Digging, Dividing and Replanting Bearded Irises

So you have some bearded iris that you want to move, or that aren't flowering as well as they did a few years ago? Chances are that they need to be dug, divided , and replanted....and now is the time to do it in late July, during the month of August and to mid-September in the Bay Area.

Bearded Iris

Cease watering a week or two before the day that you dig, to edge the plants into full dormancy. If the ground becomes so dry that it is impossible to dig, water enough to soften the soil again a couple of days before dig day. Use a digging fork and go around the clump to begin to loosen the roots, prying upward with each cut. At this point, if the clump is very large, another person with a second fork may be needed to help pry the entire clump out of the ground. Old clumps become very heavy!

Shake as much soil off the clump as possible, then use a forceful stream of water from a hose to wash away remaining soil. When the roots and rhizomes are exposed, begin breaking away the rhizomes from the clump by twisting them by hand. Save only those rhizomes that are attached to the sides of old “mother” rhizomes. The saved rhizomes should have 4 to 5 leaves in their fans. They are the ones that will be replanted to bloom the following spring.

NOTE: Once a rhizome has bloomed, that rhizome will not bloom again. Only the “increase” rhizomes that grow along the edges of the “mother” rhizome will bloom, so save the “increase”. Discard all the other rhizomes and leaves by placing in the curbside trash can. Do NOT compost iris plants.

Bearded iris before dividing

Bearded iris rhizomes before dividing

How can a “mother” rhizome be recognized? It may have a bloomstalk still attached. Or it may have a small, round, smooth callus, about the size of a thumbnail, at the leaf fan end where the bloomstalk was attached. It probably will have from one to three new rhizomes with their leaf fans growing off of it. Select the largest size “increase” rhizomes to replant. Trim their leaves to 8 or 10 inches before replanting. And this is the only time to trim leaves. Do NOT trim leaves of plants that will stay in the ground from one year to the next. . Then hold the roots of the rhizome in one fist and trim off the long roots below your fist.

If the name of the cultivar that you have dug is known, use a Sharpie pen to write that name on the central leaf of each fan. This will stay on the plant through washing, drying, and replanting ONLY. It will NOT stay on permanently. Now lay the “keeper” rhizomes aside in a shaded location, a garage or cool shed is a good storage area, while the planting beds or planting holes are readied. It will not damage the prepared rhizomes to remain out of the ground for two weeks.

Select a planting location with good drainage that receives a minimum of 5 to 6 hours of sun every day or that is in full sun. Iris may be planted in a bed or as clumps between other plants. Prepare the planting areas by digging at least 12” deep and 2 feet across. Add 1/3 the amount of compost as soil, and a handful of a complete fertilizer with all three numbers the same, such as 12-12-12, for each rhizome that will be planted in the hole. Iris are very heavy feeders. Mix soil and amendments and leave for a week or two, if possible, prior to replanting. The “wait” time is not mandatory, just gives the mixture time to mature.

Just prior to replanting, submerge the rhizomes and leaves in a solution of 1 part chlorox to 10 parts of cool water for about 20 minutes. Rinse thoroughly with running water, and allow to dry off in the sun. . This washing tells the rhizome to break dormancy and start growing again. It will also tend to prevent most fungal diseases.

For an instant clump, plant three rhizomes of the same variety together. Imagine a large clock face. Leaving an 8 inch circle in the middle of the clock face, plant a rhizome at 12 o'clock (leaf fans at the perimeter of the 8” circle, rhizomes pointing toward the outer perimeter of the clockface), plant another rhizome at 4 o'clock, and the third rhizome at 8 o'clock. Cover the rhizomes with soil, leaving the upper surface of each rhizome even with the soil level. DO NOT PLANT DEEP ! Plant clumps no closer than four feet center to center, individual rhizomes no closer than 24 inches apart. Bearded iris can stay in the original planting spot for 2 to 3 years before the gardener has to dig and divide again so they need the extra space to grow and increase.

Bearded iris after dividing 

Bearded iris rhizome after dividing

If the name of the iris is known, write it with an oil base china marker (available at office supply stores) or label maker tape on a plant marker and place it in the middle of the clump. Narrow Venetian blinds cut into 12” lengths also make inexpensive plant markers. Write the name on the top end & the bottom end of the blind using china marker or label maker, and push the blind into the soil so that only the upper end of the marker is visible. If the name should come off the exposed end, the name will remain clear on the part underground. Making a planting map with location and names of the iris is also a good idea since markers can get lost or displaced during weeding and other garden work.

Water plants deeply to settle them in. Then water only enough to keep the ground moist. The new plants will begin to send out new roots immediately. New leaves will begin growing more slowly at the center of the leaf fan . Old leaves will turn brown and may be pulled off with a sideways tug. Keep the area weeded. Continue watering as noted until the fall/winter rains take over . T oo much water or standing water will cause the rhizomes to rot. A drip watering system is best for bearded iris. Overhead watering can lead to leaf spot, a fungal disease that causes circular brown spots on the leaves. To control leaf spot, spray before the first rain with a fungicide formulated for iris leaf spot. A second spraying may be needed in early spring as the weather warms.

In February, when plants begin spring growth, feed each clump with one handful per rhizome of the same complete fertilizer used at planting time. Apply as a side dressing, scratch in, and water –or side dress and scratch in just prior to a natural rain.

At this time keep a sharp eye out for snails and slugs. Hand pick after dark or, for a heavy infestation, review the UC Pest Note on snails and slugs. Fertilize again in the same way after the bloom season is over, approximately in late May. Cut off spent bloomstalks at an angle with a sharp knife or clippers just above the rhizome. Water through the summer for development of larger rhizomes. Keep beds weeded and withered leaves removed from the plants and the beds. Older varieties may have sentimental value because Grandma had them in her yard, or because they were inherited from a good friend – that probably makes it reasonable to grow a clump of each. However, if the idea of having iris bloom in your garden all year long is appealing, or, if the modern, huge, ruffled and laced blossoms are the ones that call your name,go online and search for iris and search for "bearded iris suppliers", you'll find lots more information. 

By Philippa Alvis, MG ‘07