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Extension > Garden > Yard and Garden > Flowers > Iris

Iris

Rhonda Fleming Hayes and David C. Zlesak

Copyright © 2009 Regents of the University of Minnesota. All rights reserved.

The dazzling beauty of the iris is captured in the paintings of Monet and Van Gogh, yet this adaptable perennial is no diva. It is not unusual to see irises still blooming on abandoned farmsteads. Hardy and vigorous with flamboyant flowers, the iris has been a mainstay for gardeners throughout the years for good reason.

Bearded iris are known for their large colorful blooms.

Sound horticultural advice says “right plant for the right place”; picking from the myriad choices of iris varieties and cultivars, there are irises for just about every garden situation. Selecting for a sequence of bloom times, it is possible to extend the iris season from spring to early summer, with even an encore from re-bloomers. It is important to note that these old-fashioned flowers now serve a contemporary use in the setting of rain gardens, water gardens, or xeriscapes, depending upon the variety and its cultural needs.

Two main types of iris do well in northern climates; bearded and Siberian. However there are less common species worth investigating and growing such as Iris reticulata (dwarf or Dutch iris), I. pallida (sweet iris), I. pumila (dwarf bearded iris), I. cristata (dwarf crested iris), and the shade tolerant I. tectorum (Japanese roof iris).

Bearded iris have swollon rhizomes that grow at or near the soil surface.

Bearded iris

Bearded iris has a tan colored, tuberose rhizome that acts as a food-storage unit for the plant. Lying just at the soil surface, the rhizome sends roots downward while the shoots with sword-shaped leaves grow upward. As the leaves multiply, they fan out until a flower stem or scape emerges from the growing point and produces the spectacular flowers.

The classic bearded iris is blue or purple with yellow markings at the center, however they also come in white, lavender, yellow, peach and bronzy reds with different combinations of marking and veining, as well as ruffling. The elegant petals of the bearded iris serve very practical purposes. The upright petals or standards act as colorful flags to attract pollinating insects, while the downward curving petals or falls, function as a landing pad for them. The yellow blotches and veining work like a map that directs the insect to the nectar. While the distinctive, fuzzy beard attached to the falls helps the pollinators to hang on once there.

Bearded iris have whisker-like protrusions on their petals.

Best location for bearded iris

Bearded irises thrive in full sun with plenty of room for good air circulation. Crowding or shading by other plants can impact blooming as well as contribute to disease. Irises grow best in rich soil that has been amended with organic matter such as compost, aged manure, or peat. Well-drained soil is important to avoid root rot of the fleshy rhizome.

Best care for bearded iris

Plant selection

Iris rhizomes are typically available at nurseries and garden centers in July to August. Mail-order purchases are shipped at this time as well. Rhizomes should be firm and fresh; it is possible to trim off broken or damaged portions of otherwise healthy specimens. Potted irises are available in the spring in limited varieties.

Planting

Although the iris rhizome is planted in a shallow hole, the soil should be loosened to a depth of 10 inches to allow for optimum root growth. Place the rhizome so that it sits barely under the soil, firming the soil around it, and then water it in. Potted iris can be planted in the spring using care to plant at the same soil level as in the pot.

Containerized iris are often available in spring at local garden centers.

Water

Bearded iris should be kept consistently moist but only from spring until the end

of flowering. After blooming the plant enters a period of-semi-dormancy when too much water can actually cause rot. This watering pattern corresponds with that of a xeric garden, taking advantage of spring rains, followed by summer dryness, making the bearded iris an attractive candidate for the low-water landscape, or well drained to drier soils.

Mulch

Mulch should be used sparingly around bearded iris during the growing season; especially after bloom when a dry period is needed. Avoid mulch in or around the crown to reduce the chance of rot. Winter protection to protect from frost heaving with mulch however is important, but only after the ground freezes.

Fertilizer

Fertilize bearded iris in early spring if a soil test shows that you need to add nutrients. As growth emerges, use a low nitrogen fertilizer watering it in well; and then fertilize again when the flower stem appears. Organic fertilizers like compost, blood meal, or fish emulsion can also be used. For more information on fertilization please click here.

Best cultural practices for bearded iris

Pest And disease control

Damage is caused by iris borer.

Michelle Grabowski

Good cultural practices can help to reduce the incidence of iris borer, the most common and serious problem with bearded iris. It is the iris borer larvae which cause damage; they attack the central leaf stalk and tunnel downward into the rhizome, where they pupate and emerge as an adult moth. Gardeners can physically remove them from the leaves and rhizomes or use an insecticide. Keep beds clear of weeds and debris to avoid harboring the next generation of iris borer eggs. For more information on iris borer and other pests please click here.

Some irises with large heavy blooms and taller stems benefit from staking; use a thin bamboo or metal stake next to the plant and soft ties to attach them. Removing spent blooms stops the formation of seed pods as well as making the iris bed more attractive. Once the entire flowering stem is finished blooming, cut it off at the base.

It is a good idea to remove iris foliage showing disease or insect damage. Irises should be trimmed back carefully, leaving as much green, healthy leaf tissue as possible. This runs contrary to past suggestions to cut back the entire plant. In reality, next year’s health and vigor is determined by the ability to make and store food during the current season; the more leaves the better. Yellowed leaves can be pulled completely off without harming the rest of the plant. Any dead plant material should be discarded to prevent disease and insect problems.

The time to cut back and discard all foliage is after a hard frost. In spite of their hardiness in freezing weather; growing close to the soil surface, iris rhizomes can heave during temperature fluctuations. A six inch layer of mulch, such as chopped leaves or straw will help protect plants from repeated cycles of freezing and thawing.

Plants can become crowded over time and benefit from dividing.

Division

Dividing bearded iris is vital to the plant’s health and makes iris a relatively higher maintenance perennial than others. A decrease in bloom can indicate overcrowding and the need to divide bearded irises; this is generally done every 3-4 years. The best time to divide iris is four to six weeks after blooming, in July to early August, when the plant is semi-dormant. This will give the plant ample time for re-establishment in order to insure flowering the next season.

First cut the foliage fan back to three to six inches to prevent water loss, removing dead or yellow leaves as well. Dig up the rhizomes and cut them apart into 3-4 inch sections with a sharp knife, saving the healthy outer growth connected to the current stem tissue. The roots can be trimmed back to two inches. If disease has been a problem in the iris bed, the rhizomes can be treated with a fungicide labeled for iris before replanting. Plant the new divisions following the same bare root instructions (see above) with attention to proper spacing for good air circulation. Leftover plant pieces and debris should be discarded.

Creative landscaping with bearded iris

Siberian iris

Breeding work over the past couple decades have greatly expanded the color range available in Siberian iris.

As their name of origin implies, Siberian Irises are tough and hardy. They are sometimes known as the beardless iris. Growing from fibrous roots into tall, grassy clumps of slender leaves, the flowers unfurl without the beard but are no less beautiful. They come in various shades of blue, lavender, white, or yellow. After blooming in late spring to early summer, the foliage remains attractive throughout the season. The vase-shaped clumps multiply effectively almost as a groundcover crowding out weeds.

Best location for siberian iris

For most prolific bloom, Siberian Iris should be planted in full sun. Siberian irises flourish in rich, slightly acidic (pH 5.5-6.5), high organic matter soil. These iris have a high tolerance and actually a preference for moist to wet soils.

Best care for siberian iris

Planting

Bare root stock is available in late summer to early fall. The plants should be removed from their packaging promptly and soaked overnight prior to planting. They should be planted with the crown just under the soil surface, then watered in. Potted varieties are available in the spring; they should be planted carefully at the same depth as in the pot and watered in well. Siberian iris prefer full sun to light shade, excess shade can reduce or even eliminate flowering.

Siberian iris prefers full sun to part shade.

Water

Siberian Irises need a lot of water; consistent moisture throughout the season with occasional deep watering will help them to perform at their best. In well-drained soil it is difficult to overwater them.

Mulch

Organic mulches such as compost, shredded leaves or pine needles help to conserve soil moisture while blocking weed growth.

Fertilizer

If a soil test shows a deficiency, fertilize with a granular commercial fertilizer in early spring as new growth appears and then again just before bloom. Water soluble and organic fertilizers such as compost, aged cow manure, blood meal or fish emulsion can also be used. For more information on fertilization please click here.

Best cultural practices for siberian iris

Pest and disease control

Siberian irises are rarely bothered by pests; occasional infestation of aphids can be controlled by spraying off with the hose or using an insecticidal soap. The organic gardener will appreciate their high disease resistance. Mice will sometimes nest in the foliage over winter, trimming the leaves back after frost will make this less attractive to them. Siberian irises are deer-resistant.

Pruning and grooming

Siberian iris form dense clumps.

Removing spent blooms stops the formation of seed pods and energy to be diverted to the plant instead. However, the seed pods of Siberian iris are attractive and on established plants can be left for added fall and winter interest. If one decides to prevent seed pods from forming, remove the entire flowering stem after it is finished blooming by cutting it off at the base.

Division

There are two opportunities or dividing Siberian Iris. To insure flowering it is best to divide in early spring as new growth just appears. Waiting until the new growth has started may stress out the plants and prevent them from flowering that season. Dig up the clump and cut through the thick root system, keeping at least six growing points with adequate roots in each clump. Keep these sections moist, plant immediately, and then water in thoroughly. In late summer or early fall it is possible to cut back the foliage to about six inches, then dig and divide the plant as before. Replant, water in, and mulch well for winter survival. Siberian iris don’t require division as often as bearded iris to perform at their best; their tough crown often requires a strong arm to cut them apart.

Dwarf crested iris.

Creative landscaping with siberian iris

Japanese roof iris

More irises to consider

Iris cristata or dwarf crested iris blooms in mid to late spring appearing almost stemless at just six inches tall. It tolerates soils with low fertility.

Iris pumila or dwarf bearded iris blooms in mid spring and like dwarf crested iris is about 6” tall.

Iris ensata or Japanese iris bears an upward facing flatter version of iris flower. It thrives in damp, acidic soil. Typically hardy to zone 5.

Iris pallida ‘Variegata’ has striking foliage striped with gray-green and pale yellow to cream.

Iris pseudoacorus or yellow flag grows as a bog plant. Typically hardy to zone 5.

Iris reticulata or dwarf Dutch iris blooms in early spring. A favorite for rock gardens, it is typically hardy to zone 5 and often forced as a spring flowering bulb in mixed bulb containers.

Iris tectorum, sometimes known as Japanese roof iris, was grown on thatched roofs in Japan. It grows in damp to wet soil and will naturalize and tolerate more shade than other irises.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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