DIVIDING YOUR IRISES
Irises can be left undisturbed in your garden for many years but this will often be at the cost of reduced flowering.
When to Divide
A clump needs dividing when the rhizomes get overcrowded. An overcrowded clump will not flower well as each rhizome needs space.
The best time to divide is after flowering in spring up until autumn. Avoid the growing season or you will disrupt flowering. If you need to dig your irises to move them (such as moving house), it is possible to do this at any time if required, however summer is not ideal. They may not flower in their first spring but they will survive if reasonable care is taken after replanting. They will survive for quite a long time out of the ground if kept in cool dry place. Do not keep them moist as they will rot.
How to Divide
Lift the entire clump gently using a pitchfork. You will have a bundle of rhizomes of different sizes.
Rhizomes produce increases or offsets which start out as small buds on the side of the rhizome. Rhizomes that have flowered will never flower again, so these should be discarded. Beware of putting rhizomes in the compost as they are known for growing from tiny offsets that you may have missed.
In the photo below, there are 3 offsets developing with no leaves, one developed rhizome with a small offset or increase (with some leaves). The fully developed rhizome is an offset of the old rhizome (with the 3 small newer buds/potential increases).
In time, these buds will become flowering rhizomes, producing their own offsets.
Break up the clump by snapping off healthy rhizomes. Discard any old withered rhizomes, and any that have flower stems on them unless they have good sized offsets developing.
Trim the roots and leaves of the healthy rhizomes selected for replanting. Roots should be long enough to help the plant stay firmly in the ground but not so long that they are crumpled under the plant.
If rhizomes have started to dry out, you might like to give them a soak in some seaweed solution to stimulate root growth before planting. Soak for an hour or two or overnight if they have been out of the ground for very long.
Replant your healthy trimmed rhizomes in a sunny, well drained position in the garden. Some fertiliser placed in the hole prior to planting will ensure your rhizomes have ready access to the food they need to grow. A group of 3 rhizomes planted with "toes in" as pictured below is recommended. This will form a clump quickly.
In Australia, rhizomes need to be fully covered to protect them from the harsh sunlight. In other parts of the world, rhizomes are often exposed. Approximately one centimetre of soil covering the rhizome is sufficient. Rhizomes planted too deeply. as shown below, will not flower.