culinary use.
Growing from Seed
Iris seed needs cold and should be sown fresh in autumn. Soak it for a day, then sow in a gritty compost and leave the pan outside through winter; germination comes in spring, and some seed will wait a year. Do not let the seed dry out.
Prick the seedlings out into deep pots, since the root goes down before the shoot appears, and grow them on in a shaded frame for the first season. Plant out in late summer, setting the rhizome at the surface, in moist but free-draining soil in sun or part shade. Mulch with compost but keep it off the rhizome. Divide the clumps every four or five years after flowering, and replant the outer pieces, discarding the woody centre.
Our Take
A shade-tolerant iris with broad leaves and flat, crested flowers, and a history of growing on roofs. Give it moisture and drainage.
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