Iris kaempferi (Japanese iris) — Clump-forming, upright

Overview

Iris kaempferi 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. Iris kaempferi 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. Suggested uses include ornamental garden plant, water garden edges and pond margins. It is native to Japan, China, Korea and Russia.

About

Iris kaempferi is a clump-forming, upright perennial in the Iridaceae grown for its seasonal mid‑summer blooms. It carries sword-shaped, often glaucous leaves that create clear vertical lines in the border. It prefers full sun though some plants will tolerate light afternoon shade and will flower from May to July. Plant on neutral to slightly alkaline, moisture-retentive soil for the Japanese types, in a south- or west-facing spot.

Wikipedia contributors, CC BY-SA 4.0 — Iris ensata

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 kaempferi is a clump-forming perennial in the iris family, native to Japan, China, Korea and parts of Russia. The plant develops upright, sword-shaped leaves that are often glaucous and produces its flowers from May to July. Its architectural leaf habit makes it easy to place as a structural element in a mixed border or beside still water. Give it full sun where possible; some specimens will tolerate partial shade but flowering is best with good light. Soil can be loam, chalk or sand, and pH should be neutral to slightly alkaline. Although many irises prefer free drainage, Japanese and Siberian types perform better in moisture-retentive soils, so position them where the ground remains reliably damp without waterlogging. Note that bearded irises need much sharper drainage than the Japanese kinds. Rated H5–H7, Iris kaempferi is classed from hardy to fully hardy and is suitable for outdoor planting across most UK gardens. It suits south- or west-facing aspects and will tolerate both exposed and sheltered sites. Use it at pond margins, water-garden edges or at the front of borders where the vertical leaves can be seen; expect it to form clumps of sword-shaped, often glaucous leaves and to flower in May to July.

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 kaempferi size, habit and characteristics

Plant type: Perennial
Habit: Clump-forming, upright
Foliage: Deciduous or evergreen; sword-shaped to grassy, often glaucous
Suggested uses: Ornamental garden plant, water garden edges, pond margins
Native to: Japan, China, Korea, Russia

Care notes

Pruning and caring for Iris kaempferi

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 kaempferi?

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 kaempferi?

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

What soil does Iris kaempferi need?

Iris kaempferi 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 kaempferi?

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.

Is Iris kaempferi hardy in the UK?

Iris kaempferi 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 kaempferi?

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.

More in the Iris genus

Browse all Iris →