Hi,

Siberian iris don't require all the maintenance as a bearded iris.  As for timing, right after it has finished blooming, cut just the stalk all the way back to the base, but leave the foliage alone. When the leaves die, generally after frost you can cut those off completely.

Dutch iris come from bulbs. As soon as those flowers finish blooming, to prevent seed production and preserve the bulblet size and quantity being grown by the mother bulb, cut the stem back. The foliage is just going to wither away by summer like other spring bulbs do.

How do I care for Bearded Iris?

Bearded Iris can be dead-headed (cutting off spent flower stalks) any time they are done blooming, and with each individual flower only lasting a day or so, dead-heading may happen gradually over a period of a couple weeks. If you need to divide a Bearded Iris plant to reinvigorate the clump for better flowering or to reduce crowding, then that can also be done just after the plant has finished flowering. When dividing and replanting smaller clumps, the foliage can be cut back (Ohio State recommends "shear the foliage at 4-6 inches above the ground" while Penn State recommends cutting the leaves back by 2/3 of their length), but that can also be done without plant division if the foliage develops too much leaf spot infection or Iris Borer damage and is an eyesore.

Bearded Iris division may need to be done around once every 3 to 5 years. The oldest part of the rhizome (furthest from the young growth) doesn't keep blooming as it ages, so it can be discarded. In early winter, after freezing temps have killed the foliage, remove it all to reduce the risk of fungal spores or Iris Borer eggs from overwintering into the next year.

Miri