Every spring, I get notes and calls from gardeners disappointed by the poor showing of their tall bearded irises. These rank among the most beautiful of spring perennials, so I understand the disappointment.
While gardeners are anxious to take action immediately to remedy the problem, I always caution them to wait until July to dig and divide the rhizomes. This is the remedy that most often will solve the problem of poor bloom on bearded irises. Of course, July has been blazing hot this year, and not a lot of digging and planting has been going on. But if your plants did not bloom this year, this is the time to do it. This work will also help you spot any damage caused by rot or borers.
A garden fork is your best tool to dig up the clump. Sink the fork (or a shovel) into the ground 8 inches or so from the perimeter of the clump. Work it around the plant and then lift the clumps. A good bit of soil will come up, too. Notice at the time how shallow the rhizomes and roots sit in the ground.
Next, shake off the loose soil, then wash the rest away with water. If you have many rhizomes to divide, set the clumps on plastic, such as a leaf bag, and carry it to a cooler place, such as your porch or deck.
Separating the rhizomes can be easy. Some will be loose enough to gently pull apart. Others will be attached to the mother plant and can be cut apart with a clean knife. You should see these younger, fresher rhizomes around the perimeter of the clump. Those are the ones you want to keep and replant.
Lay out the rhizomes. If you have many more than you have time and space to replant, choose the plumpest, freshest-looking rhizomes and let the rest go.
Cut the foliage to about 6 inches. If it’s any taller, the foliage will move in the breeze and dislodge the rooting rhizomes. A good division, about 3 inches long, will have a fan of leaves at the top and some roots growing from the bottom of the rhizome.
While you are judging which of the divisions to keep, look for signs of damage by borers, which would be little holes in the rhizome. Get rid of any showing this problem. Also look for signs of soft rot in the rhizome. Cut that off with a clean knife sanitized between cuts by dipping in a solution of water and 10 percent bleach.
Replant the rhizomes in a hole 2 to 3 inches deep and wide enough to accept the rhizome.The final step is replanting your bearded iris. Choose a full-sun location and start by digging a shallow hole that will be wide enough to spread out the rhizome’s roots. Make the hole about 2-3 inches deep, then create a mound in the center of the hole to just about soil level, as shown above. Soak the soil in the planting hole. Then take a rhizome division and place it in the center of the mound. Spread the roots around and down the mound. Work soil around the division and firm it with your hand. Aim to let the top third of the horizontal rhizome rise above the soil line. Water and keep watch to make sure the rhizome does not settle and get covered with soil.