Iris

Prepared by Karen Russ, HGIC Horticulture Specialist and Bob Polomski Extension Consumer Horticulturist, Clemson University (New 06/99.)

HGIC 1167

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There are hundreds of species and cultivars of iris in all colors of the rainbow. Iris vary from tiny woodland groundcovers to dramatic flowers for the sunny border to species that thrive in swampy soil. There is an iris that will do well in virtually every garden.

Crested Iris a low growing native species
Crested Iris (Iris cristata), a low growing native species.
Joey Williamson©2007 HGIC, Clemson Extension

Height/Spread

The many different species vary from low ground covers such as Iris cristata at only 6 inches tall to some of the large Japanese iris at 3 to 4 feet tall. Bearded iris ranges from about 6 inches in the miniatures to more than 3 feet in the large types.

Growth Rate

Iris are dependable, long-lived perennials. Their growth rate varies by species and type.

Ornamental Features

Iris are grown for their graceful flowers in an endless array of brilliant colors. The bold sword shaped foliage is also an excellent contrast to the more mounded forms of many garden plants.

Landscape Use

Most iris, especially bearded iris, will grow best with full sun for at least 6 to 8 hours a day. In very hot areas though, some shade in the afternoon will help keep flower colors from fading in the heat. Iris should be planted in an area with good air circulation to help prevent disease problems.

Most iris need very well-drained soil. Japanese and Louisiana iris will grow in wet soil. If your soil is not ideal you can amend it with organic matter and build raised beds for better drainage. Do not use manure unless it is very well-composted (aged for at least one year). Manure can encourage iris soft rot.

Bearded iris prefer slightly alkaline soil. Many of the beardless iris like a more acid soil. It is a good idea to test your soil and amend the soil before planting a new iris bed.

Fertilize a new iris bed when preparing the soil before planting with a complete fertilizer low in nitrogen and high in phosphorus and potassium. Follow soil test recommendations for best results. In the absence of test results apply 1 pound of 5-10-10 per 100 square feet. Work the fertilizer into the soil and let the bed settle before planting.

When feeding established iris, do not let fertilizer touch the rhizomes. It is better to underfeed than to overfeed bearded iris. Reblooming varieties, however, are more likely to rebloom with supplemental food and water after spring bloom.

Remove old blooms and stalks promptly after flowering to allow the plant to devote its energy to growth rather than seed. Removing old blooms and stalks also encourages repeat flowering on reblooming iris.

The best time to plant bearded iris is from six weeks after bloom is finished through September, or October near the coast. This will allow them to become well-established before winter. Japanese, Louisiana and Siberian iris can be transplanted during the summer and early fall. Container-grown iris can be planted in the spring.

Iris Roots and Rhizome

Bearded iris are grown from a fleshy, bulb-like stem called a rhizome that grows horizontally just below the soil surface.

Plant Iris with the rhizome high in the soil

Plant iris with the rhizome high in the soil, and the roots well-anchored. Dig two trenches with a ridge between them, place the rhizome on the ridge and spread the roots carefully in the trenches. Then fill the trenches with soil, letting the top surface of the rhizome be just barely beneath the surface of the soil. In heavy clay soils the rhizome should be planted higher so that up to half of the rhizome is exposed above soil level. Firm the soil well and water thoroughly.

After three to five years, iris generally become crowded and should be divided.

Problems

Poor flowering is normally due to planting in excessive shade, using too much fertilizer, planting the rhizomes too deep, or plants that have become too crowded and need dividing.

Bacterial soft rot is the most serious iris disease. Soft rot causes the rhizomes to become mushy and have a disagreeable odor. Remove any yellowing leaves promptly to help prevent spread of the disease. Iris leaf spot, caused by a fungus, is the most common disease. Remove all leaf and other debris in fall, since diseases and insects often overwinter on old foliage.

Iris borer is the most serious insect pest of iris. Bacterial soft rot readily attacks borer-infested plants. Aphids can be a nuisance problem at times.

Types, Species & Cultivars

The iris most often grown in South Carolina fall into two main groups: Bearded iris and Beardless iris.

Bearded Iris: These iris are identified by thick, bushy "beards" on each of the falls (lower petals) of the blossoms. They are divided into six groups based on size. The smaller iris generally bloom earlier in the growing season.

Bearded Iris Parts

Miniature Dwarf Bearded: These are the tiniest bearded iris, with stems from 2 inches to 8 inches tall. They are also the earliest to bloom. They are grown in rock gardens or planted in low drifts at the front of the flowerbed.

  • 'Bantam' is a ruffled deep red-purple.
  • 'Scribe' is white with blue edging.
  • 'Zipper' is golden yellow with blue beards.

Standard Dwarf Bearded:These iris range in height from 8 inches to 15 inches. They bloom early in the iris season.

  • 'Bay Ruffles' is a ruffled light blue.
  • 'Software' is a pinkish cream edged in apricot.
  • 'Starlight' is a ruffled cream.
  • 'Violet Lulu' is a soft violet.
  • 'Watercolor' has yellow standards with brown falls.

Intermediate Bearded: These iris stand from 16 inches to 28 inches high. They are large enough that their individual stalks may be nicely branched, forming an elegant bouquet.

  • 'Baby Blue Marine' is light blue.
  • 'Brighten Up' is orange with coral beards.
  • 'Piece of Cake' is pink with orchid markings.
  • 'Red Zinger' is deep red.

Border Bearded: These are in the same height range and bloom size as the intermediates, but blooming later with the tall beardeds.

  • 'Chickee' is a ruffled deep yellow.
  • 'Disco Jewel' is reddish-brown with violet.
  • 'Loreley' is yellow and violet.
  • 'New Wave' is a clear white.
  • 'Rosemary's Dream' is white and orchid.

Miniature Tall Bearded: These iris have blooms that are smaller than on a border bearded on thin and wiry stems. They are well-suited for arrangements.

  • 'Just Jennifer' is white.
  • 'Pink Bubbles' is pink.
  • 'Predictions' is pink.
  • 'Tulare' is yellow.

Tall Bearded: These have stalks over 28 inches tall, extending to approximately 40 inches in height. Each individual stalk makes a stately arrangement in the garden or in a vase. Tall bearded iris have ruffled edged petals or other embellishments more often than other groups of iris. Tall beardeds are the most popular and commonly grown iris type. There are thousands of cultivars of tall bearded iris. In the South, we can grow a number of cultivars that bloom in the spring and then rebloom in late summer or fall.

  • 'Dusky Challenger' has large, ruffled, dark blue flowers on 39-inch stems.
  • 'Silverado' is a cool blue-white self that stands 38 inches tall on sturdy stems.
  • 'Jesse's Song' has white flowers with a striking violet border.
  • 'Beverly Sills' has coral pink flowers with good form on plants 3 feet tall.
  • 'Edith Wolford' has canary yellow standards and blue-violet falls, on 40 inch tall stems.
  • 'Before the Storm' is a near black, heralded as one of the darkest of the darks. It has a slight fragrance and excellent growth habit.
  • 'Laced Cotton' has pale lavender flowers fading to pure white on 36 inch tall plants.
  • 'Superstition' is a maroon-black, with three flowers open at a time on well-branched, heavily budded stems.

Reblooming Tall Bearded Iris:

  • 'Bountiful Harvest' has white standards with a violet edge and over-all peppering. It is a dependable rebloomer in late summer.
  • 'Champagne Elegance' has white standards and apricot falls on 34 inch tall stems. It consistently reblooms in late summer.
  • 'Eleanor Roosevelt' has deep-red purple blooms on a relatively short plant. This old cultivar is very dependable.
  • 'Immortality' has pure white flowers and re-blooms in late summer. This vigorous grower will reach 30 inches tall.
  • 'Sugar Blues' has large, sweetly fragrant, wisteria blue flowers. It is somewhat slow to increase, but grown because of its beautiful flower color and reliable rebloom over a long season. It grows to 3 feet tall.

Sugar Blues Iris re-blooming in August
'Sugar Blues' Iris re-blooming in August
Karen Russ, ©2007 HGIC Clemson Extension

White Flag Iris (Iris albicans): This historic iris was once very common throughout the South. It has off-white flowers that bloom in March above gray-green leaves. This old iris is heat-tolerant and durable.

Florentine Iris (Iris germanica 'Florentine'): This is a very old cultivar that has been grown for centuries for its scented rhizome, used in perfumery. The flowers are a very pale grey-blue, almost white. It blooms in April.

Dalmation Iris (Iris pallida): This historic iris is well adapted to growing in the South. It typically has pale blue flowers above gray leaves and blooms in late April. There is a beautiful variety with yellow striped leaves.

Beardless Iris: These types of iris have different growing needs than bearded iris. Siberians will tolerate light shade but Japanese and Louisiana Iris need full sun. Louisiana and Japanese iris require moist conditions during the summer months. All are moderate to heavy feeders and need to be fertilized regularly. Most do best in somewhat acid soil, between pH 5.5 and 6.5.

Beardless Iris Parts

Siberian Iris: These are excellent landscape plants, easy to grow, with elegant vertical blue-green foliage that looks good throughout the growing season. The blooms are mostly blue, violet and white with large falls and smaller standards. They grow to a height of 2 to 4 feet. Siberian iris thrive in moist soil, but do not like standing water. They will also tolerate most ordinary garden soil and are among the easiest iris to grow in most regions.

  • 'Butter and Sugar' has bright yellow falls and wide, creamy white standards.
  • 'Halcyon Seas' has large flowers with deep blue standards and shaded violet falls.
  • 'Pink Haze' is pink-lavender with darker falls and lighter edges.
  • 'Sky Wings' has very light blue standards with blue falls. A yellow blaze radiates to blue-violet veins.
  • 'Flight of Butterflies' has graceful small blue flowers on 2½ foot tall plants.
  • 'Caesar's Brother' has dark purple flowers on 36 inch tall plants.
  • 'Eric the Red' has dark wine-red flowers and grows 36 inches tall.
  • 'Orville Fay' has medium blue flowers with darker blue veins on a 36 inch tall plant.
  • 'Snow Queen' has white flowers and grows to 32 inches tall.
  • 'Super Ego' has light blue standards, with lighter falls on 30 inch tall stems.

Japanese Iris (I. ensata): These require a slightly acid soil and have the most spectacular flowers of all the iris. Blooms are usually huge, ruffled and flat in form; some are marbled with gray or white. They bloom about a month after tall beardeds. Japanese iris will flourish in wet environments, even in shallow water. These iris are heavy feeders and require lots of organic matter for nutrients. They need six hours of full sun.

  • 'Light in Opal' has large, lavender flowers and tall bright-green foliage.
  • 'Rikki Pikki' has long-lasting white flowers are on 32-inch stems.

Striped foliage is a major feature of Iris Ensata Variegata
Striped foliage is a major feature of Iris ensata 'Variegata
Karen Russ, ©2007 HGIC, Clemson Extension

  • 'Variegata' has slender, purple flowers above stunning variegated foliage. Grows to 18 to 22 inches tall.
  • 'Acclaim' has stunning red-violet standards and falls with white styles.
  • 'Azure Perfection' is a ruffled dark blue-violet with extra purple styles.
  • 'Ruffled Dimity' is a lovely, late-season, white with deep blue veins and purple styles.
  • 'Sapphire Star' has red-lavender standards, and lavender falls, with white halos.
  • 'Summer Moon' is a creamy white, ruffled semi-double flower.
  • 'Cascade Crest' is white with a wide light blue band.
  • 'Geisha Obi' is wine-red with white rays and styles.

Louisiana Iris: These iris are native to the Gulf Coast. The blooms are very wide-petaled and brightly colored. Louisiana iris need at least a half day of sun, a neutral or acidic soil, and plenty of fertilizer and water. Sandy or heavy clay soils should be amended with organic matter. New growth appears in fall, and in mild winters the foliage remains erect and green.

Louisiana iris rhizomes should be planted deeper than other iris, at least 1 inch under the soil, then mulched with 2 to 4 inches of compost.

  • 'Cajun Sunrise' features brownish-red petals edged in yellow.
  • 'Delta Star' is a dark purple cartwheel-shaped flower with narrow yellow signals. It grows especially well in water.
  • 'Dixie Deb' is tall with medium-sized yellow blooms. It increases well and grows under almost all conditions.
  • 'Marie Caillet' grows and blooms under almost any conditions. Its blue-purple flowers appear on tall, branching stalks.
  • 'Professor Fritchie' towers as much as 5 feet tall. It is one of the hardiest of the tetraploid varieties.
  • 'Professor Ike' is purple, with blue-violet standards and full petals.
  • 'Professor Neil' is the reddest of red Louisiana iris.

Dwarf Crested Iris (Iris cristata): This small native iris thrives in lightly shaded gardens. Light blue flowers in early spring with attractive miniature foliage throughout the growing season. Plant the rhizome at ground level rather than burying in the soil. Prefers infertile, well-drained soil.

Yellow Flag Iris (Iris pseudacorus): Moisture-loving vigorous iris grows 4 to 5 feet tall with butter-yellow flowers. Although it tolerates well-drained areas, it is happiest in 3 to 6 inches of water or areas that stand in water periodically.

Blue Flag Iris (Iris versicolor):Blue flag is a beautiful native iris that grows in damp areas in the eastern United States. Lavender-blue flowers on 3-foot stems during May and June.

Dutch Iris: These slender, graceful flowers are grown from bulbs. Dutch iris bloom in early summer in deep and light blue, purple, yellow and white on 24 inch tall stems.They prefer sun or afternoon shade and rich, well-drained soil. Plant bulbs 4 to 6 inches deep in October or November.

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