Growing Together: Now's the time to divide, plant iris

FARGO - Last weekend I took my wife, Mary, on a romantic getaway. I found the perfect destination. Yes, we drove to the University of Minnesota Landscape Arboretum near Chaska.

Iris grouping
For added impact, plant iris in triangular groupings. David Samson / The Forum David Samson

FARGO - Last weekend I took my wife, Mary, on a romantic getaway. I found the perfect destination. Yes, we drove to the University of Minnesota Landscape Arboretum near Chaska.

You're probably thinking we're an impulsively wild and crazy couple, but sometimes you just have to throw caution to the wind. With reckless abandon, we adventured through the arboretum, all 1,200 acres of it.

More than 5,000 plant species and varieties are organized in 20 theme gardens devoted to perennials, annuals, roses, shade plants, herbs, trees and shrubs, plus a Japanese garden. We had a great time and the arboretum was fantastic, even though the temperature was so hot we were as wilted as the huge brugmansia blossoms.

Members of the Minnesota Iris Society were selling iris divisions in every color of the rainbow. It reminded me that August is the time to divide and plant iris.

Perennial flower beds are most attractive with combinations of different types. Because most perennials don't bloom all season, an assortment provides ever-changing color from spring until fall.


Iris, blooming mostly in late May through June, are must-haves. The large, billowy blooms are the orchids of perennial beds. Even the sword-shaped blue-green leaves give foliage contrast when flowers fade.

Iris purchased in pots can be planted throughout the season, but August is the traditional time to dig and divide established plantings. Bareroot divisions are often available from garden centers now. Late August and early September are ideal for planting, so roots become established before winter.

There are different species of iris, but the most common is called bearded or German iris (from the botanical name Iris germanica). Bearded refers to the bushy "beards" on each of three drooping petal-like sepals, called falls. The true petals are called standards and are the upright part of the iris blossom.

Iris grow from thick underground stems called rhizomes that are sometimes mistakenly called iris "bulbs." Leaves grow in a fan shape arising from the top of the rhizome, and the actual fibrous roots grow below.

Eventually, iris rhizomes branch into a criss-crossing clump, with the center often choked with old leafless rhizomes. This causes a reduction in flowering and makes iris more susceptible to insects and disease. It's best to divide clumps every three to five years while they're still healthy. Dig the entire clump and reset healthy divisions in its place.

To divide and transplant iris, lift the whole clump by gently prying them out of the soil with a spading fork, which works better than a shovel because it's less likely to injure roots and rhizomes. With a sharp knife, separate the clumps into divisions, each having a single rhizome with a single fan of leaves. Cut leaves back to about 4 or 5 inches, leaving a neatly trimmed fan shape. Discard leafless old rhizomes from the clump's center.

Iris bloom best in full sun, with six hours a minimum. They also don't tolerate "wet feet." An easy way to ensure good drainage is to rake soil into a slight mound for an iris grouping. Heavy soil can be improved by adding peat moss or compost.

Dig a shallow hole for each division, leaving a ridge in the center. Place the rhizome over this ridge with the roots spread out on either side. Add soil over the roots and gently firm. Just barely cover the rhizome with soil. Iris can be planted slightly deeper in sandy soil, but never with more than 1 inch covering the rhizome. If rhizomes are planted too deeply, flowering may fail and rhizomes might rot. It's better if rhizomes are slightly visible.


Space rhizomes 12 to 15 inches apart when planting groups. Arrange several iris of the same variety in drifts with the fans pointing in the same direction, or in a triangle with the "toes" pointing inward. Water well after planting. If iris rhizomes are plump and healthy with a nice fan of leaves, they will usually bloom the first year after replanting. Small, thin rhizomes might take until the second year.

Iris should be mulched for winter protection, especially if their location doesn't receive snow cover. Apply 6 to 12 inches of leaves or straw in early November after cutting back leaves to about 5 inches.

Besides bearded iris, several other types merit greater use. Siberian iris are extremely hardy and produce a circular clump of deep green foliage up to 3 feet tall that's attractive even when they're not flowering. They can remain eight to 10 years without division. Similar to Siberian iris, and also becoming increasingly popular, are Spuria iris. Forming an attractive circle of 4 feet tall foliage, they create a nice accent at the rear of perennial beds.

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Tune in to his weekly radio segment from 11 to 11:30 a.m. Fridays on WDAY Radio 970. Readers can reach him at .

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