How to Grow and Care for Siberian Iris

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 a texture similar to that of an ornamental grass. Siberian irises are also somewhat more cold-hardy than most bearded irises, happily thriving in gardens as far north as USDA Cold Hardiness Zone 3.

Siberian iris, with its smaller, beardless flowers, might 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.

Siberian Iris Care

Here are the main care requirements for growing Siberian iris:

• Plant in full sun in moist, but not soggy soil.
• Can tolerate drier conditions in later summer.
• Fertilize with a balanced fertilizer.
• Divide Siberian iris every three or four years to prevent the plant clumps from becoming overgrown and wood, which means fewer blooms.

Light

In cooler northern climates, Siberian iris will flower best if grown in full sun, though it does tolerate partial shade. In warmer southern climates, it prefers partial shade, especially if you will not be able to water it regularly. However, if it's located in too much shade, the plant will produce fewer flowers.

Soil

Ideally, Siberian iris should have consistently moist soil but should not sit in standing water. Good drainage is essential because dense, poorly drained 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 might produce more blooms.

Water

Because 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 receives about one 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 USDA Cold Hardiness zones 3 to 8 and are borderline in Zone 9, where conditions might 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, 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 might be helpful.

Is This the Right Plant for You?

A SIBERIAN IRIS PLANT IS A GOOD CHOICE IF:

• You don't have pets (or only have pets that stay indoors, like cats) since this plant is toxic
• You have a full-sun location in your yard where this plant can thrive
• You live in a region that gets pretty cold, so you need a cold-hardy plant

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 can also be some variation in height and in hardiness range. New cultivars are regularly introduced, but here are some popular 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 variety 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 patterned 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 variety 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.

Pruning

Siberian iris need to be cut back annually to keep them healthy and to avoid any bacterial growth from lingering leaves. Once the bloom period ends, you should remove any stalks with wilted or dead flowers. Not only will this keep your garden aesthetically pleasing, but it will also help the plant redirect its energy to produce new leaves, which is essential for flowering the following season. Proper pruning will also keep insects at bay and ensure that all parts of the plant are thriving.

Propagating Siberian Iris

Like most bearded iris, Siberian iris grows and spreads from rhizomatous roots that are readily divided to propagate new plants. But in the case of Siberian iris, these roots are extremely dense and fibrous, making division a bit more difficult.

Division is not only a way to create new plants, but it 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, pitchfork, or even an axe, dig down around the entire root clump and rock the tool back and forth to loosen the rhizomes. When the clump is fully loosened, carefully lift it from the ground.
2. Prune off the foliage to six to eight inches tall, 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, spacing them one to two feet apart, and covering them with with one to two inches of soil.
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 grow true and won't be identical to the parent plants. Thus, seed propagation is not recommended.

Potting and Repotting Siberian Iris

Siberian iris is not traditionally considered to be a suitable 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 Siberian iris. 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 planting, so you might be disappointed if you are looking for an immediate display of flowers.

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 its roots. Do not bring them indoors, however, as these plants require a winter chill period in order to reset for next spring's blossoms.

To pot Siberian iris, you'll need good quality, well-draining potting soil and containers with sizable drainage holes. Begin by digging holes about 3 to 5 inches deep and 8 to 12 inches apart. Gently place each plant in the soil, with the roots fanned and pointing downwards. To avoid repotting, plant Siberian iris in a large enough container as the plant grows slim and tall.

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 (USDA Cold Hardiness 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

Bees and butterflies are drawn to Siberian iris, and deer and rabbits tend not to eat it. The plant is largely free of problems with borer worms, though it can occasionally be damaged by slugs. Siberian iris is much more resistant to the bacterial rot that is so often problematic with bearded iris. However, excessively soggy soil can still cause bacterial root rot, which will require that the plants be removed and destroyed.

How to Get Siberian Iris to Bloom

Siberian iris, once established, blooms 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 causes could be:

• 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 and planted a new 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 can cause plants to stop flowering 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 grown in alkaline soils, they might refuse to bloom. In this case, a late summer application of acidifying fertilizer might promote blooms the following spring.

Common Problems With Siberian Iris

Siberian iris is remarkably easy to grow when environmental conditions meet their basic needs. In shady locations, the foliage and flower stalks can grow excessively long, causing the plants to flop over. You might 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.

From: thespruce