How to Grow and Care for Siberian Iris

Siberian Iris bee plant with purple flowers

The Spruce / Evgeniya Vlasova

Within the very large Iris genus, the Siberian iris group (Iris sibirica) gets somewhat less attention than the more famous and popular bearded irises, but it actually has some advantages over that group. Like all irises, Siberians have strappy, sword-like foliage, but the leaves of Siberian iris remain quite attractive long after the flowers have faded, offering texture similar to that of an ornamental grass. Siberian irises are also somewhat more cold-hardy than most bearded iris, happily thriving in zone 3 gardens. And finally, Siberians are quite resistant to the borer worms and rot that can make bearded iris such a chore to maintain. Siberian iris, with its smaller, beardless flowers, may not be quite as showy as other types, but it makes up for this by being easy to grow and more drought-tolerant. Species forms usually have blue or blue-violet flowers, but there are also cultivars with yellow, white, pink, and red flowers. While not large by iris standards, the flowers are colorful and have a graceful shape.

Siberian iris is usually planted from potted nursery plants or from roots in the spring, although root division of established plants is sometimes done in the fall. These are very long-lived plants, though over time the clumps can become woody and die out in the center. Division every few years keeps the plants healthy.

All species in the Iris genus are considered slightly toxic to pets and to humans. Ingestion or handling of roots and other plant parts can cause digestive upset or skin irritation.

Common Name Siberian iris
Botanical Name Iris x, Iris sibirica
Family Iridaceae
Plant Type Herbaceous, perennial, rhizome
Mature Size 3–4 ft. tall, 2–3 ft. wide
Sun Exposure Full, partial
Soil Type Moist, well-drained
Soil pH Acidic, neutral
Bloom Time Spring, summer
Flower Color Blue, purple, white, yellow, pink, red
Hardiness Zones 3–9 (USDA)
Native Area Europe, Asia
Toxicity Toxic to humans, toxic to pets

Siberian Iris Care

Siberian iris is relatively easy to grow in a full sun location and any average soil that is moist but not soggy. Consistent moisture is important in the spring and early summer, but the plants will tolerate drier conditions in later summer. Siberian iris needs considerably less attention than bearded iris, but division every three or four years is essential to prevent the plant clumps from becoming overgrown and woody, at which point flowering may begin to fall off dramatically.

Siberian iris banish misfortune plant with purple and yellow flowers

The Spruce / Evgeniya Vlasova

Siberian iris sultan's ruby plant with violet and yellow flowers

The Spruce / Evgeniya Vlasova

Siberian iris purple mere plant with purple and yellow flowers

The Spruce / Evgeniya Vlasova

Siberian iris white amber plant with yellow and white flowers

The Spruce / Evgeniya Vlasova

Siberian iris how audacious' plant with violet and yellow flowers

The Spruce / Evgeniya Vlasova

Light

In cooler northern climates, Siberian iris will flower best if grown in full sun, though it does tolerate partial shade. In the South, it prefers partial shade, especially if you will not be able to water it regularly.

Soil

Ideally, Siberian iris should have consistently moist soil but should not sit in standing water. Good drainage is essential, as dense, poorly draining soil can result in root rot.

Siberian iris prefers a slightly acidic to neutral soil pH (6.5 to 7.0). In alkaline soils, a yearly feeding with an acidifying fertilizer may prompt better blooms.

Water

Since the Siberian iris is susceptible to crown rot, do not intentionally saturate its soil. Light, regular watering is preferable. During spring and the first part of summer, make sure it gets about 1 inch of water per week through rainfall or irrigation. Later in the summer, it can get by with an every-other-week watering schedule.

Temperature and Humidity

Most Siberian iris varieties are reliably hardy in zones 3 to 8 and are borderline in zone 9, where conditions may be too warm for most types. If you garden in a warm region, be sure to consult a local garden center or university extension service for recommendations on the best varieties to grow. Applying a layer of mulch offers two benefits: keeping the soil moist and cool, and preventing frost heaving in the winter.

Siberian iris tolerate dry and humid atmospheric conditions equally well, provided soil moisture is maintained at optimal levels.

Fertilizer

Siberian iris performs best if fertilized with a balanced 10-10-10 fertilizer in early spring, then again after flowering is complete. For the amount to use, follow the product label instructions. If you wish, an application of compost can serve as the early spring feeding. Where soils are too alkaline, a yearly feeding with an acidifying fertilizer may be helpful.

Types of Siberian Iris

The Siberian irises sold in the trade are sometimes labeled Iris siberica, but in reality, most cultivars are complicated hybrids that have both I. siberica and I. sanguinea within their parentage. Thus, you will sometimes find these plants labeled simply by cultivar name, such as Iris 'Caesar's Brother'.

In terms of size and shape, the different varieties of Siberian iris do not diverge greatly from each other, except for flower color. There may also be some variation in height and in hardiness range. New cultivars are regularly introduced, but some are now long-standing favorites:

  • 'Caesar's Brother': This established favorite has deep purple flowers on 40-inch stems. Like most Siberian iris, it is a late spring bloomer.
  • 'Butter and Sugar': This is another long-time favorite, with white and yellow flowers.
  • 'Blueberry Fair': This variety has rich blue flowers with ruffled edges.
  • 'Charming Billy': This version has reddish-purple flowers with yellow center streaking. This cultivar has better heat tolerance than most, performing well in zone 9.
  • 'Contrast in Styles': This plant is notable for its pattern flowers, which include violet, blue, and yellow hues.
  • 'Cape Cod Boys': This cultivar has periwinkle blue flowers with veins of darker blue and yellow centers.
  • 'Snow Queen': This plant has white and yellow flowers; it is slightly less cold-hardy than others, hardy only to zone 5.
  • ‘Halcyon Seas’: This type has unusually large flowers combining deep blue and violet.
  • ‘Pink Haze’: This is a pink-lavender variety.
  • ‘Sky Wings’: This cultivar combines light and dark blues with yellow centers.
  • ‘Eric the Red’: This 36-inch tall variety has dark wine-red flowers.
  • ‘Orville Fay’: Another tall variety (36 inches), this one has medium blue flowers with darker blue veins.
  • ‘Super Ego’: This is a very popular light-blue variety.

Propagating Siberian Iris

Like most bearded iris, Siberians grow and spread from rhizomatous roots that are readily divided to propagate new plants. But in the case of Siberians, these roots are extremely dense and fibrous, making division a bit more difficult. Division is not only a way to create new plants but is necessary to keep the plants healthy and productive. Divide Siberian iris when the center of the crown starts to feel woody rather than pliant. You will also get a visual signal that this is happening: The plant will start to decline in the center, with a halo of foliage surrounding a barren area.

In warmer regions, fall is the best time to perform division; in the north, it's best done in the spring. Here's how to do it:

  1. Using a shovel, dig down around the entire root clump and rock the shovel back and forth to loosen the rhizomes. When the clump is fully loosened, carefully lever it from the ground.
  2. Prune off the foliage to 6 to 8 inches, then use a sharp knife to cut away divisions, each having at least two tufts of foliage. Discard the woody center portion.
  3. Plant each division in a new location, covering it with 1 to 2 inches of soil. Space the plants 1 to 2 feet apart.
  4. Keep the new planting moist for six to eight weeks.

Growing Siberian Iris From Seed

The seed pods that remain after flowers have faded do contain seeds that can be planted, but most garden varieties are complicated hybrids, and these seeds will not "come true" to the parent plants. Thus, seed propagation is not recommended.

Potting and Repotting Siberian Iris

Siberian iris is not traditionally considered to be a container plant, but it can be successfully grown in any large, well-draining container filled with standard peat-based potting mix, which naturally provides the slightly acidic conditions preferred by Siberians. You should expect to water and feed a bit more regularly when growing them in pots. But potted Siberian iris often does not bloom for two or three years after blooming, so you may be disappointed if you are looking for an immediate display.

In large mixed containers, Siberian iris is sometimes used in the same way as an ornamental grass, placed in the center with cascading annuals around the edges. If you want to overwinter potted Siberian iris, move it into a sheltered location for the winter months to protect the roots. Do not bring them indoors, however, as these plants require a winter chill period in order to reset for next spring's blossoms.

Overwintering

In late fall or early winter, cut back the foliage to just above the plant crown. Watering should be held back in the winter months to reduce the likelihood of root rot. In colder regions (zones 3 and 4) a layer of mulch applied over the plants after the ground is fully frozen can prevent winter freeze-thaw cycles from heaving the plants upwards out of the ground.

Common Pests & Plant Diseases

Siberian iris is largely free of problems with borer worms, and much more resistant to the bacterial rot that is so often problematic with bearded iris. But excessively soggy soil may still cause bacterial root rot, which will require that the plants be removed and destroyed. And Siberian iris can occasionally be damaged by slugs.

How to Get Siberian Iris to Bloom

Siberian iris, once well established, will normally bloom adequately each spring, provided cultural conditions are right. The bloom period is relatively swift, lasting no more than a week or two. If your Siberian irises are failing to bloom, the possible reasons include:

  • Plants are not mature. It can take two to four years for a newly planted division to grow into flowering maturity. If you have just divided your iris, or just planted a nursery specimen, give the plant some time.
  • Insufficient winter chill. Siberian iris require a cool-to-cold winter period in order to reset themselves for the following spring, and in warmer regions, an unusually warm winter may cause plants to withhold flowers altogether for the following spring. The plants generally return to a normal pattern once weather patterns stabilize.
  • Plants are overgrown. After three to five years, Siberian iris can become overgrown and start to die out in the center, and flowering becomes greatly reduced. This is the time to dig up and divide the clump, discarding the woody center.
  • Soil pH is too high. Siberian iris likes mildly acidic soil, and if growing in alkaline soils, they may refuse to bloom. In this case, a late summer application of acidifying fertilizer may prompt blooms the following spring.

Common Problems With Siberian Iris

Siberian iris is remarkably easy to grow when conditions meet their basic needs. In shady locations, the foliage and flower stalks can grow excessively long, causing the plants to flop over. You may need to stake the flower stalks for plants growing in partial shade.

The grassy clumps can begin to look unkempt in late summer. A short shearing, followed by an application of balanced fertilizer, can stimulate new, sturdy upright growth and keep the foliage looking attractive into fall. This does not seem to compromise flowering for the following spring.

Although winter frost usually does not kill the roots, it can cause the root clumps to heave up out of the ground, sometimes by several inches. A thick layer of mulch applied after the ground freezes in the late fall or early winter will help moderate freeze-thaw cycles that cause heaving.

FAQ
  • How should I use Siberian iris in the landscape?

    Siberian iris is extremely versatile in the garden. It mixes well with other perennials in border gardens, offering early color as well as foliage texture.

    It can also work well planted en masse along a slope or hillside, along a path, and along the edges of streams or ponds. The flowers are excellent for cut arrangements, though they only last for a couple of days in the vase.

  • How long does Siberian iris live?

    Siberian iris can live for many decades, though the performance will gradually decline unless you divide the clumps every three to five years.

  • Do animals damage Siberian iris?

    Bees and butterflies are drawn to it, but deer tend not to eat it, and the plant is also resistant to rabbits. So you will not have to spend a lot of time on pest control when growing Siberian iris.

  • Is there a better type of iris for warmer climates?

    Siberian iris is generally better suited for cooler climates, and the same is true for most of the bearded irises, such as the very popular I. germanica. Zone 9 gardeners can have mixed results with Siberian and bearded iris, where an unusually warm winter can ruin the following spring's flowers. A better choice for warm climates is to try fringed iris (I. japonica). Others to try include Dalmation iris (I. pallida), white flag iris (I. albicans), or the hybrid "Pacific Coast" Iris (Iris "Pacific Coast Hybrid"). All of these varieties grow well in USDA zones 8 and 9, and some may be fine for zone 10.

Article Sources
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  2. Iris sibirica. North Carolina State Extension.

  3. Growing Siberian Irises in the Home Garden. Iowa State University Extension.

  4. Iris 'Caesar's Brother'. Missouri Botanical Garden.

  5. Iris. North Carolina State Extension.

  6. Iris sibirica. North Carolina Extension.

  7. Growing Siberian Iris. Iris in Canada.