Of the more than 200 species of iris, the most common garden plants are the bearded iris in intermediate (40 to 70 centimetres) and tall (70 centi-metres to one metre) forms, and the thick clumps of Siberian iris, with smaller flowers and sword-shaped ornamental foliage all season. Both kinds of irises need to be divided periodically to maintain good flower production.
Clumps of tall bearded iris carry the greatest number of flowers on the youngest rhizomes. As the rhizomes age, the nutrients they store are depleted. The plants should be divided every three to five years, when the oldest sections stop sending up stalks and flower production is limited to the outer sections of the clump. Lift the clump, discard older rhizomes and replant the younger ones to obtain vigorous stalks and flowers next season.
Siberian irises will cease flowering when they need to be divided, every three or four years. Clumps also grow from the centre outward. The underground structure is a strong fibrous root system.
Early spring is the best time to divide Siberian irises, just as the new foliage is pushing up through the soil.
Lever the clump carefully with a garden spade, sliding the spade under the roots and lifting upward. With your hands, try to separate the clump into sections along natural lines of division.
If the mass of rhizomes and roots is too strong to pull apart, use two garden forks inserted back to back in the centre and lever it apart. It is also possible to cut it in half with a blunt-nosed spade. Separate the clump into two or more sections; replant the sections 45 to 60 centimetres apart at the same level they were growing, then water them into their new hole.