Answer: If you have an overgrown and crowded clump, it is time to divide it; usually this is between three and five years after the initial planting. Division or splitting is usually done in the first half of August. Water them the day before you dig them if the weather has been dry -- this softens the ground and hydrates the plants. Work on one clump at a time so you can keep the different colors or varieties separate.
Trim the foliage down to about six inches, then use a garden fork to gently ease them up out of the ground. Try not to damage the rhizomes when you dig them up. Shake off or rinse off any clinging soil. Discard the oldest center parts and any with signs of borer damage, shriveling or otherwise imperfect. Often the rhizomes break apart almost on their own, or you can cut them with a sharp knife. You will usually have more than enough pieces to replant, so be selective. You want to keep the parts with the most vigorous new growth at the end of the rhizome close to the foliage -- just a few inches. Allow to air dry in the sun for a day or two (they can be kept for a week or two in a single layer in a dry and shady location if need be), then replant. (Mark them with tied on tags or by writing the name/color on the leaves with marker or pen.)
When you replant, select a site in full sun with rich, well drained soil. Set the rhizomes so they are only half covered with soil. Water to settle the soil. Most gardeners will set them in a group of three with the growing tips pointed either outward, or all in one direction. Allow about 18 inches of space for each rhizome for the large, tall bearded iris.
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