How to grow an indestructible iris plant.

All things green.
May 28 2007 7:22 AM

The Indomitable Iris

The tough, graceful flower endures where others wilt.

Click here for the latest installment of Slate's guide to planting a beginner's garden and here for previous installments.

Iris. Click image to expand.
An iris flower

The bearded irises used to seem to me too regal and stately to love, in fact, rather pompous. (There are other estimable kinds of irises, but the tall bearded are the most familiar and the most flamboyant—the ones Van Gogh painted.) But when I worked as a gardener for the New York City parks department, bearded irises won me over. They managed to do what I most appreciated in a plant in my urban garden days: They stayed alive.

When I walk through some of the public gardens I used to work in, I see that, of all the flowers I planted, irises are the ones that have endured in the best shape. With very little care, zero fertilizer, and no extra water, they have soldiered through city sun and heat.

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The wild ancestors of tall bearded irises grew on stony slopes or in dry grassland; thus, baking-hot sidewalks nearby do not faze them. Their most important requirement, after plenty of sun, is good drainage. Bearded irises have thick, fleshy underground stems called rhizomes that store the food made by the leaves. The trick is to prevent the rhizome from ever getting soggy, while keeping the feeder roots below it moist but not wet. The time-honored technique is to plant the rhizome at, or just slightly below, the soil surface, with the feeder roots going farther down. A good drainage-aiding strategy is to plant them in raised beds or on a slope.

Bearded iris flowers have three ruffled petals—known as standards—that stand upright, combined with three petals called falls, which hang down. The beard is a line of fuzzy hairs at the upper base of each of the falls, a way of luring pollinating insects into the heart of the flower.

Since the 19th century, people have been crossing irises to create new color combinations. You can find fabulous colors you rarely see in any other flower—strange rusts and browns, hundreds of variations on purple, and fabulous near-blacks. The petals have a velvety texture, a sumptuous pre-Raphaelite feel.

You can still find old-fashioned irises like "Missouri," a very fragrant deep blue with a touch of gold, a prizewinner in 1937. Or "Honorabile," a 19th-century variety with yellow standards and chestnut-red falls—a favorite of the Historic Iris Preservation Society.

Sad to say, the individual flowers, however gorgeous, don't look good for much more than two days. Iris fanciers plan for succession; early bloomers give way to late bloomers. Happily, the firm, upright gray-green leaves are great to have in a flower border. Right now, the leaves are highly appreciated in sophisticated garden design circles, where there's a growing esteem for things that are sculptural, structural, or architectural, rather than just flowery.

Iris pallida, also known as variegated sweet iris, has a particularly striking leaf, with elegant vertical stripes of cream and white, and the added benefit of being less prone to rot.

Rot's the reason to keep the rhizome relatively dry and close to the surface. It's one of the two plagues specific to bearded irises. The second is the iris borer. The brown moth lays her eggs on garden debris in late summer and fall. The tiny borer larvae hatch in the spring, climb up the iris leaves, chew into them, and eat their way down inside the leaves, reaching the rhizomes by midsummer. In the rhizome they grow into 1-1/2-inch-long caterpillars with a worm look—reddish brown head and pink body. After chewing tunnels through the iris rhizomes, they spin themselves a shiny chestnut-colored chrysalis, hatch, and breed a new generation. The holes the caterpillars chew make the rhizome susceptible to bacterial rot, and it turns slimy and foul-smelling. The irises I grew in Manhattan never got iris borers; I like to think the town was too tough for them.

Sure, irises come in nice colors and opulent, velvety textures, you're thinking, but why go to the trouble if borers will swoop in and ruin them? There is a safe and fairly effective organic mode of borer control. In fall, clean up and destroy the old stems, leaves, and leaf litter where the mother moth lays her eggs. This, I think, is why iris growers traditionally cut their irises back to a tidy 6-inch fan, which isn't really necessary. A chemical solution is to apply insecticide dust to the iris plants once a week from first growth until flowering. Here, you run the risk of killing off some beneficial insects.

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