Early fall is a good time to transplant or divide many perennials and ground covers, according to Michael Jesiolowski, manager of the Plant Clinic at The Morton Arboretum. They’ll have several weeks to grow some new roots and get re-established before the ground freezes.
As a general rule, fall dividing is best for plants that bloom in spring or early summer, such as woodland phlox, veronica, daylilies and daffodils. For those that bloom in midsummer or later, such as rudbeckia, coneflowers, sedum, asters and Japanese anemones, wait until early spring so you can enjoy their blooms now.
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“If you need to divide plants at a suboptimal time, it probably won’t kill them,” he said. “It just may mean that you miss a year of bloom while the plant recovers and regrows its root system.”
Why transplant a perennial? Maybe it’s in the wrong spot. A hosta in too much sun can get scorched. Lilies in the shade of a big tree may lean toward the sun. Or you may decide you want that splash of pink geraniums in the front yard instead of the back.
There could be several reasons to divide perennials. The clump may have spread until it’s too large for the site and is crowding other plants. Or you may want to get more plants by splitting up a mature plant such as a hosta.
With age, some perennials and grasses will die out in the middle and only grow around the outside, like a doughnut. In such a case, dig up the whole clump, cut it apart, and replant one of the healthy divisions.
Here are some tips for success in transplanting and dividing perennials.
Make sure it’s mature: Don’t move or divide a plant until it has a well-established root system.
Keep the roots moist: The great danger to a plant is that its roots will dry out before you replant it.
Plan ahead: Before you dig a plant up, decide where you’ll put it and prepare the new site so you can replant it quickly while the roots are moist.
Keep soil on the roots: Dig all around and then under the roots, and try to keep the soil and roots together. If you’re dividing, gently shake off just enough soil to see what you’re doing.
To divide, pull or cut the roots apart: If the plant has a fibrous root system, like a ball of hairs, you may be able to gently pull it apart along natural divisions. If it has thick or tough roots, you may need to use a sharp, clean knife or spade to divide them.
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Make large divisions: Each division should have plenty of roots. It’s better to have two healthy clumps than four smaller divisions that don’t have enough roots to survive.
Water well and mulch: Spread mulch in an even layer 1 to 2 inches deep on the soil all around the plant (not on top of it). Keep watering often enough to keep the soil moist until the ground freezes this fall and all through the next growing season.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.