Answer: Iris can refuse to bloom if they're overcrowded or if they're not receiving full sunshine. I dig and divide iris every 4-5 years, and amend the soil with some organic matter before replanting them. Iris are normally dug and divided after bloom (late August). Since yours are not blooming, you can dig and divide them in the early spring after the soil dries out enough for you to work it.
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.
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.
Your iris should bloom a year after being divided and replanted. Remember to remove the spent flower and stem to encourage flowering the following year.
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