Siberian Iris Care Guide

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A Siberian iris is a herbaceous perennial that produces beautiful blue-violet blooms in May and June that will add a burst of color to your garden. Its long green grass-like leaves and flowers with 3 large outer petals (‘falls’) and 3 smaller inner petals (‘standards’) and habit of clumping is sure to attract attention.

The Iris was made popular by French monarchs who depicted its bloom on their “fleur-de-lis” royal banners. This is the official emblem of the city of New Orleans – their Saints football team has fleur-de-lis on their helmets – and it is also the state flower of Tennessee.

If you are thinking about growing these popular perennials, keep reading for our essential care tips.

General Care Guide

The Siberian Iris is known to grow well in USDA Hardiness Zones 3 through 8.

They are resistant to deer and rabbits and are commonly used as a border plant, as their slim, green foliage provides an attractive appearance even when the plant is not blooming.

For a longer-lasting bloom season, feel free to mix and match early and late blooming varieties. For instance, the “Strawberry Fair” variety blooms in the early to mid-summer, while the “Over in Gloryland” variety is known to bloom in the late spring. Planting these two varieties together will ensure a long season of beautiful blooms.

Mix these beauties with other types of iris as well.  They bloom before the Japanese Iris and after bearded Iris. 

Siberian Iris care is generally simple and easy.

They do not require much attention apart from adequate water, sun, and nutrients.  If you choose to do so, plants can be divided and transplanted in the early spring, summer, or fall. By late fall, the plants will go dormant for the winter, reemerging to grow again when the warmth of spring arrives.

Amount of Sun

In warmer climates (approximately zones 7 and 8), it should be grown in a shady location. In colder climates (approximately zones 3 and 4), they will only thrive if grown in full sun. Otherwise, these plants do well in partial to full sun in most moderate climates.

Watering Frequency

They  prefer regular waterings and well-drained soil. However, they can withstand brief periods of drought, although this may slow or prevent growth in the plant.

Be sure not to keep the soil too moist, as the roots are susceptible to rot.

They are best planted with other perennials in your garden that receive frequent watering.  Place a 2 to 3-inch layer of mulch around your plants to preserve moisture and help them survive any dry spells.

Fertilization

Siberian Iris do best with annual fertilization. Since the flowers and the foliage utilize different nutrients, it is important to consider what type of fertilizer you will be applying.  Fertilizers that are higher in nitrogen (the first number in the NPK labeling system) will promote foliage growth but will not do as much for the flowers.  While compost or manure can be used, it is usually high in nitrogen levels.  For flower production, use a fertilizer that is higher in phosphorus (the second number).

A 5-10-10 fertilizer is a good choice. After the flowers have bloomed and died away, you can apply about ⅓ cup of fertilizer around the base of the plant. Lightly mix the fertilizer into the surface soil and water it in.

Fertilization is best performed after the blooms have appeared.

Propagation

Siberian Iris can be propagated from seed.   The species can also be propagated by root division.  They have fibrous roots, which is another way to identify them from bearded irises, which have rhizomatous root systems.

If your irises are in their first few years of growth, they will not be ready to divide. Wait until the plant is at least three years old before dividing.

Siberian irises do not need to be divided very often unless more plants are desired.  However, an old clump will need division when the center has died out and there is a ring of green foliage around the dead center.

Early spring or fall are good times to divide irises.  Avoid the heat of summer, as the new plants will have a harder time establishing themselves.  Also, it is best to wait until after the blooming season and the flowers have faded.

Dig out the entire clump and cut the new growth away from the dead.  This mass can be difficult to cut through; a small pruning saw can make the job easier.  Try to make the new clumps about 4 to 6 inches in diameter.

Transplanting

Once you have separated your root clumps, you are ready to transplant them into your garden.

Dig a hole deep enough to have the crown of the plant at soil level. It’s fine to plant them a little high, but do not bury the crowns.  Firm the soil around the roots and water well.

An application of a 5-10-10 fertilizer at ½ rate can help get the new plants off to a good start, and a 2 to 3-inch layer of mulch can

Water regularly for at least two months, giving the new plants enough time to establish a healthy root system.

As mentioned above, planting is best done in early spring or fall. If you can transplant after the first signs of growth in the spring, that will work best. If not, wait until after the blooms have faded and after the heat of summer has passed. 

Different Varieties

There is a wide range of Siberian irises that are differentiated by the color and timing of their blooms. They all enjoy the same care regimens and they can be divided in the same manner, as explained above. Most of the varieties enjoy partial to full sun.

Below are some popular varieties. As you keep reading, keep in mind the terms standards and falls. The top 3 petals of iris flowers are referred to as standards, while the bottom 3 petals of the blooms are called falls.

  • “Caesar’s Brother” is one of the oldest cultivars, but also one of the best.  It is a classic for a reason!  It has a deep purple flower in late spring and grows 36 inches tall.
  • “Flight of Butterflies” features blue standards and white falls. The blooms appear in late spring or early summer. The plant will grow to about 36 inches in height.
  • “Ruffled Velvet” features deep purple blooms with a light yellow vein at the base. These flowers appear in the late spring or early summer season and grow to around 24 inches.
  • “Butter and Sugar” features white standards and yellow falls. This variety blooms in late spring or early summer and will grow to around 28 inches tall.
  • “Silver Edge” features blue flowers with a distinctive silver edge around the petals. The unique variety blooms in late spring or early summer and can grow to around 30 inches in height.
  • “King of Kings” is one of the largest varieties available. It features large white flowers with some yellow shading in the center. It will grow to a height of around 36 inches and bloom in the early summer.

These are just a few of the many Siberian Iris varieties that are available. New varietiescontinue to appear as adventurous growers experiment with hybridization and selective breeding.

Add Them to Your Garden

As we mentioned, these irises can be used as border plants, but they can also be a strong feature in many mixed perennial gardens.  They help bridge the gap between spring bulbs and the flowers of high summer and they are a great vertical accent even when not in flower. They are lovely planted near a water feature and a popular in Japanese gardens.   Try them with roses, early blooming daylilies like “Stella d’oro”, pink peonies, heucheras and with hostas that tolerate partial sun.  

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Happy Planting!

About the author: Carley Miller is a horticultural expert at TheGreenPinky. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.

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