By Apodemus
Somerset,
United Kingdom 
I have a clump of very crowded yellow flag iris (rhizomatous ones). Only one has flowered - at the edge of the clump, so I'm guessing that they're overcrowded. Please can anyone tell me when to split them and how far apart to replant the rhizomes? I assume that those taken from the edges of the clumps will have the best chance - is that correct. I know that they will need to be reset resting on the surface of the soil.
Thanks for any advice.
9 Jun, 2012
Answers
After a rhizome has flowered it will put energy into producing new rhizomes to continue the flowering thus not flowering itself this is why digging up old clumps gives you the opportunity to discard any old, or maybe rotting rhizomes and replant the new young ones for flower. Split them after flowering but try to avoid it if hot and dry for obvious reasons.
9 Jun, 2012
Thank you both for your replies.
It looks like I'll have to put the job in the diary for either autumn or next early spring, hoping for a wet spell afterwards to settle them in.
10 Jun, 2012
Previous question
I usually split them in the spring when they are dormant. I have them in my pond so I have them about 6 plants in a 1x1ft basket.
in the ground then the rhizome should be exposed on the surface. they do better in a moisture retentative soil.
9 Jun, 2012