Answer: It is a good idea to dig up irises every 4-5 years so you can seperate out the weak and diseased ones. Then at the same time to can amend the soil a bit. They'll love you for it!
After blooming lift the entire clump with a spade and separate it into small clusters of rhizomes (this is what the roots are called). Choose only the largest and healthiest-looking ones for transplanting. Put them in a bucket while you work the soil for the new planting. You might also add some compost or rotted manure as they enjoy rich soil. Good drainage will help you avoid borers.
Place one or several of the divisions into the prepared soil. If you plant them in a circle be sure to have the leaves facing outward. If you don't do that, they will soon be crowded again. Make the holes deep and wide enough to take the roots without crowding. Have the rhizomes set so they are just below the surface of the soil.
This should do the trick. Remember to dead-head them (remove the flowers after they bloom).
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