So where did summer go? Here in Iowa it does feel like fall even though we have another month before the fall starts. Now time to think about working up the garden for winter and the next spring season. What perennials to cut back in the fall?
While it is tempting to cut back the whole flower garden in the fall, even in colder climates, it can be nice to leave some perennials standing throughout winter months. The seeds of echinacea and rudbeckia will attract and feed the birds and sedum will hold onto snow like frosting. There are also plants that like the protection their foliage provides for their crowns like asclepias (butterfly weed), chrysanthemums, and heuchera (coral bells), which all fare best if cleaned up in the spring. But, there are other plants that will need to be pruned in the fall.
No one can pinpoint when the frost and snow will come. Many gardens survive just fine with no attention at all in the fall. This listing can serve as a guideline, but all gardens are unique. What works for one, may not work for another. It never hurts to take some time and put your garden to bed in the fall.
Some perennials do not handle rough weather well. They will not remain attractive after frost, and they have recurrent problems with pests and diseases, which will overwinter in their fallen foliage and surface in the spring. These perennial flowers are best cut down in the fall. If they are diseased, throw the foliage away, do not even compost it. There will always be exceptions and time will play a factor.
The tall foliage of bearded iris begins flopping early in the season. By fall, it becomes cover for iris borers and fungal diseases. Cut back after a killing frost and dispose of the foliage, rather than composting it. It grows best in U.S. Department of Agriculture zones 3 through 10.
taken from taken from https://www.thespruce.com/perennial-plants-to-cut-back-in-the-fall
Also the question about when to divide Iris so here is that information.
Dividing and Moving Iris – How To Transplant Iris
Transplanting iris is a normal part of iris care. When well cared for, iris plants  will need ro be divided on a regular basis. Many gardeners wonder when is the best time to transplant iris and how should one go about moving iris from one place to another. Keep reading to learn more about how to transplant iris.
Signs You Need to Transplant Iris
There are a few signs that you should consider dividing iris plants.
The first sign that your iris need divided will be decreased blooming. Overcrowded iris rhizomes will produce fewer flowers than uncrowded iris rhizomes. If you have noticed that your iris are blooming less than they have, you may need to transplant the iris in your garden.
The next sign that you should consider transplanting your iris is if the rhizomes start heaving out of the ground. Overcrowded iris rhizomes will start to push on each other, which results in the entire root system of your iris plants literally pushing themselves out of the ground. The iris roots may look like a mass of snakes or a pile of spaghetti when they need to be divided. They may even stop putting up foliage and the plants may only grow foliage on the outside edges of the clump.
When to Transplant Iris
The best time when to transplant iris is in the summer, after the iris have finished blooming, up until fall.
Steps for Dividing Iris Plants
To divide your iris, start by lifting the clump of iris plants out of the ground with a spade or fork. If possible, lift the whole mass out whole, but if you are unable to do this, carefully break the clump into smaller parts and lift these out.
Next, brush of as much dirt as possible from the iris rhizomes. This will make it easier to see when you are breaking the clumps apart.
The next step in dividing iris plants is to divide the iris rhizomes. Each iris rhizome should be divided into pieces that are 3 to 4 inches long and have at least one fan of leaves on the rhizome. Do not remove the roots from the rhizomes.
As you get closer to the center of the clump, you may find large sections of rhizomes that have no leaf fans. These can be discarded.
Check all of the divided iris rhizomes for iris borers and disease. The iris rhizomes should be firm and not soft. If the rhizome feels soft, throw it away.
Steps for Transplanting Iris
Once the iris rhizomes have been divided, you can replant them. First, trim all of the iris leaf fans back to about 6 to 9 inches tall. This will allow the plant to re-establish its roots without having to support a large amount of foliage at the same time.
Next, plant the iris rhizomes in the selected location. This location should receive a good deal of sunlight and should be well draining. Dig a hole where the rhizome will settle into the ground just below the ground level. If planting several iris near each other, point the rhizomes away from each other and space them 18 inches apart.
Spread the roots out around the rhizome and then cover the roots and the rhizome with dirt. Water the newly transplanted iris plants well.
taken from https://www.gardeningknowhow.com/ornamental/bulbs/iris/dividing-transplanting-iris.
till next time this is Becky Litterer, Becky's Greenhouse Dougherty Iowa firstname.lastname@example.org