Yard and Garden: Growing Bearded Irises

May 23, 2018, 2:12 pm | Richard Jauron, Willy Klein

yellow iris bloom.AMES, 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 Iowa State University Extension and Outreach horticulturists to maintain a colorful, attractive iris planting.

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 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. 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. To prevent overcrowding, bearded irises should be divided every three to five years in July or August.  

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 five to six week period, each time removing any rhizomes visible in the soil. Replant the irises in their original location after thorough eradication efforts.  
Herbicide options are rather limited. Products containing fluazifop-p (such as Ortho Grass B Gon Garden Grass Killer) kills weedy grasses without harming iris and other ornamentals. The non-selective herbicide glyphosate (Roundup and other products) is another possibility. Glyphosate must be selectively applied to the grass foliage (possibly using a foam-type paintbrush) without getting it on the irises.  


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