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Take time to divide bearded irises

By Nancy Brachey
Nancy Brachey
Nancy Brachey writes about gardening for The Charlotte Observer's weekly Home & Garden section.
    Bearded irises come in many colors and are among the most beautiful flowers in the garden.
    A freshly dug clump of bearded iris is ready to be divided.

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  • Ask Nancy

    Q. Our KnockOut roses, which have grown and bloomed vigorously for years, have not done well. They were pruned correctly at the right time. But they are not as big as they have been in the past nor are there as many blooms as we always expect and enjoy. What is wrong?

    A. It turns out your are using a general, all-purpose fertilizer with a much higher proportion of nitrogen. I think shifting to a fertilizer specially formulated for roses – with the most favorable proportions of nitrogen, phosphorus and potassium, plus trace elements – can give these plants a fresh boost and bring them to the splendor you expect this summer and fall. They have a long season ahead, so don’t give up now.

These are slow days for gardeners. Beyond harvesting vegetables, picking flowers and watering beds, there isn’t a whole lot to do.

That is, unless you possess irises in need of division.

Fortunately, this is not a task that demands doing every year. Typically it is done every three or four years to keep clumps fresh and vigorous. And it is not too difficult, certainly easier than wrestling a large daylily out of the ground and dividing it into many pieces.

While many kinds of irises are out there, the kind that is most commonly grown is the tall bearded, often called German irises. Tall straight stems bearing the most beautiful blooms appear in mid to late spring with a peak in early May. The color range is sensational, and it would be hard to name the most beautiful. They all are. Just name your color, from white and soft pastels to vivid blues, bronzes and gold.

They are also quite vigorous. A single plant, with a horizontal, tuberous root called a rhizome, should expand to a sizeable plant in two or three years. This rhizome sends leaves and flower stem up, and smaller, threadlike roots deeper into the ground.

Because these plants are so valuable in the flower bed, it is worth taking the time at midsummer to dig and divide the clumps.

It is not difficult. First, cut back the leaves to fan-shaped foliage about 6 inches above the ground. Then, with a shovel or spading fork, dig up each iris clump and wash away the soil. Set the clump on newspaper or a plastic trash bag. Then cut or break apart the new rhizomes, which will be toward the outside of the clump. If you use a knife or shears, dip them in a solution of water with 10 percent household bleach between each cut to reduce transfer of disease organisms.

Each rhizome should be firm and have leaves and roots. Discard any showing signs of rot or damage by iris borers.

Replant the rhizomes level with the soil surface in freshly dug, weed-free and well-drained soil.

Before replanting, it also helps to dip the rhizomes in a solution of water and fungicide such as Benomyl – prepared as for a typical spraying – to reduce the likelihood of rot. Or dust the rhizomes with the Benomyl powder. This is more critical when you have seen evidence of rot in this clump or a neighboring one.

To make sure the rhizome is planted horizontally, make a ridge of soil in a shallow trench, then set the rhizome horizontally at the top of this ridge with feeder roots spread down. Then fill in with soil around the clump, but don’t cover the top of the rhizome. It likes to bask in the sun.

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