If you grow irises long enough, they will need some care and attention. Many resources suggest digging and dividing irises, but few thoroughly describe doing either. Heather frequently sees questions about this on the Iris Lovers Facebook group. Since our family has been digging and dividing recently, it seems like a worthy topic to write about.
Why do we dig and divide irises?
Irises reproduce asexually by forming new baby plants (called increases) on the sides of an older rhizome. Informally, we refer to the older rhizome as the “mother” and the smaller offspring as “daughters.” The mother plant provides a food source for daughters while they remain attached to her.
Left alone, irises will form “clumps,” and their exponential growth habit produces crowded conditions after several generations. A favorite example is ‘Peggy Sue’: a lovely reblooming iris that produces two healthy increases each year like clockwork. After one year, she had two daughter rhizomes. After two years, 4 granddaughter rhizomes appeared. After three years, there were 8 great-granddaughters. After four years, 16 great- great granddaughters formed a complete circle around their ancestors. It was impressively dense and nearly a foot and a half in diameter.
If irises grow undisturbed for a long time, rhizomes begin to grow on top of one another. If clumps are REALLY crowded, you might even see a daughter forming ON THE TOP of its mother.
Genetics and growing conditions determine how many healthy increases a mother rhizome will make, and how many of her daughters survive. For this reason, it’s hard to say exactly how often your irises need to be dug and divided. For the popular tall-bearded type, the recommendation is every three or four years.
When do you we dig and divide irises?
At Heather’s farm in Ramseur, North Carolina we try to dig, divide, and replant irises from mid-June to mid-September. In the heat of the summer, most irises go through a window of dormancy. During this time, irises can be removed from soil without disturbing processes vital to their growth and reproduction. We time digging to occur at least six weeks after bloom because irises use that time to set structures needed to bloom in the future. We try to finish replanting at least six weeks before first frost because irises need to grow new roots before temperatures get too cold.
However, North Carolina can be extremely hot and humid in the summertime. Several years back, Heather got heat rash (also known as prickly heat) during July when trying to get a new iris bed established and planted in full sun. She couldn’t garden for several weeks after that and swore never to do it again—no matter how crispy and dry her iris rhizomes look.
Now older (and wiser?), Heather avoids working in iris beds when conditions are too hot and humid for sweat to evaporate. We tend to dig iris clumps in the morning, quickly toss then into containers, and put containers of irises in the shade to divide, trim, and label. Shade is provided by a pop-up tent, carport, covered porch …the last being a joy of growing irises "Out East." Ensure access to cold beverages and a nice breeze—supplied by a construction-type battery-operated fan if Mother Nature doesn’t supply it.
Digging and dividing irises is a lot like riding a bike. With enough practice, your body will intuitively know what to do. However, if you have never seen it done it can be quite confusing.
First we look close to the clump, and take a good look at it while trying to imagine where the roots are underground and how daughters are attached to their mothers. If irises were planted too close together, it can be hard to discern where one clump ends and another one begins. Using your fingers to pull soil away from the clump can help you see and feel connected rhizomes and plan accordingly.
In California, Alleah preferred to use an ergonomic digging fork in her sandy soil and raised beds. Heather likes digging with the fork, but also uses a sharp rounded shovel to dig irises in her clay soil. The irises don’t seem to mind either one, so feel free to use whatever medium-to-long handled digging tool is in reach.
Place the tool about 6 inches away from the clump and press it deep into the ground. Next, push the handle down firmly to lift the clump of irises up. Don’t worry if this separates mother and daughter rhizomes; it was probably going to happen anyway. Place the tool in a new location and repeat until rhizomes can be picked up easily. Pulling on rhizomes can damage their leaves or roots so we avoid using too much force.
If plants and soil seem stuck together, dropping or throwing a clump root-side down onto the ground from waist-height is quite satisfying and effective. We dig one variety at a time and place its permanent tag upside down in a large plastic pot or similar container. Irises of that variety are placed in the pot, and gently stacked for further processing.
Avoid stabbing irises with your tools, stepping on them, or throwing soil where it doesn’t belong. Don’t ask how this happens… digging can be really tiring. If at all possible, train a young person to help you dig. They might get really excited about plants which can withstand being torn apart and thrown on the ground and want to grow their own. Both Alleah and Heather got interested in irises by helping to dig them. It works! You very well could inspire a youth to become a member of the American Iris Society.
How do you divide irises?
This is Heather’s favorite part. Mother and daughter rhizomes look like ginger root and can be snapped apart in a similar fashion. Some people cut rhizomes apart, but we avoid this because a knife should be cleaned between cuts. If a rhizome is soft or really long, we may cut it with pruning shears or on a cutting board with a kitchen knife which is designated for permanent garden use.
How do you replant irises?
Stay tuned for our next blog post… Growing Irises Out East: It’s Planting Time!
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How do you dig your irises?
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