You know it's time to split or thin your Irises when they start to spread into the grass (lol). As you can see by the picture below that is exactly what happened with my Iris patch.
|A clump of Irises that I dug up - they are now ready for splitting
After the Irises have finished flowering, simply remove your Irises with a shovel (you will need to dig down 6 to 8 inches to make sure you get the root base). Then look at each stalk and follow it down to the base, there should be a twisted bulb-like root for each stalk. Now separate each stalk - you might have to snap or cut the thick Iris roots apart. (see pictures below)
|I was able to get 5 plants out of 1 Iris clump
|As you can see Irises have a thick twisted root.