Growing irises: How to plant, grow and care for iris

URBANA, Ill. - There aren’t very many plants that come in a wider range of color than iris. In the past 50 years, thousands of cultivars in various colors, sizes, and forms have been developed.

"I have about 20 cultivars of bearded iris in my garden including a small white and lavender variety that has been passed down in my family for four generations," says Jennifer Fishburn, University of Illinois Extension horticulture educator. Fishburn offers these tips for successfully growing iris.

Types of iris

Iris are divided into three categories, according to The American Iris Society: bearded, beardless, and aril. Many types are long-lived perennials in Central Illinois. Iris range in height from 6-inch-tall dwarf crested iris to 5-feet-tall yellow flag iris. 

Flower color: The six-petaled flowers come in a rainbow of colors include pink, varying shades of purple, pale yellow, bright yellow, peach, pale green, light blue, white, tan, bronze, almost black, and bi-color. The three inner upward true petals of iris are called “standards.” The three outer turned down flower petals are referred to as “falls.” Many cultivars have different colored standards and falls. Be sure to remove old blooms after flowering.

The most common variety is bearded iris. This easy-to-grow iris range in height from 18 to 36 inches. Bearded iris also vary in bloom time and flower color. They grow best in well-drained soil in a full sun location. They will not tolerate poorly drained soil.

Pests and diseases

Bearded iris do have a few problems including iris borer, bacterial infections including bacterial soft rot and fungal infections of the rhizomes, leaf spots. Imagine my disappointment last year when I noticed several of my iris plants looking rather frail.  Upon closer inspection, I found that the rhizomes had turned to mush from bacterial soft rot. This bacterium needs a wound to enter a plant. Iris soft rot often enters wounds caused by iris borers. Proper sanitation is important, remove and discard infected rhizomes and plant parts. 

Iris borers are destructive and difficult to control. They can infest all types of iris. One may learn more about borers at University of Minnesota Extension

Planting and dividing

Most iris clumps become crowded and should be divided every three to four years. About four to six weeks after they flower, divide by digging up the whole clump and remove the mother plant.

Place the rhizome on a ridge of soil, placing the roots in the soil, but the rhizome just above soil level. Space rhizomes 12 to 18 inches apart to avoid overcrowding and allow for good air circulation to help prevent disease issues. Since iris have a short bloom period, consider adding iris in the middle of a perennial garden where later blooming plants can hide the iris foliage.

Learn more about various species and cultivars of iris by visiting the Plant Finder from Missouri Botanical Garden.

University of Illinois Extension is the flagship outreach effort of the University of Illinois at Urbana-Champaign, offering educational programs to residents of all of Illinois' 102 counties and far beyond. Illinois Extension provides practical education you can trust to help people, businesses, and communities solve problems, develop skills, and build a better future. Through our Agriculture and Natural Resources programs, Illinois Extension supports the economic viability and environmental sustainability of natural and managed landscapes and productive lands in Illinois. Horticulture program educators provide research-based information and training about gardening, fruits and vegetables, flowers, insects and diseases, composting, landscaping, and more.

News sources/writerJennifer Fishburn, Horticulture Educator, Illinois Extension