Many years ago, I took the Salt Lake Master Gardner's Course and got my certificate. Prior to that time, I had a great interest in knowing how to grow things, but not a lot of know-how. It was a great class, taught by Larry Sagers who was the USU Extension Agent for the Salt Lake area, as well as the expert on the KSL Greenhouse program that was featured on Saturday morning radio.
Since that time, Larry has passed away, and I have grown numerous gardens. I've had the opportunity to teach gardening at community education programs and at other gatherings. I will never know as much about gardening as Larry did, but at least I know how to grow things respectably--something that I once didn't know.
One rule of thumb that we learned was that most things are best transplanted in the cooler parts of the year. The reason being is that heat caused plants to go into stress. Fall is generally an excellent time to transplant anything because the plant is less likely to go into stress, and will go dormant for the winter within just a few weeks.
The other day, my friend asked me about how and when to transplant irises. I answered the "when" wrong, and I'm not sure I gave a very thorough explanation of the "how".
I did some further checking, just to make sure and I found that the best time to divide and transplant irises is after the flowers die back, but before fall, which generally means sometime between June and the end of summer . The reason being is that irises can get root rot pretty easily in a wet autumn and transplanting predisposes them to it. I found this great article on how to do it. Clickety-click.
There are exceptions to every rule, and when to transplant irises is one of them. Now I know.
As for the how, the above article gives a good explanation, but here's a video that should help: Dividing Irises