Iris rhizomes can grow quite quickly and outgrow their baskets. They may push against the edge of the basket or can even jump out of it. If they get too crowded or pot bound they may get starved of nutrients and flower less reliably.
Propagating by division is straightforward and a good way of refreshing and reinvigorating your irises and creating additional plants for you to use in your pond or donate to fellow pond owners. The best time to divide irises is just after they have finished flowering in mid-summer so they have time to recover from the shock, set down some new roots and establish some top growth before winter.
To divide, dig the plant up or remove it from its basket and wash all the soil off its roots. If the clump is very large it can initially be split into two smaller pieces using two garden forks, back-to-back to force it part. Then using a sharp knife divide these clumps into sections with one, two or three fans, making sure each bit of rhizome retains some growing roots and is at least 8-10cm (3-4“) long.
Wash the rhizomes and check for damaged or dead roots which should be removed and trim the remaining roots back to around 10cm (4”) in length. Trim the foliage back to about 15cm (6”) which will allow new foliage to grow more quickly.