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Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Divide (and conquer) Iris in August

David Trinklein
University of Missouri
(573) 882-9631

Published: August 3, 2017

full bed of light purple iris

Considered to be one of the more carefree perennials, bearded iris is not difficult to grow. However, when iris clumps become too large and overcrowded, fewer flowers are produced in the spring. This series of events signals it is time to rejuvenate the clumps by dividing the rhizomes into small sections. A rhizome is a thickened stem that grows horizontally at ground level or just below the surface of the soil. During July and August the growth of iris slows. It is during this period that the clump is most easily and successfully trimmed back, dug, divided and transplanted.

For the best display of flowers, iris that are growing in good conditions need to be divided every three to four years. A good sharp knife is an important tool for this procedure. Trowels, spades or dull knives may cause wounds that allow disease organisms to enter. If a disease known as bacterial soft rot is present in any of the rhizomes, extreme care must be taken to make sure that it is not accidently spread to healthy rhizomes during division.

Division of iris is the only way to propagate the plant to insure that new plants are genetically the same as the parent plant. Iris may be grown from seeds, but these vary widely and seldom give rise to flowers similar to the plant that bore them. Both color and form may be different.

Among the tall bearded iris, there is a group known as remontant or reblooming irises. These cultivars not only bloom in the spring, but repeat bloom in the fall. They often are more expensive than "normal" bearded iris and usually are available only from specialist iris propagators. However, to rebloom in the fall, excellent growth must be maintained; therefore, regular division of this group is necessary. When dividing plants into smaller section, blooms should not be expected during the fall immediately after the clumps were divided.

When planting iris rhizomes, they should be set so that the rhizome is horizontal and at or just below the surface of the soil. The tops of the rhizomes should be visible and the roots spread out and pointing downward. If sections are small, three rhizomes may be set close together so they radiate outward. When planting iris, separate plants by a distance of between 18 to 24 inches.

At the time of iris renovation or replanting, the fans of leaves may be cut back to two to three inches in height. This removal of leaf tissue is especially important if iris leaf spot, a common disease of iris, is present. Iris leaf spot seldom kills iris but it can disfigure leaves and weaken plants. Cleanliness is important. Gather up and pull off any dead or badly diseased leaves. Early in the season prior to flowering, application of a fungicide containing chlorothalonil or thiophanate-methyl as its active ingredient can help prevent or reduce incidence of iris leaf spot. Always read and follow label directions when using any pesticide.

Another potentially destructive disease that may appear in iris plants is the aforementioned bacterial soft rot. When plants are infected, the leaf base and rhizome may develop into a soft, rotten mass having a foul, objectionable odor. When this disease is detected, infected plants should be dug and all diseased parts removed from the garden and destroyed. Damaged rhizomes should be trimmed back to sound, healthy tissue and exposed to the sun for at least two days before replanting. Do not replant iris back into the spot where the diseased plant was located.

Iris borer is an insect that feeds on the rhizomes of iris and may help spread soft rot. Cleaning up debris and dead iris leaves in late fall and again in very early spring the following year is a good way to reduce the number of the eggs of iris borer. Additionally, the use on an insecticide labeled for iris borer control (e.g. acephate or spinosad) in spring near bloom time can help prevent borer damage on leaves and rhizomes.

If iris plants have not flowered well, look for a new location when renovating. Iris requires a sunny location and well-drained soil. In wet, poorly-drained soil or shady exposures, or where iris must compete with trees or shrubs, plants tend to grow poorly which makes them more subject to diseases and other problems. High soil fertility is not required, but a complete fertilizer high in phosphorus and potassium but lower in nitrogen can be beneficial.

Additional information about iris care can be found on the American Iris Society website.

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REVISED: February 21, 2017