It is best to reserve these kind of major transplanting for the spring....but if your circumstances don't permit, it can be done in the fall. Wouldn't be much different than putting in new plantings in the fall and that sure can be done successfully. I have pushed the envelope and put in new plantings in late October when the snow was flying! I don't recommend that practice, however! Hands get too cold. LOL

Some suggestions based on the assumption these are zone hardy perennials and you are moving them to well-drained soil.
Do the moves in early September. The plants don't need much of the new root thing as much as they need to get firmly established into the soil so they don't heave in the winter/spring thaws.

Take as large of a root mass with attached soil as you can possibly manage when moving a whole plant. Make large divisions rather than very small ones.

Cut back the foliage. Most of it will probably wilt and die anyway as a result of division. So don't make the plant struggle to maintain top growth at the expense of root establishment.

If the area you are moving them to has good soil and adequate amendments you probably need to do nothing in the way of fertilizer. If not...then I would probably add a little P and K....both promote root growth. Especially the K which has been shown to improve winter survival. Do NOT add any nitrogen. You do not want to stimulate top growth.

Water, water, water the transplants. Continue to water as late as possible into October.

All should reappear in the spring healthy and happy.

Incidentally, I live about 150 miles North of Duluth. Just returned home a few hours ago from a quick Duluth trip. Nice day there. Raining here.
Jan

Well said Jan!

Wow! What a great answer, Jan! Totally thorough, not a question left unaddressed. The bulk of this project is an area along a ditch. It was cut away by the city in an effort to improve drainage (which it did not) and they left a previously nice grassy slope in ragged, jagged ruin. Many, many dandelions and tansy and yucky, stinky valerian have moved in. It's not really my property, it's in the alley easement (there is no alley), so I can't do anything about this other than pretty it up. The soil is not great but adequate for what I plan to do, which is move a lot of wildflowers which I've raised from seed (asters, goldenrod, Culver's root, marshmallow, butterfly weed, nemerosa sage) and rescued from a developer's bulldozer (joe pye, wild rose, some kind of wild bluebell-looking thing)--they're all in their third summer and increasing nicely--and divide some large stands of beebalm & yarrow that I have been raising for this express purpose. I'll dig out a dandelion and plunk in a clump of something better and so on, and leave the grass intact in between as much as possible to keep the bank stabilized--besides, I like the grass. : ) There are also some nice things out there which I'll leave alone, like chamomile and daisy, and forget-me-nots at the bottom of the ditch. There are wetter and drier areas and I'll pay attention to that when I do the moves. And youbetcha, I will make these divisions bigger rather than smaller, and keep as much of a rootball as possible. I'll also amend the soil with some compost where needed, and thank you for the pointer about the P and K (no N!!)! I really appreciate your advice and am looking forward to digging in with this. Itching to get started now that the heat wave is over but everything is in bloom and I am not touching it yet. My nursery bed is absolutely gorgeous with all these guys blooming, and the hummers are enjoying it too. Thanks again!!

Bearded iris, however, usually do not do well transplanted in fall (Oct).
Rick

Diane - that sounds like quite the project. You deserve a lot of credit for undertaking a beautification project on unslightly city/easement property. Based on your plans, it will look wonderful!
Jan