Now is the time to divide those crowded clumps
Your iris will be happy in the ground for a number of years, but after a while you may find they are not flowering as well, so it's time to lift and divide the rhizomes. Or perhaps, as explained above, they are not in the best spot, and you have your eye on somewhere more suited.
The best time to lift iris is immediately following flowering, or in late autumn. Lifting in mid winter will not give your iris time to establish before blooming in the spring, and what flowers that do bloom will be small and late. Moving during summer is okay, but iris are not growing much during this time and you must be careful to not over water as you will cause rotting of the rhizome.
It is very easy to lift and divide your iris - simply use a spade or fork to lift the clump and break off the rhizomes, making sure each one has some root development. Replant shallowly, with just a centimeter or two of soil over the rhizome. Some advocate having the rhizome at ground level, but if you live in a very cold frosty winter climate, with long hot summers, a little soil over the rhizome will offer protection, but still get the chill and heat iris thrive on.
It is best at this time to trim the leaves so the newly planted rhizome has less leaf area to maintain while they are establishing. We trim our leaves by about a third, cutting diagonally.
The smaller dwarf varieties do not need as much of a trim, if at all. Water in, to ensure good root to soil contact, but from then water sparingly, as rot can be a problem in the rhizome. Generally iris only need watering once a week in the summer.
Iris clump ready to dig
Iris separated and trimmed
Newly planted iris