Prefer gritty, alkaline, sandy soil with excellent drainage making them excellent candidates for hillside or rock gardens. Grow from swollen roots called rhizomes that must remain near the soil surface when planted. Always amend soil with rock phosphate or bone meal to encourage rooting. Plant in clusters of at least 3 roots spaced 2″ apart.
German irises have blue green foliage that remains attractive all summer and is an upright, sword like design element in the perennial garden. German irises should be divided every 3-5 years to keep the clumps vigorous, avoid rhizome deterioration in the center, provide maximum bloom, and help to control the iris borer, their most common pest. The life cycle of the iris borer is as follows: Caterpillars emerge from the soil in April or May when the leaves are 5-6″ high. They climb the foliage, pierce the leaves and enter the leaves. They tunnel down (leaving linear tracks) and enter the rhizomes, growing to be fat, flesh colored worms 1½-2″ long. They eat out the center of the rhizomes then migrate into the soil, where they pupate. In late summer and fall they turn into night-flying moths that are purplish yellow in color. The moths lay eggs on the iris foliage and debris at the base of the leaves. Eggs are a creamy green, later turning lavender. The most important control measure is GARDEN SANITATION! Cut and eliminate all stalks and foliage each fall, as they are the primary over wintering site. Do this late into the fall, shaving the foliage as close to the rhizomes as possible. (In the “olden days” they used to burn off the foliage each fall to kill the larvae and clean up the foliage!) In early spring, if you see borer tunnels starting in the leaves, pinch them to kill the larvae. You can dust the base of the plants and surrounding soil with pyrethrum dust in early spring to kill the larvae as well.
German irises, unlike most perennials, are divided in July and August when they have ceased their active growth cycle. Dig up the clumps, separate them (they come apart easily by hand prying). Remove the outer, healthy “fans” of foliage each with a 2″ long rhizome attached. Cut off the older rhizomes, being sure to remove all soft rot or borer-damaged rhizomes. You will get a surprising number of divisions from each clump! Dip the rhizomes in powdered sulfur or a 50% bleach solution if there is lots of soft rot present. Reset rhizomes just below the surface of the soil in clusters of 3-5 fans set 2″ apart. They will bloom lightly the first year and beautifully the second year. You should complete this project by mid-September to allow the rhizomes to put down good roots for the winter. Never use mulch over the rhizomes, but it is a good idea to apply a loose covering of evergreen boughs or salt hay over your newly replanted iris patch in late fall as soon as the ground has frozen. This prevents heaving of the roots in the winter. The most exciting development in
German Iris breeding is the introduction of reblooming varieties. These plants bloom at the normal time in the late spring/early summer and then repeat bloom sporadically all season long.
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