Fall is a great time to thin out and divided your German iris bed. With the recent rains in the Omaha Metro area this makes it easier to work the soil.
German iris should be divided and thinned every three to four years. If you have a larger bed with multiple cultivars you may need to do this chore every third year. This will help keep all the plants happy and healthy. It will also give you a chance to take inventory of what you have and what you may have lost over the years.
Cutting back the fans of the plant will make it easier to dig and divide. This also is a great way to help reduce overwintering locations for the iris borer. When cutting back the plant leave about six inches of the fans. If your plants have any signs or symptoms of disease make sure that you clean the pruners before moving on to the next plant. When disinfecting cutting tools it is best not to use bleach on sharp tools. Bleach is corrosive and can damage the blade. You can use 70% rubbing alcohol to clean your tools. You do not need to dilute it.
Once the plants have been cut back get out the drain-tile spade and start lifting the clumps out of the ground. You will see natural breaks within the clump of the iris rhizomes where you can break off the dead and keep the fresh healthy rhizomes. The old rhizomes should be discarded because they will no longer produce. Break off the large healthy side shoots and set those aside. If you know the names of the cultivars use a sharpie to write the names on the plant. This will help you when you go to replant or share with your friends and neighbors.
If you happen to find rhizomes that have been damaged from bacterial soft rot or the iris borer it is best to discard those rhizomes if they have been severely damaged. If you want to try and save the damaged rhizomes you will need to use a spoon and scoop out the rotten plant tissue and let it air dry for a few days before planting. Some literature suggests following up with dusting the rhizome with some sulfur.
Once the rhizomes have been divided then all you need to do is plant. Iris do best in full sun locations. When planting you’ll want to keep the rhizome at the soil surface. Burying the rhizome could cause it to rot or fail to thrive. If you are ambitious try to place the taller blooming plants in the back of your bed and work forward with the bloom height. Edge the bed the the dwarf or miniature iris.
You can learn more about the culture of iris with our NebGuide: https://extensionpublications.unl.edu/assets/pdf/g1741.pdf