Bearded irises are relatively easy garden plants to grow and will give good results with a minimum of care, but like all plants, the better the culture the more magnificent the display. The following instructions are easy to implement and should lead to beautiful iris blooms year after year.
For best results, plant iris rhizomes in July, August or September. This is also the best time (plants are normally dormant during the heat of July and August) to divide and replant iris that have become overcrowded, usually after three to five years. It is important that the roots of newly planted irises be well established before the end of the growing season. Plant your iris at least four to six weeks before your first hard freeze or killing frost.
Irises require at least a half-day (6-8 hours) of direct sunlight. Some afternoon shade is beneficial in extremely hot climates, but in general irises do best in full sun. Iris will grow in deep shade, but probably not flower. Provide your irises with good drainage. A raised bed or planting on a slope are ideal places to plant iris. Good air circulation is essential and water should not stand in the beds.
Bearded irises will thrive in most well drained soils. If you have heavy soil, adding humus – compost – or other organic material – will improve drainage. Gypsum is an excellent soil conditioner that can improve most clay soils. The ideal pH for irises is 6.8 (slightly acidic) but irises are quite tolerant of less-than-perfect soils. Lime may be added to acidic soils and sulfur may be added to alkaline soils. Have your soil tested before making any correction.
Plant your rhizomes at or just barely below the surface of the ground. Irises should be planted so the tops of the rhizomes are visible and the roots are spread out facing downwards in the soil. However, in extremely hot climates or with very light soils, cover rhizomes with up to one inch of soil. Tamp the soil firmly to anchor the rhizomes until new roots begin to grow, and water well. It is a common mistake to plant Irises too deeply.
Step (1) Build up a small mound of soil in the center of the planting hole. Step (2) Center the rhizome on the soil mound and spread out the roots on either side. Step (3) Firm the soil around the roots. Newly planted rhizomes should be watered thoroughly. Be Patient — Irises are perennials and require time to grow. New growth may be noticeable within 2-3 weeks and begins with a new center leaf in the fan. Depending upon the maturity of the rhizome and the geographical location, there may or may not be blooms the first Spring.
Figure 1. Build up a small mound of soil in the center of the planting hole.
Figure 2. Center the rhizome on the soil mound and spread out the roots on either side.
Figure 3. Firm the soil around the roots. Newly planted rhizomes should be watered thoroughly.
Planting rhizomes 12 to 24 inches apart is the norm. Close planting results in immediate effect, faster clump formation, and more color but makes dividing clumps a necessity in two to three years. The photo to the below shows iris being planted in groups of three. Notice that each of the rhizome “toes” face inward towards each other about 8 inches apart as they are planted.
Newly planted rhizomes need moisture so their root systems develop. Once established, irises should be watered when the top three inches of soil dry out. The watering frequency will depend to a great extent on your environment. Over watering of Irises is a common mistake. After planting, water well and continue watering until the first good rain. If lack of rain persists, watering should be deep enough to penetrate the shallow root system. Less frequent deep watering is better than frequent shallow watering.
The soil type for your area will determine your fertilizer needs. Superphosphate, or a well-balanced fertilizer with an N-P-K ratio of 10-10-10 or 5-10-10 are recommended. Avoid anything high in nitrogen as it encourages soft growth that is susceptible to disease. Provide a light application in early spring and again a month after bloom . Place fertilizer around rhizomes, not directly on them. Alfalfa pellets (without salt) are extremely beneficial when incorporated in the soil around newly planted irises. Do NOT use Feed and Weed preparations.
When irises become crowded, usually every three to four years, bloom will decline. At this time, old clumps may be thinned by removing several divisions and leaving a portion of the clump in the ground. A better practice is to remove the entire clump, replenish the soil and replant a few large rhizomes.
Figure 5. Digging a three year iris clump.
Figure 6. Remove excess dirt and discard the old center divisions.
Figure 7. Separate the individual rhizomes for replanting.
It is extremely important to keep your iris beds free of weeds and fallen leaves so the rhizomes may bask in the sun. Spacing plants so there is good air circulation will help prevent diseases. Break out bloomstalks as soon as bloom season is over. This prevents contamination of your named varieties by chance bee crosses. These crosses would cause seedpods to form that might go unnoticed. If given time to ripen, they might drop seeds to the ground. The resulting new plants are often unattractive. So breaking out bloomstalks right away is a good garden practice.
Note: Much of the above planting information was derived from William Shear’s book The Gardener’s Iris Book published by The Taunton Press. Mr. Shear is a biologist at Hampden-Sydney College in Virginia. He has been growing irises for more than 50 years.
Beardless irises will be a beautiful asset to your garden if you meet their cultural requirements. Whe the rhizome arrives, the plants will have been wrapped in damp paper and plastic bags. Immediately remove the rhizomes and soak the roots overnight in water. This will allow the plant to replenish the water lost in transit and get the plant off to a good start in your garden. Do not let the roots dry out during transplanting.
Beardless irises are generally planted in the fall but may be planted in August, September and October depending on your climate. The fall usually brings rain to supplement watering and roots will grow well as the weather cools. If you don’t have early fall rain be sure to keep them watered so they get a good start. The new root growth is needed to anchor the plants before winter. Plant your iris at least four weeks before your first hard freeze or killing frost.
Most beardless irises need a sunny location for best performance and bloom. If plants get less than a half-day of sun they may not bloom well. Siberian irises like even moisture while Japanese irises like as much water as you can provide. These types do not, however, like to have wet feet in the winter time. Louisiana irises and several of the beardless species will grow in standing water year round in the south but will also do well in the ground as long as they get enough water and don’t dry out. Spuria irises require dry conditions during their summer dormancy in July and August. More type specific growing instructions can generally be found on the websites of their respective section. (See About AIS)
Your beardless irises will thrive in a good garden loam with added organic matter such as humus or compost. An acid soil is preferred, but Japanese irises are the only beardless irises that require an acid soil. A pH of 5.0 to 6.5 is needed for optimum Japanese iris performance.
Plant Louisiana, Spuria and Siberian irises about one inch deep. Japanese should be planted a little deeper, about two inches, as they make new roots on top of old roots. Keep newly planted rhizomes well watered until they show active new growth. Drying out at this stage will result in almost certain plant death. Once established, beardless irises grow well, but extra care is needed to start them out. After plants are showing new growth, add a mulch of two to three inches (except on Spurias) and leave it on year round. This retards weeds and allows the roots to remain cooler. Also it prevents sun scalding to Louisiana rhizomes.
The soil type for your area will determine your fertilizer needs. For optimum growth an application of a balanced fertilizer in the spring is recommended when the plants are a few inches high. Japanese are especially heavy feeders and appreciate a second feeding before bloom time.
It is important to keep your iris beds free of weeds for optimum growth. After bloom is finished, cut off the bloom stalks before any seedpods made by bees develop as seeds from these would contaminate your named cultivars. Beardless irises can grow for several years in the same location if fed on a regular basis. Japanese irises are the exception – they must be divided every three years to thrive. After fall frost, clip off brown foliage to discourage rodents from nesting in the clump. Louisania foliage in southern climates will remain green and should not be cut off.
John Weiler, an iris hybridizer and long-time professor at Fresno State University in California, wrote in The World of Irises: “Irises are free of diseases and easy to grow!” “How many times [has] that statement been made by an enthusiastic writer in a magazine article, gardening book, or newspaper [?]. In recent years, though, as more gardeners have developed iris plantings and collections have expanded, the prevalence of several iris diseases has become more obvious.”
Irises’ susceptibility to various pests and diseases often depends on your geographic location and many diseases may or may not affect your irises. Keeping your garden clean from debris goes a long way toward avoiding the conditions conducive to both pests and diseases.
Many iris types are affected by Bacterial Leaf Blight. The pathogen is Xanthomonas tardicrescens. Bacterial Leaf Blight causes large irregular spots that first appear near the margins on the leaf tips. At first, the spots are just small pale areas. The key to diagnosing bacterial infections is that the spots appear watery at first, then soon turn light brown. These brown spots become larger and develop whitish or grayish centers. The bacterial infection follows the leaf veins down the leaves and the splotches may run together. This bacterial disease is easily confused with fungal leaf spot disease. Both occur during foggy and rainy weather. Irises will get bacterial leaf spot most often during mild weather, while fungal leaf spot can occur anytime the temperature is above freezing. Bacterial leaf spot splotches are larger and more irregular than fungal leaf spots. (see Fungal Leaf Spot below)
Since there is no known cure, prevention is the only thing that will help control this disease. The bacterium is easily spread on garden tools as well as by water splashing on the plants, so beware of using any tools on healthy plants that have been used on infected plants. Wash your hands thoroughly after working on plants infected by bacterial leaf spot. Disinfect tools with a dilute solution (1 part bleach to 9 parts water) of 0.5 percent sodium hypochlorite (household bleach). Cultural measures to prevent bacterial leaf spot include removing old foliage from the garden in the fall and destroying it. This will minimize a number of potential iris problems. Since the bacteria do not infect the rhizome, transplanting can be helpful.
Fungal Leaf Spot presents as small circular to oval spots on the leaves, turning yellowish to brown and often developing a distinct red-brown border. The pathogen is Didymellina macrospora. It can affect all bearded irises, Siberians, I. versicolor and the xiphiums, known as Dutch irises. The more rain you have the more prevalent it becomes. It is easy to control with the use of a fungicide and a sticker-spreader in your sprayer. Apply in the fall and early spring before infection. If infected, tip cut the affected leaves and apply again. Maneb have been proven effective. Check with your local authorities as to pesticide and fungicide use.
Fungal Leaf Spot – Photos by Russ Rodrigue
In some years this can be a major problem that can almost wipe out a planting if allowed to spread without remedial action. The pathogen is Erwinia carotovora (yes, it affects carrots) and affects all the bearded irises. It presents as a soft, foul-smelling rhizome rot followed by wilting and dying of the leaf fans. It is prevalent in wet springs with high temperatures. If you observe this and smell it, get busy cleaning it out! Use a spoon and scrape the infected tissue out. The wounds in the rhizomes need to be exposed to the sun. Then powder the wound with a chlorine based cleanser. Instead of the cleanser some people douse the rhizomes in place with Dial antibacterial soap (with triclosan).
This can affect bearded irises, and rarely here Japanese, Siberian and Spuria irises. It presents as a gray or tan cottony mass over leaf bases and adjacent soil and rhizomes. The leaves turn yellow at the tip, rot at the base, and eventually fall over. Many times a secondary infection of bacterial soft rot will also occur. This fungus is most destructive during warm weather with moist soil. The pathogen is Sclerotium rolfsii, also know as mustard seed fungus, because it also presents as brown, spherical sclerotia, about the size and color of mustard seeds, when inactive. It is controlled by sanitary growing conditions, the cleaning out of infected tissue, exposure of the rhizomes to the sun, and at planting or transplanting time dipping the rhizomes in a fungicide or 10% Clorox solution.
This critter is the scourge of the East and Midwest. Garden sanitation is most important to avoiding this borer. Study its life cycle: Macronoctua onusta is a caterpillar, the larvae of a medium-sized, nondescript noctural brownish moth. In autumn the moths lay their eggs on old iris leaves and other nearby debris. The eggs survive the winter and hatch out with warm weather in the spring. The tiny white caterpillars search out fresh iris leaves, which are usually less than a foot tall at this time. Early damage is hard to detect but as they grow they begin to attack the edges of the center foliage making notches and leaving slimy frass. If you catch them early you can crush them in the foliage with your fingers. Iris borers are proven cannibals with only one surviving in each iris by the time they are half-grown. But they do significant damage as they proceed down the leaf sheaf or bloom stalk to the iris rhizome. The borer will hollow out the rhizome and sometimes migrate to an adjacent rhizome and do the same. They have to be dug out at this point and have grown to between one and two inches, are pinkish in appearance with dark brown heads. Ugly! In late summer they leave the rhizome entering the soil and becoming dark brown pupae. This dormant stage allows the caterpillar to transform into a moth in the autumn, when they emerge, mate and lay their eggs to start the process over.
Most pesticides are not effective against the iris borer. Garden sanitation and visual inspection of your plants is most important. Two systemic insecticides with reportedly good effect are Merit and Cygon 2E. Read the labels and apply sparingly with a spreader-sticker in early spring (when irises are about 4-6 inches tall) at least three times every 7-10 days. Eggs hatch at different times so you may get different batches dependent on the microclimate in your garden. Stay vigilant!
There have been no confirmed reports of the iris borer west of the Rocky Mountains.
These small green or gray insects are sapsuckers. They appear on the iris leaves in clusters and proceed to suck out leaf sap. They also can spread disease between plants. They are easily controlled by physical removal or crushing with your fingers. Lady bugs like them too. Spraying with an insecticidal soap or liquid dish detergent is effective.
This critter appears to only be a problem for iris hybridizers who have seed pods developing from their iris crosses. The Endothenia hebesana, bud moth attacks the seed in the pods. It has the same life cycle as the iris borer. Sanitation and cleaning up seed stalks and capsules is the only known control.
There are hundreds of species of crickets in the U. S. Crickets feed on just about everything. They will eat seeds, dead insects, leather, paper, old clothes (cotton, wool and silk), and plants. This includes the rhizome of the iris plant. They will eat holes in the rhizomes which can later be a home for pill bugs and result in soft rot from the rhizome’s injury. Adult crickets spend their days in shallow burrows beneath stones, wood or plants. They are most active in late evening and the night when males begin their serenading to attract female mates. A cricket’s life begins as one of about 300 eggs a female lays in the soil during late summer and fall. Some species overwinter as nymphs or adults, others as eggs which hatch in the spring. A year-old cricket is a rarity. Crickets can be controlled by a general purpose insecticide.
The pill bug is the only crustacean that can spend its entire life on land. Their shells look like armor and they are known for their ability to roll into a ball. The common pill bug Armadillidum vulgare normally feeds on decaying vegetation but will burrow out holes begun by crickets in iris rhizomes. Called roly-polys in some areas, these little critters are found under stones, logs or any other space where moisture is found. When assuming a defensive posture, they curl up in a ball. They can be controlled by a general purpose insecticide.
This small brown burrowing mammal is in the Talpidae family. Its diet primarily consists of earthworms, grubs and other small invertebrates found in the soil. Their saliva contains a toxin that can paralyze earthworms. Because of this, moles are able to store their still living prey for later consumption. The problem they create for plants is the air space from their burrows causing plant stunting and non-bloom. Everyone has their own methods for trying to eliminate moles. The only sure way is to move to Ireland where they don’t exist.
This small rodent is blackish brown to grayish brown, 5-8 inches long resembling a mouse but with a stouter body, a shorter hairy tail, a slightly rounder head with pointy nose, and smaller ears and eyes. They live underground in burrows (often using a mole’s) but travel aboveground through runways they make by eating everything in their path, be it grass, herbaceous plants, bulbs, tubers and even tree roots or tree bark at the ground level. They can be controlled by toxic baits, such as the anticoagulants brodifacoum and bromadiolone, and by trapping. An alternative is to use mouse traps with a piece of cheese placed right in their run. They seldom veer off these runs.
