Mid- to late summer is the time to divide irises and clear from the bed any old, diseased, or non-blooming plants. Irises should be divided every three to five years, or when flowering decreases or the clumps start crowding each other. Dividing and replanting in midsummer (or at least six weeks before the first frost) gives the plants time to put down new roots before winter.
Irises grow from rhizomes. Similar to a bulb in that it stores the energy the plant needs to grow, rhizomes are longer and tapered as opposed to round, with a fan or fans of sword-like leaves emerging at the top. Healthy rhizomes are plump, firm, light-colored, and have at least three leaves on each fan.
Once a rhizome blooms, it won’t bloom again. New rhizomes grow off the old, and these are the ones that should be cut from the old and replanted.
Photograph by Jamain, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Rhizomes that are shriveled, sunken, brown, soft, rotting, or smelly should be dug up and discarded – and not composted! Also, if you see fat white worms in the rhizome, get rid of the plant. Those are iris borers. They will kill the plant and spread to its neighbors.
To divide irises, gather your tools:
Garden fork or spade (you are less likely to slice into a rhizome with a fork than a spade)
Tarp
Knife
Bleach solution (one part bleach to nine parts water) or disinfecting wipes
Pruners
Compost, bone meal, or granular fertilizer
Cut the foliage back to one-third its height. This makes the clump easier to work with, and reduces water loss from the leaves once it’s replanted.
Dig them up and shake the dirt from the roots onto the tarp. Examine the rhizomes for decay, rot, borers, and overall health and vigor.
Using the knife, cut off the newer rhizome sections from the older one. Discard the older one and only plant the newer ones that are at least three inches long and as thick as your thumb. It should have healthy roots and four or five leaves. Disinfect the knife with the bleach solution or disinfecting wipes between cuts.
Before replanting, loosen the soil in the bed and mix in a bucket or two of compost or well-aged manure. Instead of compost, you could add bone meal, or a fertilizer that’s low in nitrogen.
Site the rhizomes 12 to 18 inches apart. You can plant them closer together, but that means they’ll get crowded sooner, and you’ll need to divide them more frequently. Replant so that the top of the rhizome is visible right above the soil line. Planting too deeply can cause the rhizome to rot.
Water them in well. Water every seven to 10 days (if no rainfall) as they reestablish and begin to produce new leaves.
Any smaller rhizomes might take a couple of years before they flower, but larger ones will likely bloom next year. Even if next year’s display is a little sparse, in subsequent years your irises will be back and blooming in all their glory.
All articles are copyrighted and remain the property of the author.
By Dan Heims, President, Terra Nova Nurseries, Inc.
Photographs courtesy of Terra Nova Nurseries, Inc.
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