Siberian Iris (Iris sibirica)

Siberian Flag, Siberian Iris

When growing Siberian iris (Iris sibirica), gardens will burst with early season color and intricate, frilly flowers. Planting Siberian iris en masse adds an elegant charm to the spring garden. Use these beautiful plants as a background border for other early spring bloomers.

I. Appearance and Characteristics 

Iris sibirica, commonly known as Siberian iris or Siberian flag, is a species of flowering plant in the family Iridaceae. It is a rhizomatous herbaceous perennial, from Europe (including France, Italy, Switzerland, Austria, Czech Republic, Slovakia, Germany, Hungary, Poland, Romania, Bulgaria, Former Yugoslavia, Belarus, Estonia, Latvia, Lithuania, Moldova, Ukraine and northern Turkey) and Central Asia (including Armenia, Azerbaijan and Siberia). 

Iris sibirica was often confused with Iris sanguinea, another blue flowering Asian iris, but I. sanguinea has unbranched stems, while I. sibirica has branched stems.

It has a creeping rhizome (approximately 0.9–1.2 cm (0.35–0.47 in) in diameter), forming a dense clumping plant. The rhizomes are covered with the brown remnants of old leaves from previous seasons.

It has green grass-like leaves, which are ribbed and can sometimes have a pink tinge at the base of the leaf. They can grow to between 25–80 cm (10–31 in) long and 0.4–0.6 cm (0.16–0.24 in) wide, normally shorter than the flowering stems. In Autumn, the foliage turns yellow and then dies back (in winter), to re-emerge in the spring.

It has a hollow, slender, 1–3 branched stem, that grows up to 50–120 cm (20–47 in) long. The stems bear 2–5 (normally three) flowers, at the terminal ends between late spring and early summer, between May and June.

It has 3 brown paper-like spathes (leaves of the flower bud), that are reddish at the base, measuring between 3–5 cm (1–2 in) long.

The flowers come in a range of blue shades. From violet-blue, to blue, and occasionally white. The flowers are 6–7 cm (2–3 in) in diameter.

It has 2 pairs of petals, 3 large sepals (outer petals), known as the ‘falls’ and 3 inner, smaller petals (or tepals), known as the ‘standards’. The drooping obovate falls, measuring 5–7 cm (2.0–2.8 in) long and 2–2.5 cm wide, have a wide (or flaring) white blade or signal (central part of the petal) with dark-blue to violet veining. The white forms of the iris have a tinge of lavender and dark veining.

The smaller narrow upright standards are between 4.5–5 cm (1.8–2.0 in) long and 1.5–1.8 cm wide.

It has a light to dark blue-violet, circular perianth tube, about 1 cm long, pale blue style (about 4–5 cm (1.6–2.0 in) long), a pedicel between 1–15 cm long and a 1.5–2 cm roundly triangular, ovary.

After the iris has flowered, it produces a short stubby seed capsule, which is roundly triangular with low ridges at angles, measuring 3–4.5 cm by 1–1.3 cm. Inside the capsule are 2 rows of seeds, which are thin, flat, shaped like a capital ‘D’ and dark brown seeds, measuring about 5 mm by 3 mm.

Like many other irises, most parts of the plant are poisonous (rhizome and leaves), if mistakenly ingested can cause stomach pains and vomiting. Also handling the plant may cause a skin irritation or an allergic reaction. However an edible starch has been extracted from the plant in China, similar to Iris ensata. The root has also been used to create an insecticide and an expectorant.

It is found growing in damp woodland, wet meadows, grasslands or pastures, reed swamps by lakes, and beside streams. They generally gain a lot of moisture from snow-melt of mountains, flooding streams and soaking areas beside them.

Within North America, it is found in the damp ditches beside roadsides.

II. How to Grow and Care

Sunlight

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.

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 well, provided soil moisture is maintained at optimal levels.

Watering

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.

Soil

Siberian iris likes soil rich in humus, so fertilize in the early spring, early fall, and before its flowering period. Before it blooms, use phosphorus-rich compound fertilizers, such as fertilizers with a 5-10-5 NPK ratio, to promote blooming and encourage more gorgeous flowers. Balanced fertilizers (20-20-20 NPK ratio) can also be used. Some organic fertilizers contain fewer nutrients but improve the soil, so it is also good for the plant’s growth. Water right after fertilizing to help the roots absorb the fertilizer.

Fertilizing

Siberian iris performs best if fertilized with a balanced 10-10-10 fertilizer in early spring, then again after flowering is complete.3 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.

Planting Instructions

It’s best to plant siberian iris in the summer. Choose a spot with ample sunlight and good drainage, and turn up the soil to 25 cm deep. Then topdress with fertilizer and mix in evenly. When planting, you can cut the leaves to 15 cm in length, which helps reduce water evaporation and increase the survival rates. Finally, cover the rhizome in the soil. Avoid letting the roots get dehydrated for too long during planting, and water right after planting.

Pruning

Cut off the bloom-bearing stalks after blooming to reduce nutrition consumption. Some varieties may bloom again. In the fall, prune old and dry leaves to improve the plant’s inner ventilation and prevent pests and diseases.

Propagation

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:

  • 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.
  • 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.
  • Plant each division in a new location, spacing them one to two feet apart, and covering it with one to two inches of soil.
  • Keep the new planting moist for six to eight weeks.

Potting and Repotting 

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.

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.

Pests and Diseases

Common Pests & Plant Diseases

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

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.

III. Uses and Benefits 

  • Ornamental uses

The beautiful and vibrant blooms of the Siberian iris mix well with various other flowering perennials.

They look incredibly planted along borders, slopes on a hillside, paths, and streams or a pond.

Add the plant in your garden design with other companion plants which grow well in the corresponding USDA Zone.

The plants grow successfully in moist locations and provide some of the best cut flowers.

Even though they usually last less than two days, they look beautiful whether placed on vases or added to a bouquet.

  • Other uses

Johan Peter Falk noted that the Tara Tartars of Russia (West Siberia) coloured cloth yellow with Iris sibirica flowers and the Votyaks, Mordvins and Kalmyks derived red dye from Galium species.

It has also been used to create a drug to be used as an emetic and laxative.

An old traditional usage before the wedding night, Polish girls eat the cooked fruit (seeds) of Iris sibirica to help improve contraception.

An illustration of Iris sibirica has been used as a Postage stamp in Poland.

Siberian Iris (Iris sibirica) Details

Common name Siberian Flag, Siberian Iris
Botanical name Iris sibirica
Plant type Bulb
Hardiness zone 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b
Growth rate Fast
Height 2 ft. 0 in. - 4 ft. 0 in.
Width 2 ft. 0 in. - 4 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition Clay
Flower color Blue
Leaf color Blue