Chisago County Master Gardener
Late August through early September is the time for planting new rhizomes and dividing those irises that have been in place for three to four years.
If your iris did not bloom as well as they used to, it may be time divide them. This is a good time to solve other problems such as improper soil, insufficient sunlight or poor drainage.
When preparing a new bed, remember that iris 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. These areas should be well mulched to keep soil temperatures even.
New beds should be free of rocks and roots. Work in organic material such as peat moss, fine compost or shredded leaf mulch to a depth of 10 to 12 inches. It may be beneficial to add 5-10-10 fertilizer, following the directions on the bag. Mixed it in well before planting bulbs.
Iris rhizomes sit just below the soil surface. To dig up a clump, slide your garden fork carefully beneath the clump and lift it out of the soil.
Spread the clump out on the lawn and wash it with a garden hose. Once you can see the rhizomes, it’s not difficult to cut each clump into smaller parts 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 attached.
Discard the old, 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. You can replant the rhizomes immediately or store them for several weeks in a cool, well ventilated place.
Plant groups of iris in drifts with their fans facing outward from the center of the garden. Leave a minimum of eight inches 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 10 to 12 inches of dried, shredded leaves.
The iris borer is a serious problem for iris growers. The borer hatches from eggs laid on the leaves in early spring.
A sign of iris borer activity could be premature yellow and brown foliage on the entire plant. The iris borer causes a bacterial soft rot that eventually kills the plant. When you lift up the iris rhizomes, trim away rotted portions or discard the entire plant.
A spring spraying of Orthene or Cygon when the leaves are five to six inches tall is the usual defense.