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Dividing Iris

by Sarah Browning, UNL Extension Educator

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Iris are one of the most popular and beautiful of garden flowers. With the wide range in plant type, size, and adaptation, there is an iris for almost any location. The standard iris, Japanese iris, Siberian, Spuria and yellow flag types are all suitable for Nebraska. By using an assortment of these types in a variety of sizes, iris bloom time can extend from early April through June.

Mid-summer, from mid-July to early-September, is the best time to divide and move iris, after plants have bloomed. This ensures adequate root growth and establishment before winter. So if your iris bed has become too thick or is producing fewer flowers, then plan to divide and conquer!

Site Selection

Before you start, decide where you will be planting the new divisions. Iris prefer a sunny planting location with good air circulation and well-drained soil. Sandy loam soil is best, but other soil types can be improved with organic matter to be suitable. Renovation is a great time to add soil amendments, such as compost, to improve soil quality.

Another possibility for areas with poor soil is to create raised beds, which makes soil improvement easier and improves water drainage. Whether the plants are grown in a raised bed or not, work the soil to a depth of at least 10 inches.

If you’re replanting in the same site, make sure it still provides enough light for good growth. As landscapes mature, it’s common for a site that was once in full sun to become shady. But if you still have approximately 8 hours of full sun each day in the original location, then your plants will thrive.

Division & Replanting

Most types of iris grow from an enlarged underground stem called a rhizome. All that is required to start a new plant is a few inches of firm, healthy rhizome with well-developed roots and at least one fan of leaves or a growing point.

Before dividing the plants, cut the leaves to about one third of their full height. Dig up the entire clump of rhizomes and wash away the soil. This will make it easier to decide where to divide the rhizome and which, if any, parts need to be discarded, such as rhizome sections with no leaves, or those that are rotten, damaged or dead.

Cutting rhizomes into small sections with only one fan of leaves will result in plants not needing to be divided again for three to five years, but these plants will also be slower to produce a good show of flowers. Larger sections with two fans of leaves will produce flower more rapidly, but will also require division sooner.

When replanting into a good, loose soil, place iris rhizomes just barely beneath the soil surface. In heavier soil, plant with the upper surface of the rhizomes exposed slightly to prevent rotting. Roots should be buried to provide good anchorage. Form a cone or ridge of soil in the bottom of each hole; place the rhizome on top of the cone and spread the roots around the outside of the cone. Don’t allow the roots to clump together.

Keep your plants well watered after division. Remember, you removed many of the plant’s roots as they were dug up, and need several months to become reestablished. Apply a 2-3 inch layer of mulch to keep the roots cool, and preserve soil moisture.

This fall, keep in mind that newly planted iris, and sometimes even those in their second or third year of growth, need to be winter mulched. Use clean hay, straw, evergreen boughs or any other non-packing material.


This resource was updated July 2013 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement

 

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office


Contact Information University of Nebraska-Lincoln in Lancaster County
Web site: lancaster.unl.edu
444 Cherrycreek Road, Suite A, Lincoln, NE 68528 | 402-441-7180