The Iris genus has more than 300 species that are often classified according to the different root structures they form. Some irises have bulbous roots (such as Japanese iris), while others have fibrous root clumps (including the Siberian iris group). But the largest group, and arguably the most important, is the bearded iris group, all of which grow from rhizomatous roots—fleshy, thickened underground structures. When a gardener purchases a new iris, especially from an online retailer, it is often just a section of rhizome that arrives to be planted, often with a trimmed section of leaves still attached. And when you propagate bearded iris, it's this rhizome that is dug up, divided, and replanted. So learning the correct way to plant an iris rhizome is crucial if you want to grow this magnificent early summer perennial.
What Is a Rhizome?
Technically, a rhizome is a thickened section of plant stem that grows underground, but in practice, a rhizome is considered to be a type of root structure, identified by its fleshy, thickened form and horizontal growth direction. This fleshy root/stem portion serves as a storage vessel for moisture and nutrients, allowing the plant to send forth vigorous leaves and flower stems directly up from the root. The rhizome steadily replenishes and expands itself in preparation for next season's flower display. Examples of rhizomatous garden flowers include bearded iris, bamboo, and lily of the valley.
When to Plant Iris Rhizomes
Most commercial growers ship iris rhizomes in late August or September when nighttime temperatures range between 40 and 50 degrees Fahrenheit. This is an ideal planting time, as the rhizomes have time to settle in the soil and get established before winter. Iris rhizomes can also be planted in spring or early summer, but try to avoid planting them very late in the fall, as they may not overwinter very well.
Equipment / Tools
- Tiller or garden fork
- Iris rhizomes
- Compost (optional)
Choose a Location
Choose a garden spot that receives full sun. While they can tolerate partial sun, bearded irises will not bloom as prolifically without abundant sunlight. With shorter types of iris, also give consideration to where they are positioned within the garden bed—avoid locations where they will be shaded by other plants.
Prepare the Soil
Iris grows best in well-draining, fertile soil that is neutral to slightly acidic. Take extra care to make sure the soil is well-drained, as irises may rot if they soak in wet soil over the winter.
Loosen the soil using a tiller or garden fork to a depth of about 12 inches. An optional step is to thoroughly mix in 2 to 4 inches of compost with the soil. This can be helpful in improving the drainage of dense soils at the same time it improves soil fertility.
Inspect the Rhizomes
When iris rhizomes are purchased commercially, they will often arrive with a good healthy rhizome section with smaller roots spreading out from the sides, and a section of leaves attached on one end, often cut into a fan shape. (If you have lifted and divided an established patch of irises, your divided pieces should also have this configuration.)
Before planting, inspect the rhizomes and reject any that are notably mushy or dessicated. With rhizomes you have newly dug up and divided, make sure there are no pieces with borer damage; these sections can be clipped away with sharp pruners.
Plant the Rhizomes
The rhizome sections are best planted in groups of three or five, arranged so that the leaf fans are oriented the same way, with the rhizomes just barely covered and all pointing the same direction under the soil. Give individual rhizomes at least 3 inches of space between them for smaller varieties, or as much as 2 feet for tall bearded varieties. Remember that these plants are fast-growing, and giving them plenty of space will allow them several years before you need to lift and divide them again.
Dig individual holes about 4 inches deep and 10 inches wide, and create a narrow mound of soil in the bottom of the hole, over which you will spread the roots of the rhizome. The fleshy body of the rhizome should rest on the soil mound.
Carefully backfill the hole with amended fill soil and tamp the soil firmly down against around the rhizome and leaves. The rhizome should be just barely covered, and may even be slightly exposed at the point where the leaves emerge.
Do not mulch over the planted rhizomes, as this can encourage root rot.
Newly planted iris rhizomes should be watered every few days until new growth is evident, then water weekly for the remainder of the growing season. Begin to withhold water when it becomes clear the new rhizome is well established, as you don't want the root to soak in wet soil as winter arrives.
Caring for Newly Planted Iris
When iris rhizomes are planted at the typical time—late summer or early fall—it is typical for some new foliage growth to begin, but do not expect the plants to flower until the following spring, at the earliest.
As the newly planted rhizomes put forth leaves in their first spring, treat them as you would when growing any established bearded iris plants, but fertilize them at about half the strength used for established plants. It's entirely possible you will get some blooms in the first season, but usually, this first year is spent developing a robust root structure that will fuel an eruption of flowers in the second full season after planting. Should your irises bloom in their first season, allow the foliage to continue growing until it begins to turn yellow and brown in late summer. The presence of green foliage is what nourishes the rhizome and replenishes it to prepare for good blooms the following year. Even if the foliage is less than attractive, leave it in place until it is no longer green.
Expect to dig up your iris rhizomes for division and replanting every three years or so, as this will keep the patch healthy and productive, at the same time it provides you with new plants. Regular division also allows you to control borer worms, the most common pest that affects this plant.