When to cut back the foliage of iris flowers depends on the date they are divided and transplanted as well as what kind of root system they have. The time for division is governed by whether the iris is grown from tuberous roots called rhizomes or the type grown from bulbs.
The foliage of rhizomatous iris needs to be trimmed to about one-third of its height when divided and replanted about once every three to four years. Trimming should be done after the flowers have finished blooming. If the plants aren't being divided, it is best to let the foliage continue growing at full height as long as possible so the leaves can produce lots of food for rhizome development.
Rhizomatous iris includes the tall, bearded German kinds that have a fuzzy, caterpillar-like growth on the bottom petal. They also include beardless species, such as Siberian and Japanese irises. The best time to divide rhizomatous iris is mid-June to September, according to the University of Illinois.
Bulb irises are referred to as Spanish iris, due to originating in the Mediterranean region. The most common kinds are the hybrid Dutch iris. Most are spring flowering varieties.
The foliage of spring-flowering bulbs usually withers before the bulbs are dug up and divided in fall. Trimming iris bulb foliage before it withers causes the developing bulbs to store less fuel for the next season's growth. Similar to rhizomes, bulbs need to be divided every three to four years. Whether grown from rhizomes or bulbs, iris become stunted and flower less when crowded. This is due to a shortage of nutrients when too many irises grow in one spot.
It is a good idea to remove bloom stalks of iris before they go to seed. This directs the plant's energy toward rhizome or bulb production and avoids self-seeding of cross hybrids that may not be as attractive as the parent plant.