By Richard Jauron, Greg Wallace
Herbaceous perennials (those that die back in the fall and emerge in new growth in the spring) are commonly divided for three reasons: to control size, to rejuvenate plants and to propagate a prized perennial. Horticulturists with Iowa State University Extension and Outreach offer tips on the proper way to divide perennials. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.
Do I need to divide the perennials in my garden?
Some perennials, such as garden peonies, can be left undisturbed for 50 or more years. Others need to be divided periodically to contain their spread or to rejuvenate declining plants. Perennials, such as bee balm and obedient plant, are vigorous growers. They grow so rapidly that they begin to crowd out neighboring plants within a few years. Bee balm and other vigorous growers should be divided every two to three years. After dividing the perennials, replant some of the divisions, give others to relatives or neighbors and discard remaining plants in the compost pile. In contrast, the performance of some perennials, such as bearded iris, begins to decline after several years.
If not divided every three to five years, bearded irises usually don’t bloom well because of overcrowding. Declining perennials should be divided to rejuvenate the plants. While division may not be necessary, many perennials can be divided on a periodic basis to obtain more plants. It’s best not to divide butterfly weed, false blue indigo and several other perennials as these plants have deep taproots. Attempts to divide perennials with deep taproots are often unsuccessful.
When should I divide my perennials?
The best time to divide perennials varies with the different plant species. Early spring (just as new growth begins to appear) is the best time to divide aster, chrysanthemum, hosta, bee balm, sedum, yarrow, obedient plant and ornamental grasses. Bearded iris should be divided in July or August. Late summer/early fall is the best time to divide peonies. Daylily, garden phlox and coreopsis can be successfully divided in early spring and late summer/early fall.
How often can I divide my perennials?
If you would like additional plants, many perennials can be divided on a periodic basis. The frequency of division depends on plant vigor. Most fast growing perennials, such as aster, coreopsis, daylily, hosta and garden phlox, can be divided every two to three years. Perennials that grow rather slowly, such as peony, can be divided every four to five years. Some perennials, like false blue indigo and butterfly weed, don’t like to be divided and should be left undisturbed in the garden.
How do I go about dividing perennials in spring?
Dig up the perennials in early spring just as new growth begins to appear. Divide each plant clump into sections with a sharp knife. Each division should contain several shoots and a portion of the root system. Replant the divisions immediately. Keep the newly divided perennials well watered through spring and summer. Most newly divided perennials do not bloom well until their second growing season.