How to Plant Iris Rhizomes

Blue Iris (Iris L.) in the green grass
Nataliia_Melnychuk / Getty Images
Project Overview
  • Working Time: 30 mins - 2 hrs
  • Total Time: 30 mins - 2 hrs
  • Skill Level: Beginner

The iris is grand and beautiful. Named after the Greek goddess who rode rainbows, it comes in more than 300 species in the genus Iris. Bearded irises (Iris germanica) are the most popular and bold, while Siberian Irises (Iris sibirica) add a gentle eastern flare to the garden. The glorious history behind the iris paves the way for a flourishing, reliable future.

Ideal for the beginner gardener, it is easy to grow and flourishes amongst companion plants that also bloom in early summer such as roses, lilies, peonies, salvia, and allium, all of which make lush bouquets. While most irises will bloom in June, some bearded hybrids may flower later in the summer for a second time. Learn to plant these rhizomes at the right time, place, and fashion. Once established, they will spread for generations to come and attract many a butterfly and hummingbird.

What is a Rhizome?

Rhizomes are underground roots that grow horizontally through the soil, sending up plant stalks along the way. While some rhizomes, such as bamboo, can become a problem due to unchecked underground growth, irises tend to grow more slowly and not invade other parts of your garden.

When to Plant Iris Rhizomes

Most growers distribute iris rhizomes in August or September when nighttime temperatures range between 40 and 50 degrees Fahrenheit. Plant at this time so they can settle in the soil and get established before winter. Yet if you are given or able to purchase them in spring or early summer, plant them as soon as possible.

What You'll Need

Equipment / Tools

  • Tiller or garden fork
  • Shovel

Materials

  • Iris Rhizomes
  • Compost (optional)

Instructions

  1. Find Full Sun

    Choose a spot that receives full sun. While they can tolerate partial sun, they will not bloom as prolifically without abundant light. Ensure that bearded irises especially are among the tallest in the bed, or planted apart in a separate bed, because any shade may stunt their growth or bloom.

  2. Plant in fertile soil that is neutral to slightly acidic. "Irises prefer wet feet, but dry knees," according to Farmer's Almanac. Therefore make sure the soil is well-drained as irises do not thrive in wet soil in winter. Loosen soil using a tiller or garden fork about 12 inches deep. An optional step is to mix thoroughly two to four inches of compost.

    Warning

    Be aware that bulb fertilizer, while encouraging healthy root growth, may be based on bone meal, which can attract critters to dig up newly planted rhizomes.

  3. Let the Tops Peek Through the Soil Surface

    Plant rhizomes individually or in groups of three with the noses facing each other. For each, dig a hole 10 inches wide and 4 inches deep. Create a small ridge down the middle. Depending on the variety, space three inches apart or up to two feet apart. Spread each rhizome's roots to encourage them to become well established. Plant bare-root irises horizontally and expose the tops above ground.

    If your zone experiences very hot summers, plant just below the soil surface and cover the backs of the rhizomes with a little more soil. Make sure not to plant too deep because they need to be exposed to the changing temperatures as well as airflow. Gently step on them with your foot. To avoid root rot, do not mulch.

  4. Water Generously

    Water well upon planting and a bit less once established the following year; while irises are drought-tolerant, regular watering through hot summers is recommended. Take care not to soak "their knees." Follow these steps and you'll find irises to be a bright spot in your future for springs and summers to come.

  5. Prune

    When your irises are done blooming, cut them down to the rhizome. It's important to do this only when the plant is done growing and to not remove any of its leaves, even after the flowers have stopped emerging. Keeping the leaves on the stalk allows photosynthesis to continue, which will nourish the rhizome and help it produce new growth in the following year.

  6. Divide

    About every 2 to 5 years you may notice that your irises have gotten clumped together and that the plants are no longer blooming the way they once were. This means it's a good time to divide the rhizomes.

    To do so, after they're done blooming around midsummer, dig up the clump. You should find that the original rhizome, known as the mother, has sent out new rhizomes. Use a sharp knife to cut the other rhizomes from the mother. Replant the newer rhizomes according to the directions above and discard the mother because it will no longer produce a blooming plant

Bulb of English iris (Iris latifolia)
 

kazakovmaksim / Getty Images