 
Source: virtual.clemson.edu
Garden irises are hardy, long-lived perennials that need a minimum of care. They are an established "backbone" of home gardens because they bloom when few other plants do. Easy to grow iris varieties adapted to every region of the United States are available. They produce graceful flowers in a wide range of shapes, sizes, and colors. |
| Description
Iris flowers have 6 petals. The 3 upright petals are called standards; the 3 that hang down are called falls. Flowers may be white, yellow, pink, purple, blue, reddish, or bi-colored. Principal types of irises are bearded, beardless, crested and bulb. Bearded irises have a fuzzy strip, or beard, that runs down the middle of the falls. They are called German iris. Iris germanica is the most commonly grown bearded species. Bearded irises live through severe droughts and cold. Flowers appear in late spring to early summer on stalks above the foliage. The leaves are stiff and strap-like, grow in fan-shaped clumps, are evergreen in warm climates, and remain green until late fall in cold climates. Bearded irises are available in a wide range of heights; miniature dwarf, up to 10"; standard dwarf, 10-15"; miniature tall, 15-25"; intermediate, 15-27"; and border, 28". Despite size differences, all bearded irises thrive in full sun and well drained soil. Beardless irises are called apogoniris or apagons. They have smooth fall petals and thin grasslike leaves. Plants grow 1-4 feet tall and most varieties bloom in June. Japanese, Louisiana, and Siberian irises are the most commonly grown beardless species. Japanese and Louisiana irises thrive in constantly moist soil or even standing water, while Siberian irises can tolerate moist conditions as well as those of a typical perennial border. They all prefer acidic soil and require regular fertilization through the growing season. Unlike bearded irises, these should be transplanted in fall or early spring and require heavy watering during establishment. All but Louisiana irises should be planted in sunny locations where they can remain for several years. They do not thrive when disturbed too often. Borers do not readily attack these irises. Crested irises have a *censored*scomb-like crest along the lower half of the falls, instead of a beard. They range in height from 6-12" and bloom in May if provided a moist, shaded location with soil rich in organic matter. |
| How Irises Grow
Irises grow from thick, underground stems, called rhizomes, that store food produced by the leaves. Rhizomes grow slightly below the surface of the ground or at ground level. Many small roots penetrate the soil deeply. Each year, off-shoots develop from the original rhizome. These off-shoots may be divided and transplanted to grow new irises. Irises grown from rhizomes should bloom the next spring after planting. Irises may also be grown from seed. A seedpod may develop below a pollinated flower that is left on the stalk after blossoming. Most seedlings do not bloom for 2-3 years after planting. The slow process of growing plants from seed is used chiefly by breeders to develop new varieties. Because irises are hybrids, flowers of seedlings rarely look like flowers of parent plants. |
| Planting Irises
The planting procedure is the same whether planting a new plant or division. Cultivate the soil 12-18 inches deep and mix in composted organic material. Dig a shallow hole large enough to receive the rhizome or clump of rhizomes you are planting. Form a cone of earth in the center of the hole for the planting base. Adjust the height of the cone, or planting depth, so that the rhizome is just beneath the soil's surface. Place the rhizome on the cone, parallel with the ground surface. Carefully spread the roots around the cone. Do not wad the roots together. Fill the hole with soil and press it firmly in place around the rhizome. Water immediately, thoroughly soaking soil around the roots. To obtain a good display of iris color, use at least 3 rhizomes of the same variety in a triangle or pattern that alternates plants in rows. Plant rhizomes about 18 inches apart. Point each fan of leaves away from others in the group. If you want to produce masses of flowers quickly, plant undivided rhizome clumps or set 3 individual rhizomes 8-10 inches apart. Before replanting a full grown iris, cut leaves to 1/3 their full height to prevent the plants from being whipped by the wind and pulled from the soil. |
| Care of Plants
Water plants often enough before blooming time to keep soil moist but not wet. Remove weeds and grass around the rhizomes. Before plants bloom, loosen the surface soil with a hoe or hand cultivator. Be careful not to injure the rhizomes or the roots. Cut flowers as soon as they fade, unless you want seeds. Plants that are growing well with good green foliage usually do not need fertilizer. If you use fertilizer, apply it immediately after the plants bloom. Work it into the soil around the base of the plants. Use about ½ cup of 5-10-5 fertilizer for 6 small plants or about 1 cup for a large iris clump. In early fall, cut leaves to 6-8 inches from the ground. All irises need mulch the first season after planting. Apply a light mulch of straw or evergreen boughs after the ground first freezes. Mulch prevents roots from freezing and stops the alternate freezing and thawing of the soil that harms plants by pushing them out of the soils. |
| Propagating Irises
When plants become crowded, divide the offshoots from the rhizomes. Irises should be divided 2-5 years after planting. Divide and transplant irises in the late summer or early fall (beardless irises should be divided in early spring), after the plants have bloomed. Cut leaves to 1/3 their full height, dig under a clump of rhizomes, and lift out the whole clump. Wash away the soil with a steady stream of water. Make small divisions if you do not want to re-divide iris for at least 3, perhaps 5, years. Make large divisions if you want many flowers the year after planting. Large divisions should be separated in 2-3 years. Cut rhizomes apart with a sharp knife. Each division must have at least one growing point, or fan of leaves, a few inches of healthy rhizome, and a number of well developed roots. When separated from the original iris clump, each division is ready to plant. |
| Iris Diseases
Iris diseases reduce the number of flowers, disfigure the leaves, and sometimes kill the plant. Prevent diseases by giving plants plenty of space, sunlight, and good drainage. Clean up dead material quickly. Do not plant irises in crowded or completely shaded areas. Contact your local extension agent for chemical control measures for specific diseases. Bacterial Soft Rot is the most destructive iris disease. Bacteria enter the plant through breaks in the rhizome. Leaf bases and rhizomes begin rotting and the plant soon dies. If this happens, dig up and destroy the diseased rhizomes. Fungus Rots include sclerotic rot and Botrytis. Sclerotic rot, or southern blight, attacks irises in warm, humid areas. A fungus affects plants at or near the soil surface. The leaves turn yellow and dry prematurely or rot off at the base. Small, yellowish brown, seedlike structures appear. Botrytis rhizome rot, occurs in cool areas. The fungus produces small, black, seedlike structures on the rhizomes and in the soil. A dry, pithy, gray rot develops in the leaf bases and rhizomes. Dig and burn plants that are seriously infected with either kind of fungus rot. Remove soil from the surrounding area and replace with new or sterilized soil. Iris Leaf Spot disfigures leaves and weakens plants. About flowering time, infected leaves are dotted with small, brown spots. Water soaked margins around the spots turn yellow. Spots later develop a grayish center with black fruiting tufts. The leaf spot fungus overwinters in old leaves and produces new spores in the spring. In mild climates, cut and burn the leaves of infected plants in the fall. If leaves are not removed, the fungus may remain active throughout the winter. In cold areas, remove dead foliage before shoots appear in the spring. Rust and Bacterial Leaf Spot weaken, but seldom kill, iris plants. Rust produces small, raised, dark red dots on iris leaves. Bacterial leaf spot causes dark green, watery spots, and streaks. The spots later turn yellow and become trans-lucent. Remove and burn all leaves that show signs of rust or bacterial leaf spot. Do not let any diseased leaves remain around the plants; infected leaves harbor spores that spread rust and leaf spot. Root-knot nematodes and meadow nematodes are microscopic worms that attack irises and a wide range of other plants. Root-knot nematodes cause distinct knots or galls on the roots. These knobby swellings on a root look like beads on a string. When meadow, or root-lesion, nematodes attack iris, the roots discolor and decay. In advanced stages of infestation, many roots rot off. Small, lateral roots that replace the rotted ones give the root system a matter or turfed appearance. Younger, newer roots are dotted with small reddish-brown spots. Remove and burn plants with knotted roots or unthrifty plants with extensive root decay. Do not replant irises in the same place until nematodes have been eliminated. Iris Mosaic, the most widespread disease of irises, is caused by a virus transmitted by aphids. Diseased flowers may be mottled or striped. Light green streaks appear on the leaves of some plants. Many infected plants do not show signs of disease. Individual plants may have typical symptoms at one season of the year and appear disease free at another season. Dig up and burn irises that show severe mosaic damage. Reduce the spread of iris mosaic by controlling aphids. |
| Iris Insects
The iris borer causes more damage to iris than all other insects. The pink, caterpillar-like larvae have rows of black spots along their sides. They are about 1 ½ inches long when full grown. Iris borer adults are large brown moths with black markings. First symptoms of borers are tear stains and chewed leaf edges that appear on leaves in early spring. Irises later develop loose, rotted bases and holes in rhizomes. Borer larvae hatch in early spring from overwintering eggs. These caterpillars pierce leaves and tunnel into the stem. Then they bore into the rhizome, where they remain to feed and grow. At maturity, larvae leave the rhizome and pupate into the soil. Bacterial rhizome rot readily attacks borer-infested plants. To eliminate overwintering eggs, clean up and destroy old leaves, stems, and debris in fall or winter. Aphids, or plant lice, are small green, pink, or mealy-white insects that attack many plants. Aphids may appear on iris plants in early spring. They pierce leaves and suck the juices. When they feed, they may transmit the virus that causes iris mosaic. Verbena Bud Moth larvae tunnel into new iris shoots and buds. Larvae are about ½ inch long and have greenish-yellow worm-like bodies and black heads. Mature moths do not attack irises. Infested shoots and buds should be cut and burned. Iris Thrip larvae and adults pierce the surfaces of young leaves and leaf sheaths. They suck juices that ooze from the wounds. Dry wounds become small, straw-colored spots, flower buds blacken, and plant tops weaken. Iris thrips are especially injurious to Japanese iris. Larvae of iris thrips are milky white. The black-bodied adults are usually wingless and are about 1/25 inch long when mature. Contact your local extension agent for recommended chemical control measures. |