Do Iris rhizomes grow bigger if they are divided in the first year
from their original rhizome or is it better to wait a year or two to divide to get a bigger rhizome?

I would not divide a first year plant. You may smash the little increases. Let them grow a few years so that you have a clump effect. Unless you had a big clump to begin with that were from that first year plant, that is.

The increases need to feed off the mother rhizome so I never divide until I have large increases (at which point the mother is starting to shrivel anyway).

I have never ever divided an iris. Do you dig up the whole fan of the increase???

I dig up the entire clump, knock the dirt off as best I can, then *snap* the increases off of the mother. There are usually enough increases that I don't need to keep and replant the mother (which will ever bloom again) -- so I usually toss her. She goes on the manure pile to become compost to benefit the life of another plant somewhere, sometime. I trim the leaves of each daughter and replant them -- most of which will be of a size to bloom the following spring.

I do what Mary Ann does. Did the whole clump and go from there. I also mark each with the name on a leaf so I know what is what.

Yup -- me too, Celia!

It's never a good thing when you look down at a pile and think 'Oh crud...what were these again?'

No kidding -- been there, done that..............

Lestv said:The increases need to feed off the mother rhizome so I never divide until I have large increases (at which point the mother is starting to shrivel anyway).

Thank you that is exactly the information I needed!