Hi Carolyn, I will cut the flower stem after they bloom just to the first leaf growth to neaten up the plant. Autumn is usually the best time to divide and it only needs done every 3-4 years.
They probably need to be divided or they could be damaged. I divided mine every three years and the ones that didn't bloom something had eaten holes through the bulbs. It also could be a compacted soil issue. When you divide I would use some Black Kow manure and till it into the soil before I put the Iris' back in the ground.
If you mean bearded iris, Dee is exactly right that late summer or very early autumn is the best time to divide them.
New to iris, can I pull off the dead flower parts? or do they nourish for next year? Thanks for suggestions
Evelyn, you can cut off the whole stalk when blooming is complete. It's the leaves that nourish the plant.
I have found that if you don't want to transplant, you can just make several straight cuts through the roots and it will revitalize them. Division without movement. But still do it in the fall.
Carolyn, I dead head mine but also cut the stem off. I like to cut into little pieces to make compost around them or other plants. Irises are one of the easiest plants to not kill. Relax and enjoy them.
It's best to not cut the leaves of flower bulbs. The leaves provide needed nourishment and growth. Wait until they start to wilt and turn brown before cutting. It they look untidy to you, you can tie the leaves together with a little twine.
I have always heard it's best to clear iris cuttings away from plants and not use them in mulch because they tend to absorb disease and fungus.