Tips for dividing and planting bearded iris

JACKSONVILLE, Ill. — Irises are some of the most popular flowers in gardens. They are easy to grow, long-lived and relatively carefree perennials. They also can be found in a variety of colors — pink, purple, yellow, peach, green, white, tan, bronze, almost black and bi-color. 

If you grow bearded irises, you may have noticed that your clumps of iris begin flowering less and become crowded over the years. Despite being relatively carefree, bearded irises should be divided every three to five years when flowering begins to decline or the clump becomes 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 before they are divided. It will also provide the plants with adequate time to develop new roots and establish themselves before the arrival of winter.  

When dividing iris, you want to lift the entire clump with a spade or digging fork. Once you have the clump lifted, 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.

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. When dividing, your new transplants (rhizomes) should be:

• Firm and light-colored
• At least 3 inches long and about as thick as your thumb
• Have healthy roots
• and have a fan of leaves (at least 4-5 leaves). 

Remove and discard the old rhizomes and only replant the younger, smaller ones that grow off the older stems. If you find rhizomes that have iris borer damage, are smelly or mushy (soft rot), discard these rhizomes too. 

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. Cover the roots with soil, so the rhizome is just slightly exposed. Do not plant the rhizome too deep because this can lead to issues with root rot. Generally, bearded irises are planted 12-18 inches apart, with the fan of leaves facing the same direction. If you have multiple cultivars, plant the different types in groups of three to seven rhizomes.