September calls us to divide our herbaceous perennial plants.
We divide these plants for one or two or three reasons:
First, the plant has become too large for its space, crowding other plants or even spilling into a walkway. When encountering such situations, remember to install new plants with their mature size in mind.
Second, the plant needs room to grow, produce blossoms and stay healthy. Many perennials need to be divided every three or four years, or will reduce blooming or become misshapen. Ideally, divide your plants when they are still at their best.
Finally, you want more of the same plant, either to place elsewhere in your garden or to give to friends. Dividing perennials yields new plants quickly and with little effort, and at lower cost than any other method except planting seeds.
Schedule a division project for cool weather, and water the plant in advance to make digging easier. Plan to replant the divisions soon, or prepare to store them in shade and kept moist.
Dig a trench around the plant, at the edge of the branches (the drip line). This will sever the longer roots cleanly, and retain the plant’s primary roots.
After trenching, dig under the plant from several angles, and lever the plant out of the ground. When dividing a large plant, slice from above to cut the root ball into two or four sections for easier handling. Re-fill the hole with compost.
Once the plant is out of the ground, decide whether to plant large or small divisions. Large divisions, e.g., halves or quarters, when replanted, will yield a full specimen in a single season. Many plants can be divided into many small plants that will grow quickly, but take longer to reach mature size.
In making this decision, remove the soil from the roots by gently shaking or washing, and examine their structure.
Perennial root types include offsets, surface roots, taproots, underground running roots, and woody roots. Dividing each of these requires a specific method that may be obvious upon inspection but for more about these methods visit ongardening.com.
Select healthy divisions for replanting or gifting. With perennial plants that develop a dead center, discard that section and choose divisions from the outer sections. Also discard any divisions that appear broken, weak or diseased, or that have minimal roots.
Plant the divisions where they will have their preferred exposure to the sun and space to grow to mature size. Dig a hole large enough to spread the plant’s roots, water in and keep moist until the rains begin.
Dividing herbaceous perennial plants can be a productive and satisfying project for the early fall.
Enjoy the season!
Fine Gardening magazine has posted Janet Macunovich’s helpful article on dividing perennials, including lists of specific categories of plants that should be divided in particular ways, as well as a few perennials that are best not divided.
The same magazine has posted a series of short video recordings that demonstrate methods for dividing perennials with different root types.