Yard and Garden: Growing Beautiful Bearded Iris


May 28, 2015, 9:41 am | Richard Jauron, Willy Klein

Iris FlowerAMES, Iowa -- It's a magical time when bearded irises unfurl their pencil-slim buds to reveal a kaleidoscope of color in spring. Bearded iris is among the most elegant, and easy to grow, flowers of spring. Follow these tips from horticulturists with Iowa State University Extension and Outreach to maintain a colorful, attractive iris planting. To have additional questions answered, contact Hortline at hortline@iastate.edu or call 515-294-3108.

How do I care for bearded irises after they are done blooming? 

As soon as bearded irises are done blooming, remove the spent flowers. Removal of the spent flowers improves the appearance of the plants and prevents seed pod formation. Cut off the flower stalk at its base when all the flowers on the stalk have faded. Bearded irises require little care during the summer months. In late fall or early spring, remove the dead iris foliage. This helps to control leaf spot and iris borer. 

My bearded irises are no longer blooming well. Why? 

The bearded irises may need to be divided. Bearded irises should be divided every three to five years as the plants quickly become overcrowded and don’t bloom well. July or August is the best time to dig, divide and transplant bearded irises.  

Lack of sunlight could be another possibility. Bearded irises bloom best in full sun. (Plants need at least six hours of direct sun per day for best flowering.) Plants in partial shade may not bloom well and should be transplanted to a sunny site. 

How do I divide bearded irises? 

Bearded irises grow from thick, underground stems called rhizomes. In July or August, carefully dig up the iris clumps with a spade. Cut back the leaves to one-third their original height. Wash the soil from the rhizomes and roots with a steady stream of water. Then cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome and several large roots. Discard all diseased or insect damaged rhizomes.  

Bearded irises perform best in fertile, well-drained soils and full sun. In clay soils, incorporate compost or sphagnum peat moss into the soil prior to planting. When planting bearded irises, dig a hole large enough to accommodate the rhizome and roots. Build a mound in the center of the hole. Place a rhizome on top of the mound and spread the roots in the surrounding trench. Then cover with soil. When planted, the rhizome should be just below the soil surface. Finally, water each plant thoroughly.  

To obtain a good flower display, plant at least three rhizomes of one cultivar in a group. Space the rhizomes 12 to 24 inches apart. 

How do I control grasses in my iris bed? 

Annual grasses, such as crabgrass and foxtail, can be controlled by hand pulling. Perennial grasses, such as quackgrass, are much more difficult to control as they spread via underground stems or rhizomes. (All of the rhizomes must be completely destroyed to control quackgrass.)  

When confronted with an iris bed infested with quackgrass, one option is to dig up the irises in July or August and replant them in a different (weed-free) location. Another option is to dig up the irises in July or August, set the irises in a shady location, and then dig up the grass and carefully remove the rhizomes and other plant parts. Dig up the area two or three times over a three to four-week period, each time removing any rhizomes visible in the soil. Replant the irises in their original location after thorough eradication efforts. Quackgrass cannot be controlled by hand pulling. Herbicides are not a practical control measure for home gardeners.  

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