Japanese iris

Japanese iris: how do you recognise it?

Habit

An upright clump about 90 cm tall and 45 cm wide.

Leaves

Narrow and sword-shaped, with a prominent midrib; they die back in autumn.

Flower

Large and flat, 10–20 cm across, with broad petals held almost horizontally, in June and July.

Colour

Purple, violet, blue, pink or white, often veined and with a yellow blotch at the base of the large petals.

Roots

A branching, creeping rhizome with fibrous roots.

Flowering: June–July · Plants: March–April and September · Pruning: July and November

Why grow it?

• Huge, flat flowers in June and July, when most other irises have finished.
• An eye-catcher for pond edges and wet borders in the sun.
• A beautiful cut flower: cut the stem when the first bud shows colour.
• The upright, sword-shaped leaves give structure from spring to autumn.
• Countless cultivars in many colours and forms, with single or double flowers.

Japanese iris: where does it grow best?

• Full sun for plenty of flowers.
• Acid to neutral soil: loam, clay or peat. It does not tolerate lime.
• Moist to wet in summer, but it must not stand under water in winter.
• Allow 40–45 cm between plants.

Light

Sun

Soil type

Loam, clay or peat

Acidity

Acid or neutral

Moisture

Normal or wet

Size

0.9 m tall, 0.45 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Japanese primrose: Flowers in May–June in the same moist, acid soil, just before the iris.
• Royal fern: Fine green fronds in the same wet, acid spot.
• Siberian iris: Flowers in May–June, so you extend the iris season.
• Giant rhubarb: Giant leaves as a backdrop beside a large pond.

Japanese iris: when and how to plant it?

Plant in March–April and September.

1. Plant in March–April or in September, when the soil is not frozen.
2. Dig a hole and mix in ericaceous compost.
3. Set the rhizome about 5 cm deep, deeper than you would plant a bearded iris.
4. Keep 40–45 cm between plants.
5. Water well straight away, preferably with rainwater, and keep the soil wet for the first few weeks.
6. Mulch with leaf mould or pine needles to hold in moisture.

Care

Watering

Keep the soil moist to wet from spring until after flowering. Use rainwater if you can: hard tap water makes the soil less acid.

Feeding

Feed in spring with an ericaceous fertiliser, as for rhododendrons.

Mulching

A layer of leaf mould or pine needles keeps the soil cool, moist and acid.

Spent flowers

Cut them off after flowering, so the plant does not put energy into seed.

Dividing

Every 3–4 years, when the centre of the clump becomes bare and flowering declines.

Tip: Water it with rainwater rather than tap water and feed in spring with an ericaceous fertiliser, as for rhododendrons. That keeps the leaves green and the flowers plentiful.

Japanese iris: when to prune it?

Pruning months: July and November.

When: It is hardly pruned. After flowering, in July, cut off the spent stems, and in November the dead leaves.

How:

• Cut spent flower stems off close to the ground.
• In November, when the leaves have turned brown, cut them back to 10–15 cm above the ground.
• Clear away the old leaves so that diseases and slugs cannot overwinter in them.

Caution: Wear gloves: the sap of the rhizome can irritate the skin.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

Every 3–4 years, after flowering in August–September or in March: lift the clump and divide it into pieces with 2–3 fans of leaves and roots. Trim the leaves to about 15 cm and replant at once; never let the roots dry out.

Seed

Sow seed in autumn in a tray and leave it outdoors. Seedlings flower after 2–3 years and do not always resemble the parent.

Japanese iris: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: cold is not a problem.
• It must not stand under water in winter, or the rhizome will rot.
• If it grows in a pot in the pond, lift the pot out of the water in autumn and stand it in the garden.
• A thin layer of leaf mould protects young plants in their first winter.

Diseases and pests

• Yellow leaves: On chalky soil or with hard tap water the leaves turn yellow. Use rainwater and an ericaceous fertiliser.
• Few flowers: Too dry, too little sun or a clump that is too old. More water, a sunnier spot or dividing will help.
• Winter rot: The rhizome rots if it stands under water in winter. Plant slightly higher or lift the pot out of the pond.
• Slugs: They eat the young leaves and flower buds. Pick them off in the evening.

Through the year

• March: Plant or divide. Apply ericaceous fertiliser.
• April: Plant. Keep the soil moist. Mulch.
• May: Water generously: the leaves grow fast.
• June: Flowering begins. Cut flowers for the vase.
• July: In flower. Cut off spent stems. Keep the soil wet.
• August: Water in dry spells. Divide if the clump is too old.
• September: Plant or divide.
• October: Lift pots out of the pond.
• November: Cut dead leaves back to 10–15 cm.

Edible and useful

Not edible. All parts of these irises are poisonous, especially the rhizome: eating any causes vomiting, stomach ache and diarrhoea, and dogs, cats and grazing livestock fall ill from it too. The main risk comes when you divide the clumps: clear up the loose pieces of rhizome straight away, do not leave them lying where children play or dogs dig, and do not throw them into a field or run. Siberian and Japanese iris like the edge of a pond, where without flowers their leaves look like those of other waterside plants: do not eat anything from a waterside plant you are not sure of. The sap can irritate the skin: wear gloves when dividing and cutting back. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Did you know?

• In Japan, cultivars have been bred since the Edo period. There are three classic groups, named after their region of origin: Edo, Higo and Ise.
• In June, iris fields in Japan draw many visitors, such as the iris garden at the Meiji Shrine in Tokyo.
• The species name ensata means ‘sword-shaped’, after the shape of the leaves.