Irises come in a variety of forms and colors with large, showy blooms that make them one of the most popular flowering plants in gardens across the globe. Fairly easy to grow and care for, these spring blooming, perennial rhizomes do require annual pruning to keep them healthy and able to put on the flower display for which all types of iris are so-well known.
Why You Should Cut Back Irises
Maintaining the thick-spathe type leaves of the iris and removing spent flower stems are important for several reasons.
Most varieties send up a flower stalk with several blooms that open incrementally. It isn't necessary to deadhead individual spent blooms, but you do want to remove the stalk at the base once the two to three week bloom period has ended. Stalks with wilted blooms or bare of any bloom aren't attractive and detract from the foliage which, by itself, adds form and glossy green color in the flower garden.
The spent flowers also will direct energy into producing seedheads. While this is perfectly okay, it does divert the plant's energy from producing new leaves; a necessary function that insures a good flowering period the following year.
Pruning or trimming the foliage protects the plant from fungal and bacterial diseases caused by too much moisture on the leaves. Irises are vulnerable to blight, leaf spot and soft rot. Left to overwinter, they are also vulnerable to a major iris pest-the iris borer. The adult (a brown moth) lays eggs which overwinter in dead foliage. Larvae hatch in spring and eat through the leaves eventually boring into the plant's rhizome causing rot.
Proper pruning allows the opportunity to inspect your plants, remove any diseased or dead leaves and check for insect pests.
How to Cut Back for Best Blooms
Irises rhizomes and bulbs store energy needed to develop new leaves which are the key element for consistent bloom. This energy is generated through photosynthesis in the plant's long, thick, narrow leaves and allows the rhizome or bulb to begin producing new leaves to support next season's bloom period. Cutting back the leaves the correct way at the right time, along with dividing your iris every two to three years, are the best ways to achieve consistent annual bloom.
Remove Spent Flower Stalks
Bearded Iris, one of the most popular types, often sends up a second bloom in the fall. Regardless of variety, whenever your iris has completed it's bloom cycle, the plant benefits from removing spent stalks. Use a sterile hand pruner or clipper to cut the stalk at it base. Angled cuts are best because they allow for water run-off which helps prevent moisture-based disease.
Fan Trim in Mid-Fall
Fall is the time to trim back the clump of iris leaves which continue to produce energy as long as temperatures remain above freezing. The fan trim is a good garden practice for maintaining and encouraging consistent annual blooming. Clean, sterile scissors, hand trimmers or a small garden shear all work well for this task. Starting in the middle of the clump, cut at a downward angle along each leaf. leaving a graduated fan shape of 4 to 5 inches tall at the peak. The outside leaves should be 2 to 3 inches tall. This fan shape helps keep leaves healthy by preventing moisture from accumulating in the center of the clump, but allows for continued photosynthesis. At this same time, go ahead and remove dried, discolored and diseased leaves.
Remove All Foliage After Frost
Once cold temperatures are consistent, most, if not all, of the iris leaves will die back naturally. Now dead foliage should be removed leaving just a half-inch to an inch above the rhizome. Use the angled cut and a sterile tool to cut back and avoid breaking or pulling the spent leaves since this can damage the rhizome.
This is a good time to run your hands over the plant's base. In a good sized iris clump you will find a number of rhizomes fairly close to the soil surface. Check for spent or soft ones and cut them out with your clean tool. Cut between the soft or damaged one and any healthy ones, lift it out and add it to your compost or dispose of it.