Siberian iris

Siberian iris: how do you recognise it?

Habit

A dense, upright clump about 60 cm wide, with flower stems up to 90 cm.

Leaves

Narrow, grass-like and upright, fresh green; they turn yellow-brown in autumn.

Flower

Slender and smaller than a bearded iris, without a beard; the drooping petals are often marked with white or yellow and finely veined. A few flowers per stem, in May–June.

Colour

Mostly blue to violet; there are also white, pale blue and white-and-yellow varieties.

Fruit

Elongated brown seed pods that split open in autumn and stand into winter.

Rhizome

Thin and branching, below ground; it forms a dense, fibrous clump that widens slowly.

Flowering: May–June · Plants: March–April and August–September · Pruning: February, June and November

Why grow it?

• Elegant flowers in May–June that seem to float above the leaves.
• Grows where many perennials would be too wet: in moist borders and at the edge of a pond.
• The narrow, upright leaves stay fresh all summer and give a grassy structure.
• Brown seed pods decorate the border into winter and look good in dried arrangements.
• A good cut flower, and bees collect nectar and pollen from it.
• Extremely hardy and long-lived; dividing is rarely needed.

Siberian iris: where does it grow best?

• Full sun to partial shade; it flowers best in sun, as long as the soil stays moist.
• Humus-rich loam, clay or peaty soil, preferably acid to neutral. It grows less well on chalky soil.
• Normal to moist: ideal in a damp border or at the edge of a pond, but not with its base permanently under water.
• Allow 50–60 cm between plants; a group of three makes a fine, full clump after a few years.

Light

Sun and partial shade

Soil type

Loam, clay or peat

Acidity

Acid or neutral

Moisture

Normal or wet

Size

0.9 m tall, 0.6 m wide

Hardiness

Hardy (to -40 °C)

Goes well with

• Hosta: Broad leaves next to the narrow iris foliage, for the same moist spot.
• Astilbe: Pink, red and white plumes that flower after the iris.
• Daylily: Takes over the flowering in July and also likes moist soil.
• Purple loosestrife: Tall purple spikes in summer, also at the pond edge.
• Lady’s mantle: A soft green edge with small yellow flowers at its feet.

Siberian iris: when and how to plant it?

Plant in March–April and August–September.

1. Plant in March–April or in August–September, when the soil is moist.
2. Dig a generous hole and mix in compost or leaf mould.
3. Set the rhizome 2–5 cm deep, so below the surface, unlike the bearded iris.
4. Space plants 50–60 cm apart.
5. Water well straight away and keep the soil moist for the first weeks: the roots must not dry out.
6. Mulch with compost or leaf mould to hold in moisture.

Care

Watering

Keep the soil moist, especially in spring and while flowering. Water generously in dry summers.

Feeding

A layer of compost or a little organic fertiliser in spring is enough.

Mulching

A layer of leaf mould in spring keeps the soil cool and moist.

Spent flowers

Cut the stems off if you do not want seed, or leave them for the attractive seed pods.

Dividing

Only when the centre of the clump goes bare, which often takes many years.

Tip: Never let a division dry out: unlike the bearded iris, the Siberian iris wants moist roots. Replant the pieces straight away and water regularly for the first weeks.

Siberian iris: when to prune it?

Pruning months: February, June and November.

When: Cut off the dead leaves in November or, if you leave them over winter, in February. Spent flower stems can go in June.

How:

• Cut the brown leaves down to just above the ground before the new shoots appear.
• Cut spent flower stems off at the base if you do not want seed pods.
• Leave the seed pods if you like them: they decorate the winter border.
• No other pruning is needed.

Caution: If you leave the seed pods, seedlings may appear that flower differently from the parent plant.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In August–September or in spring. Cut the tough clump with a sharp spade into pieces with several leaf fans each, trim the leaves to 15–20 cm and replant at once. Keep them moist until established.

Seed

Sow fresh seed in autumn in a pot left outdoors: the seed needs a cold spell to germinate. Seedlings vary and only flower after two to three years.

Siberian iris: how does it get through the winter?

Hardy to −40 °C.

• Extremely hardy, to about −40 °C: no protection is ever needed in Belgium.
• The leaves turn yellow-brown in autumn; leave them as a winter silhouette or cut them off in November.
• In a large pot it stays hardy too, but never let the pot dry out.

Diseases and pests

• Few flowers: Too dry, too much shade or recently divided. Water more, move it to a lighter spot or be patient: after dividing it takes a year or two.
• Bare centre: An old clump dies out in the middle. Divide it and replant the young outer pieces.
• Slugs: They eat young shoots and flower buds in spring. Pick them off in the evening.
• Brown leaf tips: A sign of drought. Water generously and mulch the soil.

Through the year

• February: Cut off old leaves, if you left them over winter.
• March: Plant or divide. Add compost.
• April: Plant. Mulch. Watch for slugs.
• May: Flowering begins. Cut flowers for the vase.
• June: Flowering. Cut spent stems or leave them for the seed pods.
• July: Water in dry spells.
• August: Plant or divide.
• September: Plant or divide. Pick seed pods for drying.
• October: The leaves turn yellow-brown. Sow fresh seed in a pot outdoors.
• November: Cut off the dead leaves or leave them for winter.

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.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Despite its name, it does not only come from Siberia: it grows wild from central Europe to Russia, mostly in wet meadows.
• Unlike the bearded iris, it has no beard on its drooping petals: it belongs to the beardless irises.
• The dried seed pods stay attractive for a long time and are popular in dried arrangements.