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Columns

Learning to Grow: It’s time to divide irises and peonies

"Although many perennials do best if divided in spring, the iris and peony (pictured) should be divided in August and September," writes columnist and master gardener Darlie Simerson.
"Although many perennials do best if divided in spring, the iris and peony (pictured) should be divided in August and September," writes columnist and master gardener Darlie Simerson.

Other than making sure plants have sufficient water and the weeds are not taking over, some believe there is not much gardening to do in late summer. But there are several reasons to continue enjoying time in the garden. Here’s just one: Some perennials want to be divided this time of year. Although many perennials do best if divided in spring, the iris and peony should be divided in August and September.

How do you know if a perennial needs to be divided? Reasons to divide include plants have begun to produce smaller flowers, growth in the center of the plants has died, plants are less vigorous or if plants have outgrown the area that you intended. This can vary from three to 10 years, depending on the perennial.

The process of division is as simple as digging around the plant and lifting out the clump of roots or rhizomes. Divide by using a knife or shovel to cut the clump down to the size preferred for transplant. Some perennials have dense root systems but don’t be afraid to cut through and remove any dead or damaged areas. Replant divisions as soon as possible to prevent roots from drying out. Dividing during dry, hot weather lessens the occurrence of bacterial soft rot.

The types of iris that can be divided in fall include all rhizomatous species, such as the common bearded, beardless Siberian and Japanese iris. It does not include Dutch and reticulate irises that form from bulbs.

The iris commonly needs dividing every three to five years. Before dividing, cut back leaves to one third of the plant's height and replant young rhizomes that grow off older stems, leaving the leaves intact. Dig a hole 5 inches deep and sit the rhizome on a small mound letting the roots hang down. Cover with soil, allowing the rhizome to be slightly exposed so it does not rot.

Unlike irises, peonies do not require regular division to bloom. Often they are divided to spread their beautiful flowers to more areas in the garden. Each divided section must include three to five “eyes,” which serve as shoots for next season. Plant divisions at the same depth they were growing originally. Planted too deeply, peonies will not bloom.

Add perennial division to your to-do list in the late summer garden and your perennials will reward you with more color and vigor next spring.

Darlie Simerson​ is a University of Illinois Extension master gardener for Kane County. The “Learning to Grow” column runs weekly during warmer months of the year. Call the extension office at 630-584-6166 for more information. Feedback on this column can be sent to editorial@kcchronicle.com.

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