Irises need to be rejuvenated every few years. Irises that produce spring foliage but do not bloom is a clear indication that rhizomes need to be divided.
Rhizomes are specialized stem structures that grow horizontally at, or just below, the surface of the ground. Rhizomes grow by elongation of growing points that are located at the terminals or the lateral branches of the structures.
As the iris rhizome grows and extends, the older part dies. Several lateral branches can develop, each producing a new iris.
The growth cycle of an iris begins with the initiation and growth of a lateral branch on a flowering section of the rhizome. The flowering stalk will die but new lateral branches that have developed will produce leaves and grow new foliage during the vegetative phase after flowering.
Keep top growth undisturbed until after flowering and into late summer since next year’s foliage depends on photosynthesis during the vegetative phase this year. Leaves will stop growing in late summer, indicated by browning of tips. It is at this time of the year that irises can be divided.
Steps to divide iris rhizomes:
• Undercut the plants by digging under the complete clump with a garden fork, gently lifting the rhizome clump.
• Keep roots intact after lifting. Shake off residual soil.
• Trim each rhizome by cutting off the old part. This is easily distinguished from flower tissue as the old part has no roots, is darker, and often smaller than flowering tissue. Cut off with a sharp knife or pruners. Discard the old tissue as it will not reflower.
Replanting iris rhizomes:
• Replant in full sun.
• Prepare soil by amending with compost, adding slow-release fertilizer like bone meal.
• Place rhizomes in a circle or if preferred as a border, in a shallow trench.
• Space rhizomes 12 inches apart. If rhizomes are planted in a large circle place the rhizomes facing each other with the tops to the outside.
• Place rhizomes on mounded soil, spreading roots out over the mound. Roots will dry but keep when replanting to help anchor the rhizome.
• Cover rhizomes with
• Water well to settle soil around roots; continue to water deeply once a week until new growth appears.
• Fertilize early in spring and after blossoming with a low nitrogen fertilizer.
Ellen Peffley Harp, a retired professor of horticulture at Texas Tech University, writes about gardening for several Texas newspapers.
