I had a note from a disappointed gardener recently, sad about the poor showing this spring of her bearded irises. The clump that had performed well in previous years produced no flowers.
This was a reminder that bearded irises require 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 dig 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.
Divided iris clumps will probably produce more 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 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 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, choosing 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 be covered by it.