Iris tectorum (Japanese roof iris) — Clump-forming, upright

Overview

Iris tectorum is a clump-forming, upright perennial in the genus Iris, part of the Iridaceae family, with deciduous or evergreen; sword-shaped to grassy, often glaucous foliage. It typically grows around 0.3 m tall. Iris tectorum grows best in full sun; some tolerate partial shade, and tolerates loam and chalk or sand; sharply drained for bearded kinds soils and neutral to slightly alkaline pH. Its UK hardiness is rated H5–H7. It is native to China.

About

Iris tectorum is a clump-forming, upright perennial in the iris family, distinguished by sword-shaped leaves that are often glaucous. It reaches about 0.3 m in height and flowers in the UK from May to July. It is native to China and is suited to sunny positions with neutral to slightly alkaline soil. Wear gloves when handling as the plant is harmful if eaten.

Wikipedia contributors, CC BY-SA 4.0 — Iris tectorum

About the genus

Iris takes its name from the Greek goddess of the rainbow, and for once the naming feels entirely earned. I can't think of another genus that covers so much ground — inky purples, burnt apricots, clean whites, blues that seem lit from inside — or so many situations. There are bearded irises that want their rhizomes baked in full sun, Siberian types happy in ordinary border soil, and species that will sit contentedly at the edge of a pond. What they share is the flower: three petals standing up, three falling away, built with a kind of architectural confidence most plants never attempt. Even out of bloom the foliage earns its place — those upright fans of blue-green leaves hold a border together for months. I grow them for the fortnight in May and June when nothing else in the garden can compete, and I've made my peace with that. Some plants are for the long haul. An iris is an event.

Written by Henry

In the garden

Iris tectorum is a compact, clump-forming perennial from China with erect fans of sword-shaped leaves that are often glaucous. In the UK it flowers from May to July, offering a defined flowering window for late spring and early summer interest. Position it in full sun where possible; some plants will tolerate light afternoon shade. A south- or west-facing aspect suits it well and it tolerates both exposed and sheltered sites. Soil should be neutral to slightly alkaline; while irises commonly grow in loam, chalk or sand, the Japanese and Siberian types are best given moisture-retentive soil rather than bone-dry composts. Generally provide free drainage but steady moisture through the growing season. Iris tectorum is rated H5–H7 for UK hardiness (hardy to fully hardy) and corresponds to USDA zones 3–9, so it will survive typical UK winters across most regions. At around 0.3 m tall it suits the front of a sunny border, edging, or gravel bed where its upright, sword-like leaves form compact clumps. Take care when planting and handling: the species is harmful if eaten and gloves or other protective equipment should be worn.

Growing conditions

Best soil and position for Iris

Sunlight: Full sun; some tolerate partial shade
Soil type: Loam, chalk or sand; sharply drained for bearded kinds
Soil pH: Neutral to slightly alkaline
Soil moisture: Well drained; Siberian and Japanese kinds need moisture-retentive soil
Aspect: South or west facing
Exposure: Exposed or sheltered
UK hardiness: H5–H7

Plant details

Iris tectorum size, habit and characteristics

Plant type: Perennial
Habit: Clump-forming, upright
Foliage: Deciduous or evergreen; sword-shaped to grassy, often glaucous
Height: 0.3 m
Native to: China
Toxicity: Harmful if eaten. Wear gloves and other protective equipment when handling. Pets: Harmful if eaten. For further information and contact numbers regarding pets, see the HTA guide to potentially harmful plants

Care notes

Pruning and caring for Iris tectorum

Cultivation: Plant bearded irises with the top of the rhizome exposed so it can bake in summer sun; beardless kinds sit just below the surface in richer, damper ground. Feed with a low-nitrogen fertiliser after flowering and keep the rhizomes clear of mulch and encroaching neighbours.
Pruning: Remove spent flower stems at the base. Trim the outer leaves back by about half in late autumn to reduce overwintering fungal spores, leaving the central fan intact.
Propagation: Divide rhizomes six to eight weeks after flowering, discarding the old woody centre and replanting vigorous outer sections. Species come from seed, but named cultivars will not come true.
Pest resistance: Generally trouble-free, though iris sawfly and slugs damage foliage and thrips can disfigure buds.
Disease resistance: Prone to leaf spot, and to rhizome rot wherever drainage is poor or planting too deep.

Frequently asked questions

How do I grow Iris tectorum?

Plant bearded irises with the top of the rhizome exposed so it can bake in summer sun; beardless kinds sit just below the surface in richer, damper ground. Feed with a low-nitrogen fertiliser after flowering and keep the rhizomes clear of mulch and encroaching neighbours.

Where is the best place to plant Iris tectorum?

Iris tectorum prefers full sun; some tolerate partial shade. It suits exposed or sheltered positions.

What soil does Iris tectorum need?

Iris tectorum grows best in loam and chalk or sand; sharply drained for bearded kinds soil. It prefers neutral to slightly alkaline soil. Aim for soil that stays well drained; siberian and japanese kinds need moisture-retentive soil.

How do I prune Iris tectorum?

Remove spent flower stems at the base. Trim the outer leaves back by about half in late autumn to reduce overwintering fungal spores, leaving the central fan intact.

How big does Iris tectorum grow?

Iris tectorum reaches a height of 0.3 m.

Is Iris tectorum hardy in the UK?

Iris tectorum has an RHS hardiness rating of H5, meaning it is hardy in most of the UK, tolerating -15 to -10°C.

How do I propagate Iris tectorum?

Divide rhizomes six to eight weeks after flowering, discarding the old woody centre and replanting vigorous outer sections. Species come from seed, but named cultivars will not come true.

Is Iris tectorum toxic?

Harmful if eaten. Wear gloves and other protective equipment when handling. Pets: Harmful if eaten. For further information and contact numbers regarding pets, see the HTA guide to potentially harmful plants.

More in the Iris genus

Browse all Iris →