Iris: planting rhizomes, dividing clumps and why it does not flower

In brief: Bearded irises are planted in summer, from July to mid September, and shallowly: the upper half of the rhizome must stay above ground. They need sun and free-draining soil without fresh manure. Small bulbous irises, by contrast, are planted now in autumn, 8–10 cm deep.

An iris is a perennial that you need plant well only once and then divide only occasionally for decades. Most failures have one cause: the rhizome ends up in the ground as deep as a tulip bulb. A bearded iris then grows leaves but does not flower, or the rhizome rots in a wet winter.

Under the name iris, however, plants with completely different requirements are sold, from drought-loving bearded irises through the moisture-loving Siberian iris to small bulbs that flower in February. So we start with which iris you actually have.

Which iris you have: four groups, four different plantings

You recognise bearded irises by the "beard", a strip of hairs on the lower petals, and by broad grey-green leaves arranged in a fan. Siberian iris has narrow, grass-like leaves and forms a dense clump. If you bought a bag of tiny bulbs, they are bulbous irises and the same autumn calendar applies to them as to crocuses; details in the article autumn planting of bulbs.

Why bearded irises are planted in summer

After flowering a bearded iris slows down for a few weeks and then begins to form new roots. If you divide or plant it in July and August, it manages to root before winter and the rhizome anchors itself well. The cut-off is roughly mid September. A rhizome planted later does not have enough roots before the frosts, frost can heave it out of the ground and in spring it usually does not flower.

Now at the end of September, therefore, rather plant container plants that have a root ball. They can go into the ground in autumn and in spring. Bare rhizomes bought at the end of September are not lost, but expect them to flower weakly or not at all in the first year. After planting, in a frosty winter without snow cover them lightly with evergreen boughs, not with a layer of leaves under which the rhizome rots.

A rhizome in the sun: planting a bearded iris

1. Choose a place with at least 6 hours of sun a day. In shade an iris grows but flowers little.
2. Loosen the soil to a depth of 25–30 cm. Lighten heavy clay soil with coarse sand or fine gravel; an iris does not tolerate standing water.
3. Do not add fresh manure or nitrogen fertilisers. A little well-rotted compost is enough, and in poor soil a handful of bone meal in the hole.
4. Dig a shallow hole and heap a small mound of soil in the middle.
5. Place the rhizome on the mound, spread the roots downwards on both sides and cover them.
6. Leave the upper half of the rhizome above ground. Turn the fan of leaves to the north, so that the rhizome gets more sun.
7. Press the soil firmly around the roots and water. Afterwards water only in dry weather until the plant roots.

In a group plant three rhizomes in a triangle, with the fans pointing away from each other, 30–45 cm apart. Dwarf bearded irises need only be planted 20–25 cm apart and are excellent for a rock garden, as the article building a rock garden describes.

Siberian iris is planted differently: the rhizome goes completely underground, 2–5 cm deep, in soil that does not dry out in summer. Yellow flag and other marsh species belong in baskets at the edge of a pond; more in the article how to make a garden pond and what to put in it.

Dividing an overgrown clump

After 3–5 years a bearded iris spreads into a ring and the centre of the clump stays bare. The old rhizomes in the middle no longer flower; only the edge does. This is the signal to divide, ideally 4–6 weeks after flowering, that is in July or August.

• Lift the whole clump with a digging fork; a spade easily cuts through the rhizomes.
• Shake off the soil and use a knife to separate young side rhizomes with one or two fans of leaves. Each piece should have healthy roots.
• Throw away the old woody centre and soft or smelly pieces.
• Trim the leaves to about 15 cm in an inverted V; the plant then dries out less and the wind does not rock it.
• Plant the pieces as described above, even in a new place.

Siberian iris is divided either in spring, when it starts to sprout, or at the end of summer. Its clump is dense and tough, and often has to be cut with a spade or two forks back to back. The general principles of dividing perennials are summarised in the article ornamental perennials.

Why an iris does not flower or rots

The rhizome is buried

The most common mistake. A rhizome under the ground, under bark mulch or under a layer of leaves gets no sun and the iris flowers weakly. Pull the soil back so that the upper half of the rhizome is visible.

The clump is overcrowded or in shade

After years an iris becomes exhausted and surrounding shrubs shade it. Divide it and replant it in a sunnier place. The first year after dividing the flowering is usually weaker; in the second year it returns.

Too much nitrogen

Manure and lawn fertilisers promote leaves and at the same time soft rot of the rhizome. Irises need only one light feeding in spring with a fertiliser low in nitrogen.

A soft, smelly rhizome

Bacterial rot starts at the base of the leaves, which can be pulled out easily, and the rhizome is mushy and smells. Cut the affected part out back to healthy tissue, leave the wound to dry in the sun and improve drainage. Destroy badly affected plants and do not put them on the compost heap.

Brown spots on the leaves

Leaf spot appears mainly in wet years. Remove affected leaves, clear away old dead leaves in autumn and do not plant clumps too densely, so that air can circulate between them.

Irises in the Czech countryside and safety

Several wild irises are among the specially protected species in the Czech Republic. Under Annex II of Decree 395/1992 Coll., Iris humilis is critically endangered, and stemless iris, dwarf iris, variegated iris, Siberian iris and grass-leaved iris are highly endangered. So do not pick or dig them up in the wild. You may grow Siberian or dwarf iris from a garden centre, as these are plants raised in a nursery.

Irises are poisonous to people and animals. The rhizome contains the most active substances. In dogs and cats, chewing causes drooling, vomiting, diarrhoea and lethargy. The sap from the rhizome can irritate the skin of sensitive people, so wear gloves when dividing clumps. Do not leave the rhizomes you discard when dividing lying in the garden, where a dog could dig them up. If you suspect poisoning of a child, call the Toxicological Information Centre on 224 919 293 or 224 915 402.

Frequently asked questions

When should I plant irises?

Bearded irises with a bare rhizome from July to mid September, ideally 4–6 weeks after flowering. Container plants can also be planted in autumn and in spring. Small bulbous irises are planted in September and October like other spring bulbs.

How deep should I plant an iris?

A bearded iris shallowly: the upper half of the rhizome stays above ground and only the roots are covered. Siberian iris 2–5 cm below the surface. Bulbous irises 8–10 cm deep.

Should irises have their leaves cut in autumn?

Leave the green leaves, as the plant draws nutrients from them until autumn. Remove only brown and spotted leaves. Trim the leaves into a V only when dividing and transplanting.

Why does my iris flower only at the edges of the clump?

The centre of the clump consists of old rhizomes that no longer flower. The clump is overgrown and needs dividing. Plant only the young edge pieces and throw away the old centre.

Is an iris poisonous to dogs and cats?

Yes. The whole plant is poisonous, most of all the rhizome. After chewing, drooling, vomiting and diarrhoea appear. If you suspect it, contact a vet.

Recommended products

We currently do not have iris rhizomes on offer. The following suit planting and dividing:

• Bone meal 500 g – phosphorus and calcium without excess nitrogen, a handful in the hole under new rhizomes.
• Planting trowel – for the shallow holes with a mound in which a bearded iris is planted.
• FORESTINA DEKOR perlite 2–4 mm, 2.5 l – lightens the substrate for dwarf irises in a container, where drainage is the most important thing.

Sources

• Royal Horticultural Society: Iris
• Decree No. 395/1992 Coll., Annex II: list of specially protected plant species
• ASPCA Animal Poison Control: Iris
• Toxicological Information Centre, General University Hospital in Prague

Updated: 27 September 2026