I had a note from a disappointed gardener recently, sad over the poor showing this spring of her bearded irises. The clump that had performed well in previous years produced no flowers.
This disappointment is a reminder that bearded irises require digging and dividing every few years or so for best performance.
The good news is this is not a difficult job, largely because the clumps are easy to lift out of the ground, followed by dividing and replanting.
The bad news – and please don’t blame me – is that it is a job for now until August, the hottest time of the year.
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But it will probably solve the problem of a poor showing of flowers because the clump is refreshed by removal of the oldest growth and given room to develop anew.
A garden fork is an excellent tool for this job, but a shovel will work. Just sink it into the ground a few inches beyond the outside of the perimeter. Work your way around the clump and it should rise easily. The roots do not grow very deep below the fat, horizontal roots called rhizomes, which send up the flowers.
Once the clump is up, take time to shake off loose soil, then rinse off the rest so you get a clear view of the packed rhizomes. This is where the real action begins. It helps to put the clump on a sheet of plastic, such as a leaf bag, or on newspaper.
Look over the packed rhizomes, noting ones that show any signs of rot or holes caused by the iris leaf borer. Discard those rhizomes. Soft rot can actually be cut away, but if you have plenty of fresh, healthy rhizomes, don’t bother doing that.
The younger, fresher looking rhizomes are the ones you want to save, and they tend to grow from the older ones, which are usually the largest in the clump. Some of these new rhizomes will be loose enough for you to gently pull them away from the mother rhizome. Others will be attached more firmly; cut them away with a knife.
As you do, cut and separate, choose the plumpest, freshest ones to save and replant. Cut back the foliage to about 6 inches. If taller, the leaves will move in a breeze and dislodge the rooting rhizomes. A promising new division not only looks fresh and plump but is about 3 inches long, has a fan of leaves at the top and roots growing from the bottom.
It is critical to reset the rhizomes with care. They should sit horizontally with the top of the rhizome – about one-third of it – still visible so sun can reach it. Dig a shallow hole, 2 to 3 inches deep, that is wide enough to spread out the rhizome’s roots. Make a little mound in the hole so the rhizome will sit on it. This will keep the rhizome at ground level as you fill in the sides with soil around the roots.
Water the newly set plants and watch them to make sure the rhizomes don’t settle into the soil and get covered with it.