Rain Garden Plants: Iris sibirica

Sarah A. White, Clemson University
Amy L. Dabbs, Clemson Extension Service
November 2015

Rain gardens are functional, ornamental landscape features designed to protect downstream water quality. By capturing stormwater runoff and allowing it to soak into the ground, these gardens function as natural filters that improve water quality, provide wildlife habitat, and feature beautiful perennial plants.

Siberian iris (Iris sibirica), from the Iridaceae family, is an example of a low maintenance, well-adapted plant that is perfectly suited for rain gardens, as well as other locations throughout the landscape.

Figure 1. Range of flower colors of Iris sibirica in the garden.

Figure 1. Range of flower colors of Iris sibirica in the garden.

History and Traditions

The genus Iris shares the same name as the mythical Greek goddess of the rainbow who ruled over both sea and sky. Just like the goddess, Siberian iris bridges the gap between water and land by thriving equally well in both moist, boggy areas and dry garden soils.

King Louis VII adopted the iris-inspired fleur-de-lis as the French national symbol in the 12th century. The purple iris became recognized as the official cultivated flower of Tennessee by the state’s legislature in 1933 (Anonymous 2015).

Benefits

Siberian iris, also known as beardless iris, boasts beautiful fluttering petals in a rainbow of colors ranging from white, yellow, and nearly every shade of purple imaginable (Figure 1). The velvety petals are often marked “signals” of white or gold radiating from the center of the flower to the middle of the petals.  Many cultivars also have lush ruffled flowers.

In addition to adding beauty to the garden, solitary native bees use the tall-pithy flower stems as nesting material during the winter (Vorel 2010). Hummingbirds gather nectar from the flowers of Siberian iris, acting as pollinators as they visit each flower. Humans and livestock should not consume iris as the rhizomes and rootstocks contain iridin, (irisin or irisine) a resinous glycosidic compound that is poisonous. Iris are typically deer-resistant in the landscape.

Planting and Care

Table 1. Plant preferred site conditions

Light: Full sun (cooler climates < zone 7), full sun - light shade (> zone 8)

Zones: 4 – 8 (zone 9 heat limits flowering)

Origin: Eastern Europe and Russia

Type: Herbaceous perennial

Moisture: Drought tolerant, performs better in moist to wet soils.

Moisture timing: Prefers moisture, tolerates drought. Better flowering with adequate moisture. In zone 8b and higher adequate moisture needed for flowers.

Soil: Prefers acidic soils (5.5-6.5), tolerates other soil types if organic matter present.

Siberian iris have tough fibrous root systems that make them less prone to pests and diseases than bearded iris. They tolerate a wide range of soil types, although gardeners can improve drainage in heavy soil or increase water-holding capacity in sandy soil by amending the entire planting area with compost. To maintain even soil moisture and prevent weeds, apply 1 - 1 ½ inches of organic mulch such as pine straw, leaves, or shredded bark.

Although Siberian iris will tolerate drought, they thrive in areas with consistent moisture, particularly in spring and summer months. Throughout the southeast, afternoon shade provides relief from the heat of the summer.

Siberian iris flower most profusely when they receive a cold (vernalization) period over the winter. When planted in USDA hardiness zone 8b and higher, warm winters may hinder vernalization, reducing the plant’s ability to initiate flower buds.  Therefore, in the coastal region the floral display may be less plentiful than plantings in the upstate.

To ensure flowering the following spring, divide and transplant Siberian Iris in the fall. For more information, see The Clemson Home and Garden Information Center Factsheet # 1150, “Dividing Perennials” (http://www.clemson.edu/extension/hgic/plants/landscape/flowers/hgic1150.html)

Garden Design

Table 2. Design considerations - growth habit and plant interest

Height & Width: 1.5 - 4’ h x 1’ w

Spacing: 1’ – 1.5’

Growth rate: Slow (dry), moderate (adequate moisture)

Habit: Dense, vase-shaped clumps.

Foliage: Tall, grass-like slender leaves arising from fibrous root system.

Flower: 1.5 – 2” white, cream/yellow, lavender-pink, lavender, blue, deep purple, or a combination thereof of delicate “beard-less” flowers. Zone 8 – Bloom late April through May.

Fall interest: Seedpods provide fall and early winter interest. Foliage is pale tan to golden in color in fall.

Siberian iris are versatile in the garden as both a back of the border perennial and a show-stopping specimen (Figure 2).  With grass-like foliage and dramatic flowers and seedpods (Figure 3), Siberian iris can be planted en-masse to create a stunning wall of color or a planted as a single clump or as a specimen to create a focal point.

The beautiful flowers and seedpods can be used in cut flower arrangements, although the blooms typically only last two days after cutting. Some cultivars are fragrant.

Due to their adaptable nature, Siberian iris may be planted in traditional perennial gardens, cottage gardens, rain gardens, or in riparian buffers around ponds or water features.  

Companion Plants

Perennials:

  • Common yarrow (Achillea millefolium, yellow, pink and red flowers in mid-summer, full sun)

  • Butterfly milkweed (Asclepias tuberosa, orange-yellow summer blooms, full sun)

  • Red columbine (Aquilegia canadensis, spring blooming, full sun – light shade)

  • Scarlet rosemallow (Hibiscus coccineus, summer – fall blooming, full sun to part shade)

Deciduous shrubs:

  • American beautyberry (Callicarpa americana, vivid purple fruit in fall through early winter, full sun to part shade)

  • Dwarf fothergilla (Fothergilla gardenii, white flowers spring, red/orange fall color, full sun to part shade)

Small trees:

  • Ozark witch hazel or witch hazel (Hamamelis vernalis or H. virginiana, red, yellow, or orange flowers in mid- to late-winter or early spring, full sun to part shade)

Recommended Cultivars

‘Bennerup Blue’ – 2 foot moss green foliage with taller flowering stalks from which deep ice-blue flowers arise.

‘Butter and Sugar’ – 2.5 foot flowering stalk, flowers have white standards and yellow falls.

‘Caesar’s Brother’ – 3 foot flowering stalk with dark, velvety purple flowers arise from moss-green foliage. The foliage is an excellent contrast in the garden.

‘Eric the Red’ – 3 foot flowering stalk with reddish-purple flowers rising from gray-green foliage.

‘Ruffled Velvet’ – 2.5 foot foliage with flower stalks adding additional height. Flower falls are deep purple with violet standards.

‘Snow Queen’ – 2.5 – 3 foot foliage and 3.5 foot flowering stalks from which white flowers arise to dance above fine, grass-like foliage, re-blooms.

Figure 2. Iris sibirica foliage and flowers in the garden.

Figure 2. Iris sibirica foliage and flowers in the garden.

References:

Anonymous. 2015. State Symbols. Web. 28 Apr. 2015. <http://www.tn.gov/state-symbols.shtml>

Vorel, Cory. 2010. Encouraging Native Pollinators in your Yard and Garden. Utah Pests News Quarterly Newsletter. Vol. IV, Winter 2009-10. <http://utahpests.usu.edu/htm/utah-pests-news/winter2010/native-pollinators/>

Cornell University. 2015. Plants Poisonous to Livestock. Web 28 Apr. 2015. <http://www.ansci.cornell.edu/plants/php/plants.php?action=indiv&byname=scientific&keynum=47>

All images: S.A. White.

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