Iris sibirica (Siberian flag) — Clump-forming, upright

Overview

Iris sibirica 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 0.6 m tall and 0.3 m wide. Iris sibirica 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 Europe to C Asia.

About

Iris sibirica is a clump-forming, upright perennial in the iris family that carries tall flower stems above narrow, sword-shaped leaves. It bears violet-blue to blue, occasionally white, flowers from May to July. Plants reach about 0.6 m tall with a roughly 0.3 m spread and form dense clumps from creeping rhizomes. Leaves are often glaucous, giving a blue-green cast to the clump.

Wikipedia contributors, CC BY-SA 4.0 — Iris sibirica

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

A native of Europe to Central Asia, Iris sibirica is a herbaceous perennial valued for its vertical flower stems and narrow, grass-like foliage. Flowering occurs from May to July, when two to five blossoms open on branched stems; colours range from violet-blue through blue and occasionally white. The plant grows from a creeping rhizome and forms neat clumps about 0.6 m high and 0.3 m across. Position plants in full sun where possible, though some specimens tolerate light shade. Soil should be neutral to slightly alkaline; loam, chalk or sand are suitable provided Siberian irises receive consistent moisture — unlike bearded types they prefer moisture-retentive conditions while still draining freely. A south- or west-facing aspect suits flowering, and plants tolerate both exposed and sheltered sites. Iris sibirica is rated H5–H7, i.e. hardy to fully hardy for the UK, and will withstand cold winters in most British gardens. Use it where slim, upright foliage and seasonal blue flowers are needed: in mixed perennial borders, alongside moisture-loving herbaceous planting or at the back of a sunny bed. Allow the narrow, glaucous leaves to form a compact architectural clump that continues to give structure to the border after the blooms have finished.

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 sibirica size, habit and characteristics

Plant type: Perennial
Habit: Clump-forming, upright
Foliage: Deciduous or evergreen; sword-shaped to grassy, often glaucous
Height: 0.6 m
Spread: 0.3 m
Native to: Europe to C Asia
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 sibirica

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 sibirica?

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 sibirica?

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

What soil does Iris sibirica need?

Iris sibirica 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 sibirica?

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 sibirica grow?

Iris sibirica reaches a height of 0.6 m and a spread of 0.3 m.

Is Iris sibirica hardy in the UK?

Iris sibirica 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 sibirica?

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 sibirica 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 →