From the Ground Up: Now's the time to divide irises

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By Lisa Slattery, Iowa State University Extension

Midsummer gardening chores center around weeding, deadheading and fertilizing those tomatoes that are coming soon. It’s not the ideal time to plant or transplant due to the hot temperatures of midsummer. However, there is an exception to this and that is iris. The ideal time to divide and replant iris is July and August.

To divide irises, carefully dig up iris clumps with a spade. Cut the foliage back to one-third of the original height. Cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome and several large roots coming off the rhizome. Check closely for any signs of disease or insect damaged rhizomes and discard those.

To obtain a good flower display, plant three or more rhizomes of one variety in a group. Space the rhizomes about 12 inches to 24 inches apart. Point each fan of leaves away from the other irises in the group. Newly planted or transplanted bearded irises are susceptible to injury their first winter from repeat freezing and thawing of the soil so make sure you cover plants with several inches of mulch, pine needles or straw in late fall. Remove the mulch in early spring. The transplanted irises might bloom sparsely the first spring after transplant but will be in full bloom in their second and third years.

When dividing irises you might see signs of iris borer. The iris borer is the most prevalent pest of iris. The cycle starts when the adult moth lays eggs on iris foliage and nearby plants in late summer and early fall. The eggs hatch in the spring and the small caterpillars bore into the iris foliage and feed. Signs of feeding are brown streaks on the leaves as the caterpillars grow and tunnel their way to the rhizome.

If brown streaks are detected, pinching the terminal end of the brown streak should destroy the caterpillar or cut off the leaf below the terminal end of the brown streak. If the caterpillar gets to the rhizome, it feeds inside the rhizome and destroys it. At this point, soft rot sets in and creates a soft, foul smelling mess. Discard any rotting rhizomes.

To prevent iris borer, remove dead foliage in late fall or early spring, which eliminates the eggs. Also carefully examine iris in late summer or early fall for soft rhizomes by gently probing rhizomes with a stick. Remove any soft rhizomes before the larvae move into the soil and start the entire process over again.

Iris need at least six hours of sunlight to ensure good blooms. Bearded iris perform best in fertile, well-drained soils in full sun. While they tolerate light shade, they won’t bloom as well. Iris benefit from regular division every three to five years. If they become overcrowded, flower production decreases.

For gardening questions, call the Linn County Extension Master Gardener Hortline at (319) 447-0647.

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