By Cindy Haynes
Extension Horticulturalist
Iowa State University Extension
Even though it is the start of the school season, don’t let the title fool you, this article is not about long division or other math-related problems. It is, however, related to the multiplication of some of our favorite landscape plants – perennials. Experienced gardeners know that many perennials can be easily divided and shared with friends. If done properly, dividing perennials is relatively simple and painless for the gardener and the plant.
Why Should I Divide Perennials?
While perennials can’t talk, they can communicate when division might be needed. Division often rejuvenates plants. When they start to grow more slowly or even decline, division is recommended. If a perennial is not blooming well, especially in the center of the plant, division can improve the size and number of blooms. When a perennial is overtaking its designated area or encroaching into other plants, division is a way to control or limit unwanted growth. Some perennials have problems with diseases like powdery mildew (white fungus that coats leaves). Division helps increase air circulation around the plant, thus limiting the incidence and spread of some diseases. And finally, sometimes we are greedy and want more of our favorite perennials. Division is a great way to get more of them in the landscape.
How Do I Divide a Perennial?
Division is the act of separating one plant into many. The actual procedure is really as simple as the definition. While there are many methods of division, a method that works for most plants is to start by digging up the entire plant (if possible). Examining the root system and overall health of the plant is easier to do when you can see it all. For some perennials, like bearded iris, this is extremely important. Look at the swollen roots for signs of borers or rot. Destroy any root sections that have evidence of insect or disease.
Digging up the entire plant also allows you to notice any spaces in the plant crown or clump which could be natural areas for separation. Sometimes perennials will separate easily into multiple pieces with little effort. Other times sharp knives, shovels, even an ax may be necessary to cut the plant's root system apart. If the tools are required, it is often best to cut the perennial into sections like you would a piece of pie. This works well with ornamental grasses that are starting to die out in the center. These wedge-shaped pieces contain more of the vigorous “outer” portions and less of the dead “inner” portions of the plant.
After carefully separating the perennial into many smaller plants or divisions, simply place one of the divisions back in the original planting hole. Be sure the division is planted at the same depth as the original plant. Perennials planted too deeply will not perform well. Some perennials like peonies do not bloom well when the buds (located where the roots and crown connect) are planted more than 2 inches below the surface of the soil. Good care and maintenance like regular watering and weeding will be needed to re-establish a good root system.
When Should I Divide Perennials?
Now is a great time to divide many of the spring blooming perennials. Division in late summer and early fall gives the perennial plenty of time to re-establish in the landscape before winter. Wait to divide the summer and fall blooming perennials in early spring as they begin to emerge from the ground. When to Divide Perennials (RG 319 from ISU Extension) will give you times to divide specific perennials. Generally there is only one rule on this subject – don’t divide a perennial when it is currently in bloom.
What Size Should I Make the Divisions?
The ultimate size of a division depends on three things; 1) the plant species, 2) the amount of time you have to care for the divisions, and 3) your level of patience. Some perennials like hosta, iris, and daylily will grow and re-establish quickly after division, regardless of size. Small divisions with only 2-3 buds, eyes, or fans are perfectly acceptable for these species. Other perennials like peony grow and bloom better when divided into larger sections.
The smaller the division, the more time and effort will be needed to re-establish the plant in the landscape, regardless of species. Small divisions have smaller root systems and need more frequent care. While small division will give you more plants, you increase the risk of failure or death of the divisions without proper and sometimes constant care.
How long are you willing to wait for blooms on your newly divided perennial? Since many perennials will need a year to fully re-establish after division, blooms are typically poor the year after dividing. Some plants may take 2-3 years to bloom after division. Small divisions will take longer than large divisions to re-establish and bloom again.
How Often Should I Divide?
The frequency of division depends greatly on the species. Most vigorously growing perennials can be divided regularly, if wanted or needed. Fast growing daylilies, hosta, bee balm and mums can be divided every 3 to 5 years. More slow growing perennials such as phlox and peonies can be divided every 5 to 7 years. Some perennials like False Indigo and Gas Plant prefer to be left alone in the garden and can live happily without division for decades.
So, knowing a bit about each of the perennials in your landscape can increase your success in division and save you a lot of effort in the future. Fortunately math skills are not needed.
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