Irises are easy to grow, long-lived, and relatively carefree perennials, making them some of the most popular flowers in gardens. They can also be found in a variety of colors, ranging from pink, purple, yellow, peach, green, white, tan, bronze, to almost black, and bi-color.
The American Iris Society divides irises into three main classifications: bearded, aril, and beardless irises. The most common type of iris grown are bearded irises.
When and why should I divide bearded iris?
If you grow bearded irises, you may have noticed that over the years, your clumps of iris begin flowering less and become crowded. Despite being relatively carefree, bearded iris, and others that grow from rhizomes like Siberian and Japanese iris, should be divided every three to five years when flowering begins to decline, or the clumps become crowded. This will help keep the irises blooming and help prevent issues with iris borer and soft rot.
While they can be divided any time after they are done blooming, July and August are the best time to divide. This will give the new rhizomes enough time to fully develop. It will also provide the plants with adequate time to produce new roots and establish themselves before winter.
How to divide bearded iris
Remove and discard old rhizomes and only save newer ones that grow off the older parts. If you find rhizomes that have iris borer damage, or are smelly or mushy (soft rot), discard these, too.
When dividing iris, lift the entire clump with a spade or digging fork. Then, remove as much soil as possible from the roots and rhizomes.
You’ll also want to cut back the leaves to about one-third of their height. This will make the plants easier to work with and help reduce water loss while plants are establishing. This can be done before or after you lift your clumps.
Rhizomes can be broken apart by hand or cut apart with a clean, sharp knife. If a knife or pruners are used, make sure to sanitize them with a ten percent bleach or disinfecting wipes. Your new transplants (rhizomes) should be firm and light-colored, at least three inches long (about as thick as your thumb), have healthy roots, and have a fan of at least four to five leaves.
Planting bearded iris
Plant your rhizomes relatively soon after dividing. (It’s okay to wait a few days). Bearded irises grow best in full sun, at least 6-8 hours of sunlight a day. They also prefer well-drained soils. Wet soils may lead to the rhizomes rotting.
Once you’ve selected your location, you can add organic matter or other amendments if needed. To determine if you need soil amendments it’s a good idea to conduct a soil test.
When planting iris, dig a hole about four inches deep with a small mound in the middle of the hole. Place the rhizome on top of the mound and let the roots fall down the sides of the mound. After spreading the roots out, cover them with soil, so the rhizome is just slightly exposed. Do not plant the rhizome too deep because this can lead to root rot.
Generally, bearded irises are planted 12-18 inches apart, with the fan of leaves facing the same direction. Plant the different types in groups of three to seven rhizomes.
Your transplanted irises may only bloom sparingly their first year, but you should end up with a beautiful display in subsequent years.
Ken Johnson is a Horticulture Educator serving Calhoun, Cass, Greene, Morgan and Scott counties. He is one of the authors of the “Good Growing blog” at go.illinois.edu/goodgrowing. The Green Thumb page is coordinated by DeKalb County Master Gardener Janice Weber.
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