Linda Geist
  • Bearded irisPhoto by Andrey Korzun via Wikimedia Commons. Shared under a Creative Commons license (BY-SA 3.0).
    Bearded irisPhoto by Andrey Korzun via Wikimedia Commons. Shared under a Creative Commons license (BY-SA 3.0).
Your Show-Me Garden: MU Extension brings you gardening tips from experts around the state.

COLUMBIA, Mo. – Most of the year, irises grow easily without a care in the world. Rarely do they need attention to thrive.

But gardeners should give the carefree beauty their undivided attention in August. By dividing and replanting clumps that have grown too large, gardeners can increase spring displays of the bearded beauty, said University of Missouri Extension horticulturist David Trinklein.

In Greek mythology, the goddess Iris was associated with the rainbow. Few flowers come in as many colors or bring such joy on a spring day.

But when iris clumps become large and overcrowded, they bloom less.

Rejuvenate clumps by dividing rhizomes into small sections. A rhizome is a thickened stem that grows horizontally at ground level or just below the soil’s surface, said Trinklein. They grow slowly during July and August. Dig, divide and transplant during these months, Trinklein added.

Divide iris every three to four years. Trinklein prefers a sharp knife to trowels, spades or dull knives that may open wounds that allow disease to enter. Disease-infected rhizomes should be separated from healthy roots to prevent spreading the disease.

“Division of iris is the only way to propagate the plant to ensure that new plants are genetically the same as the parent plant,” said Trinklein. Iris can be produced from seeds, but color and form may differ and disappoint.

Tall bearded irises include a class known as reblooming irises. These cultivars bloom in both the spring and fall. They need regular division to rebloom, Trinklein said. Don’t expect blooms in the fall right after you divide clumps in August.

When planting, place rhizomes horizontally at or just below the soil’s surface. Rhizome tops should be visible. Spread out roots and point them downward. Three rhizomes may be set close together so they radiate outward if they are small. Space rhizomes 18 to 24 inches apart.

Cut leaves back to 2-3 inches when replanting. This is especially important if iris leaf spot, a common disease that weakens and disfigures plants, is present.

Keep your iris bed clean, Trinklein said. Gather up and pull off any dead or diseased leaves. Before flowering, apply a fungicide containing chlorothalonil or thiophanate-methyl as its active ingredient to prevent or reduce iris leaf spot. Follow label directions when using any pesticide.

Bacterial soft rot is another common iris disease. The leaf base and rhizome of infected plants develop into a soft, rotten mass with a foul odor. Dig and destroy infected plants and remove them from the garden. Trim damaged rhizomes back to sound, healthy tissue and expose 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. Clean up debris and dead leaves in late fall and very early spring to reduce the number of iris borer eggs. Near bloom time, use an insecticide labeled for iris borer control, such as acephate or spinosad, to prevent borer damage on leaves and rhizomes.

Look for a new location for irises that do not flower well. Irises need sun and well-drained soil. They do not grow well in shady areas. This makes them subject to diseases and other problems. High soil fertility is not required, but irises benefit from a complete fertilizer high in phosphorus and potassium and lower in nitrogen.

Find more about iris care on the American Iris Society website at

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