Answer: Basically, you are on track with your procedure. Be sure to plant them no deeper than they grew before, because planting too deeply will cause them to rot. It is usually better to replant the largest of the new sections of rhizome because they are more vigorous. Also, dispose of any that show signs of borer or other problems. Trim the healthy foliage to about six inches and remove any that is discolored.
Bearded iris need to be divided about every three years to keep them vigorous and blooming their best. Traditionally, they are lifted and divided and replanted in mid summer after they have bloomed. Moving them in early spring will work, but it usually sacrifices the blooms that year.
They should be replanted into a good rich soil that is well drained and has a pH near neutral. They also prefer a location in full sun, so I am surprised your shaded iris are blooming the best. Perhaps the other clumps are in exhausted soil or are badly in need of thinning or are in a spot with poor drainage or have some other type of problem and this would explain the poor performance.
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