You will find a huge variation between cultivars, as to the amount of increase. I planted about 300 iris last year, and while some were "early spring "plantings, and some were "late fall", the majority were all planted from, say, June 20th, to July 15th. I'll post a pic or 2 that I have already in the file( I'd take a few just of "plant size", but it's raining!):
With the exception of the yellow in the foreground, all these were planted as single rhizomes, last year
These were all" single " plantings from last year,( except for 3 plants in foreground )
I think you will find that after about 2 years in the ground, there are some that will have increased to the point of 'crowding their neighbors", and will need dividing/moving, and others will just be making their first stalk! That's just the "nature of the beast"!
