I have heard different opinions on how to treat iris foliage. Some say to cut it down after flowering, and others say to leave it. Which is correct?
Unlike most spring bulbs, bearded iris don’t require a period of cold to produce blossoms. They are also extremely drought tolerant – good for areas with little or no summer rains. Bearded irises are not actually bulbs, but rhizomes – a kind of thick brown root. The rhizomes need plenty of sun to bloom well and regularly. They do best in well-drained soil. To plant a rhizome, dig a large hole, fill the center with a mound of soil and set it on top. If planting in the fall when dormant, cut back existing roots to about three inches. Iris planted in the spring should have their roots fanned out around the sides of the mound. The rhizome itself should sit at about ground level. Growers differ as to whether it is best to cover the rhizome completely with a light layer of soil or to leave the tops a bit exposed.
Irises multiply out from the center, which gradually becomes dry and dead. Every three to four years, they need to be dug up, divided into new plants and replanted to provide the most flowers. Cut off dead blossoms and dead bloom stalks when the iris has stopped flowering, but don’t cut back the leaves until they begin to turn brown in the fall. The leaves are needed to produce next year’s blooms.
A common problem is the iris borer, one of the most destructive insect iris pests. Females lay their eggs on old iris leaves and other plant material in the fall. These eggs spend the winter on leaves and hatch in April or May. Cutting iris leaves back to a four-inch fan and removing other garden refuse during the fall can save iris plants from serious damage next season. Feed your irises a teaspoon of bone meal every fall and when you replant. You can feed again a month before they bloom if you like, but they are quite tolerant of just one feeding.