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Extension > Garden > Yard and Garden > Flowers > Iris for northern gardens

Iris for northern gardens

Deborah L. Brown, Extension Horticulturist Department of Horticulture

Copyright © 2013  Regents of the University of Minnesota. All rights reserved.

Few garden flowers have enjoyed the spectacular development and improvement that the iris has in the past fifty years. Today we have thousands of beautiful cultivars of bearded, Siberian and Japanese iris in many colors, sizes and forms. These modern hybrids may require more attention than older types, but in return they offer larger, more numerous flowers and an expanded color range.

There are more than 300 species of iris worldwide; many types are suitable for northern gardens. Most common are the tall bearded iris hybrids with flowering stems over 28 inches high. Shorter bearded iris are also available. Siberian iris are somewhat less common, though they’re actually easier to grow successfully than bearded iris. And for gardeners who like more of a challenge, Japanese iris, which demand special care, might be just the thing to provide an unusual touch of elegance to the garden.

Proper division for replanting. Discard the old central portion of the rhizome. Plant the side "fans" for bloom next year.

Growing Bearded Iris

Bearded iris are available in sizes that range from only a few inches (miniature or dwarf) to nearly four feet. The flowers themselves come in almost every color and combination imaginable, from white to near black, bronze to green, blue to nearly red, pink to yellow. They get their name from the fuzzy patch or “beard” on their down-turned petals or “falls.” The true petals or “standards” arch upward to form the center of the ruffled blossoms.

Bearded iris bloom in early to late spring in Minnesota (depending on type), then go into a period of “summer dormancy” soon afterwards. To prevent seed production, remove flower stems as soon as they’re through blooming, but do not cut back the foliage until it yellows. This allows the plants to store food for the next season of bloom. A few bearded iris may rebloom in fall if left undisturbed, although the late blooms can be touched by early frost.

Like other iris, bearded types are sold as bareroot divisions or as potted plants bearing a cultivar name. A single division of a bearded iris plant will have a thickened rhizome with a “fan” of leaves at one end and fat roots coming off the rhizome. The best time to plant these divisions is in late summer, from mid-July to Labor Day, when the plants will have time to develop new roots and become established before winter sets in.

Choose a sunny, well-drained site for your iris garden—the sunnier, the better. Make sure standing water does not collect there in spring or after heavy rainfall. Work plenty of compost or peat moss into the soil before planting, and add two pounds of 5-10-10 fertilizer for every 100 square feet of garden.

Dig a shallow hole for each division, leaving a ridge in the center. Place the rhizome over this ridge with the roots spread out on either side. Check the depth to be sure the rhizome is just below the soil surface. In very sandy soil, the rhizome may be planted a little deeper, but no more than one inch below the surface.

Space the rhizomes at least eight inches apart to allow for future growth. When planting several iris of the same variety, arrange them in drifts with the fans pointing in the same direction, bearing in mind that the new plants will come from the fan end and sides of the rhizome. You could also plant rhizomes in a tight circle with the fans pointing outward in a circular clump.

Bearded iris need the same good care that most flowering perennials require: deep watering in dry weather, regular weeding to reduce competition for moisture and nutrients, and fertilizing once or twice each year with 5-10-10 or similar fertilizer. Do not fertilize after mid-August, so plants have time to prepare or “harden off” before winter.

Summer dormancy provides an opportunity to lift and divide older plants. Established plants need to be divided every third or fourth year or whenever the clumps become crowded and flowering decreases. Use a spading fork to lift the entire clump, being careful not to break the fat feeder roots. Wash off the soil and use a sharp knife to separate the rhizomes into individual fans. Discard the bloomed-out center portions and use only the vigorous healthy fans from the outside of the clump. Cut back the leaves to about six inches and trim off any broken roots. You can label these divisions with the cultivar name and store them for a few weeks in a cool well-ventilated place before planting, if you wish, or you may plant them outdoors immediately.

After the first hard frost in fall, cut back the tops of iris plants to about six inches and clean up any remaining iris debris. Once the ground freezes in November, mulch the bed with four to six inches of loose weed-free compost, straw or leaves. As ice and snow melt in spring, remove the top layer of mulch to allow air and sunlight to dry the surface. Wait a few days before removing the rest of the mulch, taking care not to break the center portion of the fans, where flower buds are developing. Do your final clean-up on a sunny day, after the soil has dried.

Iris borers are the most destructive pests of bearded iris. Eggs which are laid in fall hatch early the following spring. Then, as tiny caterpillars, they bore through the leaves and eat their way down into the rhizome. As they feed, borers introduce bacteria that cause a soft rot that ultimately turns leaves brown and destroys the plants.

To control iris borers, remove and dispose of any infected rhizomes and leaves. Clean up dead leaves and debris after hard frost in fall to remove the eggs and reduce borer numbers the following year. It may be necessary to spray with dimethoate, a systemic insecticide sold as Cygon. Apply it in spring when new growth is only four to six inches tall. Be sure to read and follow label directions carefully.

Growing Siberian Iris

Siberian iris are enjoying increasing popularity, which is understandable given how easily they may be grown. Unlike bearded iris, they tolerate a wide range of soil moisture conditions, thriving in damp and even wet areas, since they grow from fibrous roots rather than fleshy rhizomes. They also bloom in partial shade—though not as well as in full sunlight—and they’re not bothered much by iris borer, bacterial soft rot or other iris problems.

Known for their graceful appearance, Siberian iris produce copious amounts of arching grass-like leaves that stay green all summer. Late each spring they burst forth with delicate beardless flowers in a rainbow of colors: blues, purple, maroon, white, pink, and yellow.

You can divide or plant Siberian iris either in spring or in late summer, the same time as bearded iris. If they’re sold in pots they can be planted any time before Labor Day. Like bearded iris, Siberian iris may need to be divided every three or four years. Select a sunny site that receives ample moisture in spring. Be sure to dig a hole that’s wide enough to allow plenty of room for the spreading roots and the large clump that will develop over time.

Other Types of Iris

Japanese iris (Iris ensata or Iris kaempferi) flowers are distinctly different from those of bearded or Siberian iris; they’re very large and frilly with flatter flowers than bearded iris. They are also fussier in that they only thrive in acidic soil, they require ample watering prior to blooming, and they need good winter protection in our climate.

Two other beardless iris are sometimes planted in wet areas. The yellow flag of Europe (Iris pseudoacorus) and our native blue flag (Iris versicolor) are both moisture-loving plants that can be naturalized along streams or lakes. Both are tall, growing two to four feet in height, and are good for providing late spring color in a naturalized landscape. However, there are better choices for perennial displays where the site is not moist.

Crested iris offer another flower variation; instead of “beards” on their falls, they have raised serrated ridges. Iris cristata is a dwarf iris with light violet petals and whitish crests outlined in purple. roof iris, Iris tectorum, a native of China, has bright lilac blossoms borne on branched flower stalks about a foot and a half tall.

Buying Iris

It takes just as much time, energy and space to grow an inferior iris as it does to grow superior varieties. The table below lists just a few of the hundreds of cultivars that grow well in Minnesota. Local nurseries and iris experts can help you choose others that will thrive in your garden. For information about local and national iris societies, garden books, or for help finding a specific iris cultivar or species, contact the Anderson Horticultural Library at the Minnesota Landscape Arboretum in Chanhassen by calling 612-443-2440.

Some Recommended Iris Cultivars for Minnesota

Cultivar/species Type Color Height Time of Bloom
Lemon Puff miniature dwarf bearded light yellow 8" early May
Iris cristata crested iris light violet 3" early May
Beechfield standard dwarf bearded yellow 12" mid-May
Cheers intermediate bearded white 14" mid-May
Batik border bearded purple & white 24" late May
Bride's Halo tall bearded yellow & white 36" late May
Beverly Sills tall bearded pink 35" late May
Edith Wolford tall bearded yellow & blue 35" late May
Laced Cotton tall bearded white 36" late May
Ruffled Violet Siberian deep violet 28" early June
Kingfisher Blue Siberian light blue 32" early June
White Swirl Siberian white & yellow 36" early June
Joretta Siberian deep blue 40" early June
Iris techtorum roof iris bright lilac 16" early June
Iris pseudoacorus yellow flag yellow 36" mid-June
Henry's White Japanese white 36" mid-June
Iris versicolor blue flag blue 30" mid-June

 

This publication is based on an earlier version written by Anne Hanchek, former extension horticulturist, and Leon C. Snyder, former director of the Minnesota Landscape Arboretum.

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

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