Tuesday, August 17, 2010

Recalcitrant Rhizomes

The time has come to face the music and talk about the difficult but delectable rhizomatous irises. Iris germanica is the most familiar and probably what you remember seeing growing with wild abandon in hot sunny climates. Almost all bearded iris hybrids descend from wild forms of Iris pallida (Sweet Iris) and I. variegata (Hungarian Iris).

The model for the fleur de lis, blossoms of this stately beauty are instantly recognized. Three upright petals are called “standards”, and three pendulous petals are called “falls”. The falls are adorned June 005with bristly hairs that earn this lovely lady the moniker “bearded.”  Bloom stems carry multiple buds that flower in succession and make them desirable for the vase. Many of them are wonderfully fragrant. "Remontant" Iris, or "Reblooming" Iris  have a genetic tendency to bloom a second time in late summer or fall.

Bearded Iris have a reputation for being sturdy, long lived and low maintenance. Why do they turn into such prima donnas in the PNW? They don’t like clouds and they don’t like wet feet. Bearded iris will endure droughts with finesse, but they cannot tolerate soggy soil. They require baking in the hot, full sun with their rhizomes exposed like sunbathers on the beach at Nice. (Some people should not go topless. I’m just sayin.)

Still, they can be grown successfully here. Make sure the spot you choose has full sun and good drainage. As much as 50% of the rhizome should be above ground; planted too deeply they produce leaves but no blooms and are more susceptible pink iris to soft rot and iris borer. Do not mulch them or they will sulk. Planting on a slope or in raised beds improves drainage. It may help to dig your hole, form a mound at the bottom, and lay the rhizome with the roots splayed down the sides. Firm the soil around the rhizome and then water to help settle the soil. Once established, Iris don't require watering. Overwatering is strictly verboten.

Irises thrive in almost any type of soil provided it is well-drained. You may fertilize in the early spring, if you like. Iris enjoy bone meal, superphosphate or any good 6-10-10 fertilizer. Be sure to avoid using anything high in nitrogen, as nitrogen encourages rot problems.

If, despite your best efforts, your iris leaves look tatty after bloom, remove them immediately. In the late fall or early winter, cut healthy leaves back to about six inches. Avoid using mulches, ground covers, or anything that covers your rhizomes or they will rot. Excessive moisture and rainy or humid weather can lead to yellow and brown leaf spots. Always remove old dead leaves, and cut off and destroy any leaf or part of a leaf that is affected.

When your iris clumps become crowded, the bloom will taper off and they must be divided. Dividing every 3-4 years should prevent a season without bloom. The best time to divide your013 iris is August. Save divisions from the perimeter of the clump and cut the leaves back to one third when replanting. You can dig the clump or, if you are confident in your excision skills, you may remove the centers of the clumps with your spade and leave the newer growth undisturbed. Remember that about 25% of Iris won’t bloom the  first year after planting. Sometimes they need an extra year to become established.

Give your German Iris plenty of sun, great drainage and room to bloom and they will make you proud. A little Wagner played full blast in early summer with the windows open won’t hurt.

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