Though iris do not suffer from many problems, routine lifting and dividing is necessary to combat iris borer worms and keep plants healthy and productive.
Light
Irises need full sun to thrive. Irises in full shade produce fewer blooms and may suffer from an increase in diseases.
Soil
Heavy clay soils do not work well for growing iris, but sandy or gravelly soils are excellent. If your native soil is heavy, you can plant irises in raised beds to help drainage. You may also amend your soil with gypsum or organic matter like compost to lighten the soil.
Water
Although irises like moisture, they also need good drainage to prevent rot problems. Water them when the top 2 inches of the soil feels dry. These plants have a good tolerance for drought.
Temperature and Humidity
Irises are notorious for their hardy disposition. They don’t mind temperature extremes, as long as the soil allows excess rain or snowmelt to drain away. Irises that are damaged by heavy winds or hail can be susceptible to iris borer larvae.
Fertilizer
In the spring, apply a low-nitrogen 6-10-10 fertilizer around your irises. Too much nitrogen will encourage foliage at the expense of blooms. Bone meal is also a good fertilizer.
Types of Bearded Iris
Within the broad category of the popular bearded iris group, there are also subcategories: tall bearded, intermediate bearded, short bearded, miniature bearded, and border bearded. The main distinguishing feature of these subcategories is their stature, although they also vary somewhat in bloom time. Read plant labels carefully so you understand what type of iris you are buying. New named varieties of bearded iris are developed for commercial sale every year, and the ranking of the most popular irises, conducted annually by the American Iris Society, changes frequently. When looking for irises to plant, it’s a good idea to look for those that have won well-known awards, such as the Dykes medal. There are many dozens of fine irises to choose from; here are some award-winners that are widely grown:
“Celebration Song” is a pink and lavender tall bearded iris, growing to 37 inches.“Abiqua Falls” is a true blue tall bearded iris that grows to 39 inches.“Again and Again” is a yellow reblooming tall bearded iris growing to 36 inches.“American Classic” is a white with blue-violet iris, a tall bearded that grows to 36 inches.“Beatnick” is a dwarf bearded iris growing to 14 inches. It has purple flowers with white highlights.“Big Blue Eyes” is a dwarf bearded iris with white and violet flowers. It grows to 14 inches.“Dusky Challenger” is a very dark purple iris, growing to 39 inches tall.“Thornbird” is a beautiful yellow iris that grows to 37 inches.“Queen’s Circle” has white flowers with blue-lavender fringes. This iris grows to 32 inches.“That’s All Folks” is a towering 40-inch yellow-gold iris.“Jesse’s Song” is a 35-inch tall violet and white iris.
Pruning
Faded, withered flowers should be pinched off immediately. This may promote additional blooms or even a repeat bloom period later in the season. Every four or five years, clumps of iris should be dug up, divided, and replanted in order to cull out diseased roots and iris borer damage. This will keep iris plants vigorous and also allow you to propagate new plants.
Propagating Bearded Iris
Dividing irises not only yields more plants for your garden, but it also keeps your existing irises healthy and vigorous. Here’s how to do it: Divide irises every three to five years—or more often if iris borers have infiltrated the plants.
How to Grow Bearded Iris From Seed
Propagation by seed is generally only done by nursery professionals when they are seeking to breed new varieties by careful cross-pollination. But it is certainly possible to grow iris from the tiny seeds found in the green oval pods left behind after the flowers fade. If you collect the seeds and plant them about 1/4-inch deep in the garden, they often will sprout and mature into flowering plants within three years. But be aware that most garden iris are hybrids, and plants propagated from their seeds usually do not “come true.” The iris plants you propagate by collecting seeds may look quite different from the mother plant. Propagation is almost always done by dividing root clumps—a speedier and more reliable method.
Potting and Repotting Bearded Iris
Although it’s not a common method, you can grow irises in pots that are at least 12 inches wide. Use a loose, soil-free potting mix, and leave the tops of the rhizomes exposed or just barely covered. Make sure not to overwater the plants. In colder climates, you may need to move the containers to sheltered locations for the winter to ensure the plants’ survival. Irises in containers may need dividing and transplanting more frequently than those growing in the ground—about every other year or so.
Overwintering
Trimming back foliage fans to about 6 inches in the fall makes the garden look tidy, reduces leaf surface area that might host fungal disease, and removes caterpillar eggs. Destroy all foliage that you remove, and do not add it to the compost bin. If plants show signs of rot or damage from borers, the roots should be dug up and inspected in early fall. Cut away any diseased or worm-damaged sections and replant the remaining root sections. Ideally, this should be done several weeks before freezing weather sets in, to allow the roots time to become reestablished before frost.
Common Pests and Diseases
The iris borer is the most serious insect pest of irises. In the spring, caterpillars hatch and tunnel through leaves, reaching the rhizome by summer. Feeding tunnels allow the rhizome to become infected with bacterial rot, compounding the damage. Remove all iris leaves after frost to remove caterpillar eggs, as well. Where rhizomes are penetrated by worms, dig them up, cut away soft, infested portions, and replant the pieces. This is also a good time to propagate your irises by division. Major disease problems include bacterial soft rot, crown rot fungus, and fungal leaf spot. Mottling of leaves and flowers suggests the presence of mosaic virus. Affected plant material should be removed and destroyed (not placed in compost). Good hygiene is usually enough to control these diseases; fungicides aren’t usually needed.
How to Get Bearded Iris to Bloom
When an iris fails to bloom, it can usually be traced to one of four reasons:
The rhizomes are planted too deep. When you replant divisions, make sure the crown of the rhizomes are just barely covered with soil.Plants are not getting enough sun. Make sure iris are getting at least six hours of direct sunlight each day.Plants have been overfed. Too much fertilizer causes the iris to put energy into leaf growth at the expense of flowers. A single feeding in the spring is all that irises require.Plants are overly crowded. When root clumps become too dense, flowering will fall off radically. Dig up your irises every few years to divide and replant them.
Common Problems With Bearded Iris
Unique among spectacular flowering plants, bearded irises are largely trouble-free. But do watch for these issues:
Leaves Turn Yellow or Brown, Fall Over
This is usually a sign of root rot caused by excessively wet soil. It can also be the result of extensive damage from iris borers. It is best to immediately lift affected root clumps to cut away any soft, decaying areas and replant in soil that is porous and well-draining.
Flower Stalks Topple Over
Some bearded irises grow as much as 40 inches tall, and staking may be required to support the huge flowers. Irises that grow in shady conditions may get even leggier, making staking a necessity.
Leaves Have Yellow and Brown Spots
Spotted leaves are the result of bacteria or fungal infections. Bacterial leaf spot usually begins on the edges of the leaves, gradually enlarging. Fungal leaf spots usually appear on the inner part of the leaves, and they do not enlarge in size. Affected plant parts should be removed and destroyed. Keep the ground around irises free of mulch and debris. Good garden hygiene usually controls leaf spot diseases. Serious fungal infections can be controlled with fungicides. Both types of iris are hardy in zones 3 to 9, and both types bloom in spring. But bearded iris have the larger flowers, with the namesake fuzzy “beards” on the downward-facing fall petals. Siberian iris have smaller flowers without beards, and they typically bloom a little later than bearded iris. The foliage of the Siberian iris is grasslike, remaining attractive in the garden after the flowering period is done. Siberian iris is more tolerant of shade, and the tough fibrous root clumps do not need division as frequently as with the bearded iris.
