As far as the tall bearded iris, it carries the largest number of flowers on the youngest rhizomes. As those rhizomes age, the nutrients stored are depleted, which is a good reason for the plants to be divided every three to five years. After that, the oldest sections stop sending up stalks and flower production is limited to the outer sections of the clump.



By lifting the clump and discarding the older rhizomes, then replanting the younger ones, the result will be vigorous stalks and flowers for the following season.

Gardeners should divide their bearded irises in late summer, about six weeks after the blossoming stops. The dividing can be done after the new rhizomes are well established.

Lifting the entire clump is necessary and the fact that the rhizomes are shallowly anchored by short fibrous roots makes this easier.

Removing the upper half of all leaves is a first step, the intention being to remove moisture stress.

Following that, easing a garden fork under the clump and working from all sides will gently lever it out of the ground.

Once the clump is out, use a garden hose to spray soil debris from the rhizomes and their roots, then examine the rhizomes for holes and signs of decay. It's at this point where you are looking for signs of the iris borer, a fat larval worm that eats into rhizomes, hollowing them out. The borer carries bacterial soft rot into tubers and triggers an infection with a strong odor. The best thing to do is to discard any material that is damaged, hollow or mushy.

In order to prevent the spread of disease, prepare a 10 percent solution of bleach (nine parts water, 1 part liquid laundry bleach). Splitting up the clump, dip the blades in the bleach solution after each cut, separating the younger sections from the older, interior sections, discarding the oldest ones in favor of firm, young ones.

Replant the rhizomes, allowing sufficient room for each to develop new sections in the next three to five seasons of growth.

All should be transplanted with a slight curve in their foliage fans, facing in the same direction.

Each transplant should be in a hole with a mound of soil in the center, with compost, composted manure and peat moss added if the soil needs renewing.

For Siberian Irises, which cease flowering every three to four years, spring is the best time to divide.

Clumps can be separated by hand, once the clump is leveraged with a garden spade.

If the mass of rhizomes is too strong to pull apart, two garden forks inserted back to back in the center can help leverage it apart. It may also be possible4 to cut the clump in half with a blunt-nosed spade.

The clump can be separated into two or more sections, with the sections replanted at the same level as they were growing, watering them into their new hole.