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Home , Iris (plant), Rhizome

Utah State University DigitalCommons@USU

Gardening Current USU Extension Publications

Winter 1-1-2005

Growing

Utah State University Extension

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Recommended Citation Utah State University Extension, "Growing Iris" (2005). . Paper 12. https://digitalcommons.usu.edu/extension_curgarden/12

This Article is brought to you for free and open access by the Current USU Extension Publications at DigitalCommons@USU. It has been accepted for inclusion in Gardening by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. GROWING IRIS 51 South University Provo, UT 84601 801-851-8460 bearded iris grow 2 to 3 feet tall. Garden irises are hardy, long-lived perennials that need a minimum of care. Because they are easy to grow, tall bearded They are an established “backbone” of home irises are recommended for beginning gardens because they bloom when few other gardeners. plants do—after spring-flowering and before , delphinium and phlox. Usually, tall bearded irises bloom in May and June. Several varieties bloom in both Easy-to-grow iris varieties adapted to every spring and fall. region of the United States are available. They produce graceful in a wide A group of bearded irises that naturally range of shapes, sizes and colors. grow 4 to 9 inches tall are called dwarf iris. The two most common are I. pumila DESCRIPTION and I. chamaeiris. Both are well adapted to rock gardens because they spread quickly Iris flowers have 6 . and form dense mats of foliage. They bloom The 3 upright petals are in March, April and May. called standards; the 3 that hang down are called Beardless irises are called apogoniris or falls. Flowers may be white, yellow, pink, apogons. They have smooth fall petals and purple, blue, reddish or bicolored. thin, grasslike . Plants grow 1 to 4 feet tall. Most varieties bloom in June. Principal types of irises are bearded, beardless, crested and . Japanese (I. kaempjeri) and Siberian (I. sibirica) irises are the most Bearded irises have a fuzzy line, or beard, commonly grown beardless species. that runs down the middle of the falls. They are called German iris or pogoniris. Iris Japanese irises have soft, drooping germanica is the most commonly grown standards and wide falls. Plants grow bearded species. 2 to 4 feet tall. Flowers are borne on long stems. Bearded irises live through severe droughts Siberian irises have stiff, narrow falls and and cold. The sword- narrow, upright standards. Stems grow 18 shaped leaves are inches to 2 feet tall. Beardless types, which evergreen in warm thrive in moist soil, frequently are planted climates and remain on stream and lake banks. green until late fall in cold climates. Most Crested irises have a small raised area, or crest, on the middle of each fall. Often, the Many new varieties are introduced each color of these crests contrasts with year. Growers’ and breeders’ catalogs and colors. One of the more popular crested garden magazines describe and picture many irises is a dwarf species, I. cristala. popular irises.

HOW IRISES GROW PLANTING IRISES

Bearded, beardless, and crested irises grow The best time to irises is in late from thick, underground stems—called summer or early autumn. They should be —that store food produced by the established in the soil before winter. leaves. Most garden supply stores sell rhizomes Rhizomes grow slightly only during the planting season. If you order below the surface of the irises by mail, usually you will receive ground or at ground level. rhizomes at the planting time recommended Many small penetrate for your locality. the soil deeply. Plant rhizomes as soon as practical after you Every year, underground offshoots develop receive them. from the original . Offshoots may be divided and transplanted to grow new irises. Where to Plant Irises need full sunshine. Select a site with A rhizome that will produce a plant has at southern exposure and good air circulation. least one or growing point. Each bud produces a large fan of leaves and a Bearded and crested irises need lime soil stalk. Irises grown from rhizomes should with good drainage; rhizomes may rot in soil bloom the next spring after planting. that holds water around them. Beardless types need moist soil that is slightly acid. Irises may be grown from . A seedpod may develop below a pollinated flower that Preparing the Soil is left on the stalk after blossoming. Most Prepare the bed 1 to 2 weeks before planting seedlings do not bloom for 2 or 3 years after irises to allow the soil time to settle. Dig and planting. loosen the soil at least 18 inches deep. Thoroughly break up all lumps. The slow process of growing plants from seed is used chiefly by breeders to develop Use commercial fertilizer to enrich poor soil new varieties. Because irises are hybrids, in the iris bed; use organic matter to flowers of seedlings rarely look like flowers improve soil structure and productivity. of parent plants. For poor soil, add ½ pound of a 5-10-5 VARIETIES fertilizer for each 5- by 10- foot area, or ½ cup for every 6 or 7 rhizomes. Thoroughly From the several thousand varieties of irises mix fertilizer into the soil so that lumps of it available, select varieties that will provide do not touch iris roots. the colors you want in your garden. Spading organic matter—compost, well- rotted manure, or peat moss—into relatively leaves to one-third their full height. heavy soil may improve drainage. CARE OF PLANTS How to Plant In a well-prepared bed, dig a shallow hole Water plants often enough before blooming large enough to receive the rhizome or time to keep soil moist but not wet. Remove clump of rhizomes you are planting. Form a weeds and grass around the rhizomes. cone of earth in the center of the hole for the planting base. The height of the cone—or Before plants bloom, loosen the surface soil planting depth—is determined by the type of with a hoe or hand cultivator. Be careful not garden soil. to injure the rhizomes or the roots.

In the medium soil, make the cone high as soon as they fade, unless you enough so that the planted rhizome is just want to obtain . below ground level. In light or well-drained soil, build a low cone. The top of the planted Plants that are growing well with good green rhizome should be 2 inches below ground foilage usually do not need fertilizer. If you level. use fertilizer, apply it immediately after plants bloom. Work it into the soil around In heavy soil, build a cone even with the plant bases. Use about ½ cup of 5-10-5 ground surface. The top of the planted fertilizer for 6 small plants or about 1 cup rhizome should be slightly above ground. for a large iris clump.

Place the rhizome on the cone, parallel with In early fall, cut leaves 6 to 8 inches from the ground surface. Carefully spread the the ground. roots around the cone. Do not wad roots together. All irises need mulch the first season after planting. Apply a light mulch of straw or Fill the hole with soil and press it firmly in evergreen boughs after the ground first place around the rhizome. Water freezes. Mulch prevents roots from freezing immediately; thoroughly soak soil around and stops the alternate freezing and thawing roots. of the soil that harms plants by pushing them out of the soil. Irises in northern states To obtain a good display of iris color, use at may need mulch every year, even after they least 3 rhizomes of the same in a are established. triangle or a pattern that alternates plants in rows. Plant rhizomes about 18 inches apart. PROPAGATING IRISES Point each fan of leaves away from other plants in the group. When plants become crowded, divide the offshoots from the rhizomes. Irises should If you want to produce masses of flowers be divided 2 to 5 years after planting. quickly, plant undivided rhizome clumps or set 3 individual rhizomes 8 to 10 inches Divide and transplant irises in the late apart. summer or early fall, after plants have bloomed. Cut leaves to one-third their full Before replanting a full-grown iris, cut height. Dig under a clump of rhizomes and lift out the while clump at once. Wash away Another fungus disease, Botrytis rhizome soil with a steady stream of water. rot, occurs in cool areas. The fungus produces small, black seedlike structures on Make small divisions if you want many the rhizomes and in the soil. A dry, pithy, flowers the year after planting. Large gray rot develops in the bases and divisions should be separated in 2 or 3 years. rhizomes.

Cut rhizomes apart with a sharp knife. Each Control - Dig and burn plants that are division must have at least one growing seriously infected with either kind of fungus point (or fan of leaves), a few inches of rot. Remove soil from the surrounding area; healthy rhizome, and a number of well- replace it with new or sterilized soil. Cut out developed roots. When separated from the the rotted areas of slightly damaged original iris clump, each division is ready to rhizomes. plant. Iris Leaf Spot IRIS DISEASES Iris leaf spot disfigures leaves and weakens plants. About flowering time, infected Iris diseases reduce the number of flowers, leaves are dotted with small, brown spots. disfigure the leaves, and sometimes kill the Water-soaked margins around the spots turn plant. yellow. Spots later develop a grayish center with black fruiting tufts. The leafspot fungus Prevent diseases by giving plants plenty of overwinters in old leaves and produces new space, sunlight and good drainage. Clean up in the spring. dead material quickly. Do not plant irises in crowded or completely shaded areas. Control - If iris leaf spot has been a problem in your area, spray or dust the plants every 2 Bacterial Soft Rot weeks from the time leaves emerge until Bacterial soft rot is the most destructive iris they stop growing. Use a copper fungicide disease. Bacterial enter the plant through spray or zineb dust. Follow label directions. breaks in the rhizome. Leaf bases and rhizomes begin rotting, and the plant soon In mild climates, cut and burn leaves of dies. infected plants in the fall. If leaves are not removed, the fungus may remain active Control - Dig up the diseased rhizome. If rot throughout winter. is extensive, destroy the iris. Cut out and discard diseased parts on less seriously In cold areas, remove dead foliage before affected plants. appear in spring.

Fungus Rots Rust and Bacterial leaf Spot Sclerotic rot, or southern blight, attacks Rust and bacterial leaf spot weaken, but irises in warm, humid areas. A fungus seldom kill, iris plants. affects plants at or near the soil surface. The leaves turn yellow and dry prematurely or Rust produces small, raised, dark red dots on rot off at the base. Small, yellowish, brown iris leaves. seedlike structures appear. Bacterial leaf spot causes dark green, watery spots and streaks. The spots later turn symptoms at one season of the year and yellow and become translucent. appear disease free at another season.

Control - Remove and burn all leaves that Control - Dig up and burn irises that show show signs of rust or bacterial leaf spot. Do severe mosaic damage. Reduce the spread of not let any diseased leaves remain around iris mosaic by controlling aphids. plants. Infected leaves harbor spores that spread rust and leaf spot. IRIS INSECTS

Nematode Infection Iris borer -knot nematodes and lesion nematodes The iris borer causes more damage to iris are microscopic worms that attack irises and than all other insects. a wide range of other plants. The pink, caterpillar-like larvae have rows Root-knot nematodes cause distinct knots or of black spots along their sides. They are galls on the roots. These knobby swellings about 1 ½ inches long when full grown. Iris on a root look like beads on a string. borer adults are large brown moths with black markings. When lesion nematodes attack iris, the roots discolor and decay. In advanced stages of First symptoms of borers are tear stains and infestation, many roots rot off. Small, lateral chewed leaf edges that appear on leaves in roots that replace the rotted ones give the early spring. Irises later develop loose, root system a matted or turfed appearance. rotted bases and holes in rhizomes. Younger, newer roots are dotted with small reddish-brown spots. Borer larvae hatch in early spring from overwintering eggs. These caterpillars Control - Remove and burn plants with pierce leaves and tunnel into the stem. Then knotted roots or unthrifty plants with they bore into the rhizome, where they extensive root decay. Do not replant irises in remain to feed and grow. At maturity, larvae the same place until nematodes have been leave the rhizome and pupate in the soil. eliminated. Bacterial rhizome rot readily attacks borer- Treat infested soil with a nematicide. Use infested plants. according to manufacturer’s directions. Control - To eliminate overwintering eggs, Mosaic clean up and destroy old leaves, stems, and Iris mosaic, the most widespread disease of debris in fall or winter. To kill young, irises, is caused by a virus transmitted by hatching larvae, apply dimethoate spray to aphids. the iris beds at 2-week intervals from time first growth starts until June 1. To kill older Diseased flowers may be mottled or striped. larvae in fans and rhizomes, spray with Light green streaks appear on the leaves of dimethoate. Read the directions and heed all some plants. precautions on the container label. With aid of pointed stick or pencil, locate and destroy Many infected plants do not show signs of borers in young leaf sheaths that escape dust disease. Individual plants may have typical treatment. Transplant infested iris after it flowers; destroy larvae and infested June. Do not spray during flowering. rhizomes and chestnut brown pupae in soil before replanting. Community effort is PRECAUTIONS important in iris borer control. Pesticides used improperly can be injurious Aphids to man, animals and plants. Follow the Aphids, or plant lice, are small, green, pink directions and heed all precautions on the or mealy-white insects that attack many label. plants. Store pesticides in original containers—out Aphids may appear on iris plants in early of reach of children and pets—and away spring. They pierce leaves and suck the from foodstuff. juices. When they feed, they may transmit the virus that causes iris mosaic. Apply pesticides selectively and carefully. Do not apply a pesticide when there is Control - To kill aphids, spray plants with danger of drift to other areas. Avoid malathion. Repeat if aphids reappear. Read prolonged inhalation of a pesticide spray or the directions and heed all precautions on dust. When applying a pesticide it is the container label. advisable that you be fully clothed.

Verbena Bud Moth After handling a pesticide, do not eat, drink, Larvae of the verbena bud moth tunnel into or smoke until you have washed. In case a new iris shoots and . Larvae are about pesticide is swallowed or gets in the eyes, one-half inch long. They have greenish- follow the first aid treatment given on the yellow, worm-like bodies and black heads. label, and get prompt medical attention. If a Mature moths do not attack irises. pesticide is spilled on your skin or clothing, remove clothing immediately and wash skin Control - Cut and burn infested shoots and thoroughly. buds. Dispose of empty pesticide containers by Iris Thrips wrapping them in several layers of Larvae and adults of the iris thrips pierce the newspaper and placing them in your trash surfaces of young leaves and leaf sheaths. can. They suck juices that ooze from the wounds. Dry wounds become small, straw-colored It is difficult to remove all traces of a spots. Flower buds blacken; plant tops herbicide (weed killer) from equipment. weaken. Iris thrips are especially injurious Therefore, to prevent injury to desirable to Japanese iris. plants do not use the same equipment for insecticides and fungicides that you use for a Larvae of iris thrips are milky white. The herbicide. black-bodied adults usually are wingless; they are about one twenty-fifth of an inch NOTE: Some states have restrictions on the long when mature. use of certain pesticides. Check your state and local regulations. Registrations of Control - Spray plants with dimethoate 4 pesticides are under constant review by the times at weekly intervals during May and U.S. Environmental Protection Agency. Use only pesticides that bear the Federal regulations number and carry directions for home and garden use.