Prepared by Karen Russ, HGIC Horticulture Specialist and Bob Polomski Extension Consumer Horticulturist, Clemson University (New 06/99.)
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 (Iris cristata), a low growing native species.
Joey Williamson©2007 HGIC, Clemson Extension
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.
Iris are dependable, long-lived perennials. Their growth rate varies by species and type.
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.
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.
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, 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.
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.
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.
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.
Standard Dwarf Bearded:These iris range in height from 8 inches to 15 inches. They bloom early in the iris season.
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.
Border Bearded: These are in the same height range and bloom size as the intermediates, but blooming later with the tall beardeds.
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.
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.
Reblooming Tall Bearded Iris:
'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.
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.
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.
Striped foliage is a major feature of Iris ensata 'Variegata
Karen Russ, ©2007 HGIC, Clemson Extension
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.
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.
Page maintained by: Home & Garden Information Center
This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.