An Iris Primer, by Iris Craig

Jun 3, 2018

An Iris Primer, by Iris Craig

My mother loved flowers: she named me Iris. Having a flower name has affected my life and perhaps led me to be a Master Gardener.

When I purchased my house 40 years ago, friends arrived with iris rhizomes for my garden. Not sure how to plant them, I asked for advice from my friend, Alice, who inherited an iris garden from her mother. Some of those gifts—often neglected, rarely transplanted—still bloom today. Abandoned houses sometimes have iris blooming long after the owners have left.

Iris is the largest genus in the Iridacadae family. Iris have thrived in the wild for thousands of years and figure in cultures throughout the world. In Greek mythology, Iris is the rainbow goddess who brings messages from humanity to the gods. In Roman mythology, she walks the rainbow to the heavens carrying messages to the gods.

Egyptian and Indian cultures used iris rhizomes for healing and for making perfume. Orris root, made from the rhizomes of bearded iris, was burned to create scents pleasing to the gods. Dried iris rhizomes were an industry in Florence in the 19th Century, and the flower became the emblem of Florence.

Iris represents the renewal of life. Clovis I of France chose the fleur-de-lis for his emblem in the sixth century. The fleur-de-lis continues today as the emblem of New Orleans and the state flower of Tennessee.

There are about 280 species of iris. Moors brought iris to the Alhambra in Spain. When Columbus set sail for America, Holland had already developed many new species. Some American iris came across the Atlantic with early settlers and date back to the 1600s.

Twenty-eight native species are found today in North America. The three main natives are Blue Flag, Louisiana and Pacific Coast iris. This last is native to California and Washington State. All attract pollinators and are a colorful addition to your garden.

The Siberian bearded iris, with more than 200 species and a multitude of colors, is the one often found in Napa Valley gardens/ These non-natives can be divided into two main groups: bulbs and rhizomes.

Rhizomatous iris grow horizontally close to the surface with underground stems that supply food for the plants. The common bearded Siberian and Japanese iris grow from rhizomes that are planted in the summer. Bulbous iris, among the most reliable, are repeat bloomers in our area. These include Dutch iris and the dwarf reticulated iris, both of which are best planted in October with other bulbs.

When iris have been in the ground for a couple of years and have fewer blooms, it is time to divide and replant. Rhizomatous iris do well when divided every three to five years. Here's how:

With clean garden shears, cut back the leaves by one third. Next, lift out the entire clump with a shovel or pitchfork. Using a sharp knife dipped in 10 percent bleach solution after each cut, separate the rhizomes.

My friend Alice called the main, larger rhizome with last season's bloom stem at one end the ‘mother.' The smaller rhizomes attached to her are her children and, thus, the pieces to be broken off and transplanted. The ‘mother' rhizome is discarded. The new healthy (children) transplants are firm, with roots, and a fan of five or more leaves.

Check the new rhizomes for soft-rot, a bacterium (erwinia carotovora). Victimized iris have wilted leaves, and the rhizome emits a foul odor. It occurs when the plants are over-watered. If rot is evident, while digging up the plant, cut off the affected areas with a clean knife, expose the healthy rhizome to the sun, and wash with an anti-bacterial soap. Do not put the leaves and diseased rhizome in your compost.

Next, check for the iris borer (Macronoctua onusta). This moth lays eggs on the leaves of old iris leaves and debris. In midsummer, the caterpillars chew through the leaves, find their way down to the rhizome and begin eating it, causing soft rot and possibly death. The best way to prevent damage by this pest and others is to water lightly and keep the garden free of debris and weeds.

Water and excessive moisture are the source of soft-rot. Iris do well when planted in the heat, during the dry weather of summer. In Napa Valley, plant late July through September to minimize rot. Choose a sunny, well-drained area in the yard. A few varieties found on the edge of ponds can manage moisture; however, most iris cannot.

Plant groups of the same variety 18 to 20 inches apart. Dig a hole five inches deep. Place a mound of dirt in the center. Set the young, healthy rhizomes on the top of the mound with the roots hanging down the sides. Cover the roots but leave the top of the rhizome exposed to the sun. If planted too deep, iris may not bloom and could rot.

Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.