Dividing Tall Bearded Iris
Tall bearded iris are breathtakingly beautiful perennials that are long-lived and provide a bright splash of color to the garden in the spring. Their attractive, sword-like foliage provides a strong vertical effect even after flowering is finished.
Why Divide Iris?
Over time, clumps of iris become overgrown and they don't bloom well anymore or they stop blooming all together. Old undivided iris are also more prone to disease, insect damage, and root rot. This is one plant that really benefits from being divided on a regular basis - maybe every 3 or 4 years.
When is the Best Time to Divide Iris?
Iris should be divided in the late summer after they have stored energy in their rhizomes all summer. If you wait too long into the fall to replant the rhizomes, the roots may not have enough time to establish before the ground freezes and the plants will be subject to frost heaving. August or September is a great time to do this.
How do I Divide My Clump of Iris?
To divide iris, loosen the soil with a digging fork and carefully lift the rhizomes from the ground. Gently loosen and remove the soil from around the rhizomes or use a garden hose to wash the soil off so you can more clearly see the rhizomes.
Trim the foliage to about 6 inches with a sharp blade and discard it or destroy it - do not compost. We cut the foliage in a fan shape that is more attractive for replanting in the garden. To help with disease and insect control, dip the rhizomes in a 10% bleach solution (1 part bleach to 9 parts water) and let dry.
Break the rhizomes apart at the joints where they will snap naturally in your hands. You can also use hand shears to do this. Break off and discard the old, withered rhizomes. These sections will never produce foliage or flowers again. Cut out or remove any mushy or rotten areas.
Replant in a shallow hole spreading the roots out and leaving the top of the rhizome (which is actually a modified stem) exposed at the surface of the soil.
Do not mulch iris. If you are planting later in the fall and you want to protect them from frost heaving, you can cover them with a layer of composted leaves or other mulch but be sure to remove this mulch in the spring.