The why, when and how of dividing perennials
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Many gardeners love flowering perennials because you don’t have to plant them every year, saving labor. However, most perennials do require occasional maintenance. A well-maintained perennial will have healthy growth and larger blooms.
Herbaceous perennials produce new plantlets each year, which become crowded, competing for resources. In time, herbaceous perennials will often become a dense clump of plant material. Blossoms may lose their size and vigor. Often the center of the plant will die out. Dividing perennials will help to overcome these problems. Division is also a way to “produce” more plants.
Most herbaceous perennials should be divided every three to five years—some more often, and others, never. Careful observation of the plants will give good indicators of when they need to be divided.
Typically, fall-blooming perennials should be divided in the spring and spring/summer-blooming perennials should be divided in the fall. If possible, avoid dividing them when they are in bloom. Immediately after bloom is much better than before bloom. Most important is the care they receive during division. Avoid dividing during hot, windy, sunny weather. Fall division should take place at least six to eight weeks before the ground freezes.
The mother plants should be watered a day or two before the operation. To minimize time out of the soil, prepare the new location before transplanting. If the roots are going to be out of the soil for more than a few minutes, wrap them in moist paper or wood chips.
Most plants will survive the shock of division if the leaves are cut back to about six inches, to reduce dehydration stress. The procedure may differ slightly depending on the root system, but in all of them you lift the mother plant by slicing all around the crown, then slide the spade under the root ball so it can be lifted in one piece.
Spreading roots systems such as astilbe, bell flower, blanket flower, coneflower, or black-eyed Susan roots can often be pulled apart by hand, or tool, into individual plants. Include at least three to five buds with each new clump.
Clumping root systems such as daylilies, rhubarb, red-hot-poker, or lily of the valley will often require a spade or machete to divide the crowns. Each new division should have several eyes, or buds.
Rhizomes (underground stems) such as bearded iris, peony, or hosta can be separated with a sharp knife, making sure there are a few buds with each division.
The new plants should be buried as soon as possible with the crown at or slightly above the new soil level. Be sure to settle the soil in around the roots and water well immediately after transplanting. Do not fertilize the first year.
Care after division
Keep the new plant well watered, but not waterlogged, for a couple of weeks after it has been planted. A thick mulch after the ground freezes will help protect the new plants from frost heaving. Some perennials (oriental poppy, columbine, lupine, monkshood, bugbane) should not be divided. Here is a list of common perennials with division information from University of Minnesota Extension.
May your perennials be ever beautiful.