@snowbear this popped up in my feed today. Thought it might be helpful.
IRIS 101
Many people know how to prune roses, or cut back their black eyed Susan's, dead head petunias, but I'm amazed at the missing knowledge for iris care, so I thought Id share this information hoping that it will help someone have amazing blooms year after year!
IRIS 101. Once a rhizome blooms it will never bloom again, it will then create daughters, they will bloom next year if growth conditions are good, they will then become mothers and so on. This is why is very important to dig and prune out iris every 3-4 yrs. Also why iris will appear to stop growing and become thin with less blooms, they're over crowded. Sometimes you'll notice iris will often create a circle with an empty area, this is the mothers offspring.
Dig up your clumps of iris, starting about 6" away from the plant if possible, trim back the foilage to 6-8" in an upside down V shape to prevent breakage, and put it on a concrete area if available. Take the garden hose and go to town! You will not hurt them, get them nice and clean so you can see what is what.
I prefer to use bypass prunes, but you can use strong scissors or a sharp knife. You want to trim off your daughters, ideally leaving about 3" of rhizome. You can toss the mothers or "old toes" or replant them, they will usually grow more daughters but at a slower pace as they get older. Meaning each mother will take at least 2-3 years to see a bloom. If the daughters are small, you can leave them on a young mother to help them grow bigger.
When you replant use the long roots as an anchor, I work up the dirt, and put the root (not rhizome) about 3" deep, and the rhizome ( the ugly toe part) at the surface barely covered by dirt, when the dirt settles and you water, the rhizome will show, that's ok.
It's best to divide in the fall after the rhizomes have had all summer to grow, if you need to divide sooner, it may slow your next bloom but won't hurt the plant over all.
If you bury too deep they will suffocate and take years to bloom. Do not mulch, they will suffocate and get fungus from the held in moisture.
If you're worried about fungus or parasites from cutting your iris, you can dip them in neem oil before replanting.
Photo courtesy of Daves Garden