Bearded irises are best divided in late summer, about six weeks after blossoming ceases and after new rhizomes, which grow outward from the centre of the clump, are well established. It's necessary to lift the entire clump; the rhizomes are shallowly anchored by short fibrous roots that are easily lifted.
First remove the upper half of all leaves, using secateurs to cut through the thick foliage. This reduces moisture stress. Then carefully ease a garden fork under the clump, working from all sides until you can gently lever it out of the ground.
Set the clump down and use a garden hose to spray soil debris off all the rhizomes and their roots. Examine the rhizomes carefully for holes and signs of decay. This is the best opportunity to discover the destructive work of the iris borer, a fat larval worm that eats its way into rhizomes and hollows them out. The iris borer carries bacterial soft rot into tubers, triggering an infection with a strong unpleasant odour. Discard any that are damaged, hollow or mushy.
To prevent the spread of bacterial pathogens, prepare a 10 per cent bleach solution (nine parts water to one part liquid laundry bleach). Use secateurs or a sharp knife to split up the clump, dipping the blades in the bleach solution after each cut, and separating the younger sections of rhizomes from the older inter-ior sections. Discard the oldest sections and select firm young ones up to 15 centimetres long, with roots and a fan of leaves to be the new transplants. The smallest sections of young rhizomes (less than eight centimetres long) can also be transplanted, but will require two years of growth to reach flowering size.
Replant the rhizomes 30 centimetres apart, allowing sufficient room for each to develop new sections in the following three to five seasons of growth. All the transplanted rhizomes should be positioned with the slight curve in their foliage fans facing in the same direction.
Dig a hole for each transplant about 12 centimetres deep, with a mound of soil in the centre. Compost, composted manure and peat moss can be added to the soil if it needs renewing. Set the rhizome on the mound and allow its roots to hang down the sides. Fill in with soil, covering the roots entirely and halfway up the sides of the rhizome, leaving its top surface exposed. Covering over the top with more than two centimetres of soil will cause it to rot. Water the transplants in and keep the soil moist while they establish new roots.