If your bearded iris are overcrowded like this or failed to bloom in spring it is time to dig, divide and replant for better bloom next season.
Start the process about 6 to 8 weeks after the iris bloomed or in some cases should have flowered. Carefully lift the clump out of the ground with a shovel or garden fork.
Now separate the thick fleshy rhizomes. Cut away and discard old leafless, shriveled, borer-infested or rotted rhizomes.
Next cut the leaves down to 4 to 6 inches to reduce moisture loss.
Amend the soil with compost or other organic matter before planting your divisions.
Set your iris in place, spread out roots and cover with soil. The rhizomes should be even with the soil surface in heavy soils and just slightly lower in sandy soils.
Water to help remove air pockets and get your iris off to a growing start.
A bit more information: Discovered iris borer while transplanting? You can help reduce future problems with a bit of fall cleanup. The adult borer is a day flying moth that lays its eggs in the leaf debris. By removing all the dead iris leaves in fall, you will break the life cycle and usually eliminate the problem.