September 15, 2011 at 9:03 a.m.
If you are preparing a new bed, remember location is important. In addition to the cold exposure during the winter months, they also need warmth and bright light to trigger proper growth in the spring. The soil near foundations will warm up faster, particularly on southern and western exposures, resulting in earlier flowering. Areas should be well mulched to keep soil temperatures at a constant level and sheltered from winds.
The soil should also have good drainage. New beds need not be free of debris, such as rocks and roots. Work in organic material such as peat moss, fine compost or shredded leaf mulch. This should be worked in to a depth of 10 to 12 inches.
Any good garden soil does not require fertilizing when you first plant your iris. We do not recommend the use of manure or heavy nitrogenous fertilizers, as they promote rapid, soft growth that is subject to rot. Steamed bone meal is O.K. We like 5-20-20 or 6-24-24 fertilizer better because iris are ravenous feeders and need some nitrogen, and sufficient potash to develop strong firm roots which are more resistant to disease and inclement weather.
Iris rhizomes do well just below the surface. To dig up a clump, slide your garden fork carefully beneath the clump and lift it out of the soil. Spread the clumps out on the lawn and wash them off with a garden hose. Once you can see the rhizomes, it's not difficult to cut each clump into smaller pieces for replanting.
Each clump of iris will have two or more fans of leaves growing out of its rhizome. Use a sharp knife to divide the rhizome and discard the oldest woody part of the original rhizome, along with any parts that are soft or rotted.
Next, trim away broken roots and cut the green foliage back to four to six inches in height. You can replant the rhizomes immediately or store them for several weeks in a cool, well-ventilated place. Plant groups of the same iris in drifts with their leaf fans facing outward from the center of the garden. Leave a minimum of eight inches apart between each rhizome for expansion. Rhizomes should be positioned horizontally, right below the surface. Dig a shallow hole for each rhizome, with a small ridge of soil for it to sit on. Spread the roots to both sides of the ridge. Put more soil over the rhizome and roots and water it thoroughly. When the soil begins to freeze in November, mulch the iris with four to six inches of straw or marsh hay, or ten to twelve inches of dried leaves.
The iris borer is a serious problem for iris growers. The borer hatches from eggs laid on leaves in early spring. A sign of iris borer activity could be the premature yellow and brown foliage on the entire plant. When you lift up the iris rhizomes, trim away rotted portions or discard the entire plant. The iris borer cause a bacterial soft rot that eventually kills the entire plant.
Parts of this article were taken from earlier writings by Bee Tolzmann, former Chisago County Master Gardener and Deb Brown, U of M horticulturist.