While a lush border of bearded irises can fill in nearly any space quickly, one drawback of this is that an iris can crowd itself, and the plant will begin to flower less before it dies. If iris plants are struggling for flowers and you can no longer see soil between the leaves, it may be time to separate the bearded iris. After separating and replanting, more irises will be available to use anywhere.
Dig up the bearded irises with a hand trowel in mid- to late summer by starting 4 inches from the base and working up and under the large rhizome that makes up the clump.
Pull the iris clump out of the flower bed. Shake off any large clods of dirt from around the roots and rhizome to expose the entire base of the plant.
Cut apart the large rhizome into smaller clumps with a clean, vertical slice of a knife. Ideally, each new clump should have at least three leaves extending from the rhizome.
Clip the leaves from each clump to 4 or 5 inches tall with a clean, horizontal slice of a knife.
Dig a hole for each new clump three times as large as the clump to be planted. Set the clump in the hole and back fill it with soil around the plant, keeping the iris at the same level in the soil as it was planted before.
Water the iris plantings to keep the soil moist for two to three weeks as they adjust to their new area. Water as needed thereafter if the area experiences a lack in rainfall.
Dust a light application of fertilizer on the surface around your irises in late fall to early winter to feed the irises for the following year. Apply the same fertilizer again in the spring. Follow the label instructions for details.