After the pond Iris have flowered in late May or June it is time to look at the size of the clump and decide whether to divide any Iris in your pond this year - usually every 3 years is sufficient.
Immediately after flowering is the best time to cut back the foliage to 6” high and cut the spread of the clump back - keeping the strong, young rhizomes and discarding the old central sections.
Repot the prepared Iris rhizome sections in aquatic compost and a mesh basket and add an aquatic feed tab deep into the compost and place the planted basket back on a shallow pond shelf so that the roots will form strongly.
Divide Iris now to give time for the plant to recover root growth and settle before Winter and allow it to flower next year. If you leave it until next Spring to divide it you will probably not get any flowers that year.
By staggering the cropping of the Iris foliage and only dividing some Iris each year you are leaving some plants for the dragonfly and damselfly to use for emergence.
Iris sibirica and Iris ensata in the moist garden should be divided after flowering too. The younger shoots can be replanted back in the ground and the older sections discarded in the same way as described for the pond Iris. Give them a good mulch of well rotted garden compost too.