Rhizomatous irises
Lift and divide after three to five years. Dividing and replanting
is ideally carried out six weeks after flowering, to give
sufficient time to produce new growth for the following season.
Preparation:
Cut away each fan, using a sharp knife, with
a portion of young rhizome (up to 15cm/6in long in tall bearded
or smaller sections in miniature tall bearded). Select the
largest fans with the healthiest rhizomes. Discard the smaller
fans along with old rhizomes. Cut the leaves back horizontally
about 15cm above the rhizome, trimming old roots.
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, a
little below on light sandy soil, as they will work to the
surface. Replant in groups 30cm (12in) apart, 15cm (6in) for
dwarf kinds.
Siberian irises
Divide large clumps shortly after flowering, using two forks
inserted back to back in the centre of the clump. Remove old
rhizomes and roots, avoiding breaking the clump into small
sections. Replant the younger outer sections.
Iris unguicularis
Divide clumps in autumn or wait until after flowering has
finished in spring. Split the clump into largish sections
to avoid disturbing the roots excessively.
Pacific Coast irises
As they dislike root disturbance, lift only part of the clump in either autumn or spring when new roots have appeared.
Bulbous irises
These produce bulbils which can be removed when dormant. These will take two to three years to reach flowering size.