Both clump forming irises and those with

Rhizomes are creeping swollen root-like structures that are actually adapted stems. Roots, stems with leaves and flowers are produced along its length. See plants such as Anemone nemorosa, bamboo, canna, border iris.

These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes.

bulbs

• Rhizomatous irises (having fleshy stems at soil level) are divided just after flowering
• Clump forming irises are divided in early autumn or early spring

Bearded rhizomatous irises

Bearded irises (sometimes sold as Iris germanica cultivars) have large fleshy stems (rhizomes) at soil level and flowers with soft hairs (the ‘beard’) on their lower petals (falls).

• Lift and divide rhizomatous bearded irises every three to five years
• This is ideally carried out six weeks after flowering, to give sufficient time for the plants to produce new growth for the following season before they enter winter dormancy

Division:

• Cut away each fan of leaves from the clump, using a sharp knife. Each fan should have a portion of young rhizome (up to 15cm/6in long for tall bearded irises, smaller for miniature tall bearded irises)
• Select the largest fans with the healthiest rhizomes
• Discard smaller fans and old, withered looking rhizomes
• Shorten the leaves to about 15cm (6in) above the rhizome and trim the roots to shorten them

Replanting:

• Dig a hole, large enough for the rhizome and roots, mounding the soil slightly if this makes placing the rhizomes easier, but otherwise working the soil back between the roots
• The rhizome should be placed at soil surface on heavy soils, but a little below the surface on light sandy soils, as they will work their way back to the surface
• Replant the divisions in groups, with 30cm (12in) between larger plants and 15cm (6in) between dwarf plants

Siberian irises

Siberian irises are clump-forming irises with beardless flowers. Large clumps can be divided to rejuvenate them if flowering has become reduced at the centre of the clump.

• Divide large clumps shortly after flowering, using two forks inserted back to back in the centre of the lifted clump
• Remove any old rhizomes and roots; avoid breaking the clump into small sections
• Replant the younger outer sections to the same depth as they were planted before

Unguicularis irises

Iris unguicularis are low-growing clump-forming irises with beardless flowers that flower in late winter and early spring. Large clumps can be divided to rejuvenate them if flowering has become reduced at the centre of the clump.

• Divide clumps in autumn, or wait until after flowering has finished in spring
• Use your hands to pull apart the divisions, or, if this is insufficient, try two hand forks inserted back-to-back in the centre of the lifted clump
• Split the clump into largish sections to avoid disturbing the roots excessively
• Replant the divisions to the same depth as they were planted before

Pacific Coast irises

Pacific coast irises are low-growing clump forming irises with beardless flowers and grass-like leaves. Large clumps can be divided to rejuvenate them if flowering has become reduced at the centre of the clump.

• As these irises dislike root disturbance, lift only part of the clump in autumn (the best time) or in late spring once new roots have appeared
• Take divisions from the healthy and vigorous areas at the edge of the clump, replanting them to the same depth as they were planted previously

• Those irises divided and re-planted in summer are at risk of drought during dry spells
• Watering the area and allowing it to drain overnight before planting, then watering every five days during dry periods after planting, can help in these conditions