In early summer, I drove from nursery to nursery to buy a certain color of bearded irises. But no luck! I assumed that I was too late - until one of the staff explained the real reason: the Iris Borer which had wiped out a huge part of the crop in many nurseries and the plant wholesale.
It is a pest that carries bacterial soft rot into tubers, triggering infection with a strong unpleasant odor. So, I decided to divide the Irises I had on hand. I hadn't done it for a couple of years. Bearded irises and Siberian irises need to be divided periodically - every three to five years - to maintain a good flower production.
Bearded irises are best divided in late summer, about six weeks after blossoming ceases, while Siberian irises need to be divided in spring just when the first small shoots show up. First remove the upper half of all leaves, using pruners to cut through the thick foliage. This reduces moisture stress. Lift the clump, discard older rhizomes and discard tubers that are damaged, hollow or mushy. Use a garden hose to spray soil debris off all the rhizomes and their roots. Examine the rhizomes carefully for holes and signs of decay. Replant the younger tubers 30 centimeters apart, to obtain vigorous stalks and flowers next season.
Don't Plant Too Deep!
Dig a hole for each transplant about 12 centimeters deep, with a mound of soil in the centre. Compost, composted manure, and peat moss can be added to the soil if it needs renewing. Set the rhizome on the mound and allow its roots to hang down the sides. Fill in with soil, covering the roots entirely and halfway up the sides of the rhizome, leaving its top surface exposed. Covering over the top with more than two centimeters of soil would cause it to rot. Water the transplants in and keep the soil moist while they establish new roots.
See a short video:
https://www.youtube.com/watch?v=KkaO-mplU6M
How to Divide Siberian Irises
Siberian irises will cease flowering when they need to be divided, every three or four years. Clumps also grow from the center outward and give way to a "bald" center.
Siberian irises have thick, strong roots; you may need garden forks and elbow grease to separate the clumps. Early spring is the best time to divide Siberian irises, just as the new foliage is pushing up through the soil.
Lever the clump carefully with a garden spade, sliding the spade under the roots and lifting upward. With your hands, try to separate the clump into sections along natural lines of division. If the mass of rhizomes and roots is too strong to pull apart, use two garden forks inserted back to back in the center and lever it apart.
It is also possible to cut it in half with a blunt-nosed spade. Separate the clump into two or more sections, replant the sections 45 to 60 centimeters apart at the same level they were growing, then water them into their new hole.
Happy Gardening!
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