How do I divide perennials?
Fall is a great time to divide many overgrown perennials.
In general, summer-flowering perennials are divided in fall and fall-flowering perennials are divided in spring. Dividing perennials when they are not blooming reduces stress on the plants. The less stressed the plant is, the more likely it is to survive division, especially with some tough perennials.
Most perennials are divided every three to five years. Some may wait eight to ten years and others resent division and will let you know by refusing to thrive or becoming afflicted with the “rise and die” disease.
Perennials often send signals to let you know when they need to be divided: reduce blooming with smaller flowers; The center of the plant dies, leaving only green growth on the edges; The plant loses its strength. The plant needs staking due to housing. Otherwise, if perennials are growing and flowering well, they should be left alone, unless more plants are needed or the plant has outgrown its limits.
Before you rush to divide perennials, it is important to first prepare the plant.
- Water plants should be divided a day or two before you plan to do the work.
- Plant plum stems and leaves six inches off the ground to facilitate splitting and reduce moisture loss.
- Choose a cloudy day, ideally with light rain expected for several days, to reduce heat stress on uprooted perennials.
- Using a sharp, pointed shovel or scraping fork, dig deep on all sides of the plant and about four to six inches from the plant.
- Lift the entire clump and shake or shake off the loose soil. Remove dead leaves and stems, trim any damaged roots, and discard the old, dead center of the plant. This will help clean the plant and expose the roots.
Plants are divided according to their root system. The list below includes several common perennials, their type of root systems, and division steps:
Spread: (Astilbe, Beebalm, Bellflower, Black-eyed Susan, Cranesbill, Coreopsis, Garden Plox, Lamb’s Ear, Purple Cone-Flower, Yarrow) These plants have a tangled root system with no distinct pattern. They have a tendency to die at the center as they get older and some can become invasive. Divide these plants into groups of three to five strong shoots each. The roots can usually be cut with a knife, but thick, tangled roots may require forceful separation.
Agglomeration: (Delily, Hosta, Lily of the Nile, Red Hot Poker) These plants arise from a central clump, and many have thick, fleshy roots. It is often necessary to cut the crowns of these perennials with a sharp knife. Keep at least one bud with each division; Leave several shoots for larger plants.
Rhizome: (bearded iris, blackberry lily) Rhizomes are stems that grow horizontally at or above the soil level. When dividing the rhizomes, cut and discard the oldest parts and those damaged by disease or insects. In the case of iris, your new section should have a few inches of rhizome and one fan of leaves. Replant so that the top of the rhizome appears just above the soil level.
Keep your new sections moist until they are planted. Replant the sections at the same depth as they were originally and tamp down the soil around the roots to remove air pockets. Fall can be dry at times, so be generous with water.
Adding mulch to garden beds is always a good idea, and with new plants, this may give them some extra survival insurance this winter. A light application of 10-10-10 will also help give the roots of new transplants a healthy start.
Division will rejuvenate older plants and keep them strong and thriving freely. Dividing perennials is an easy and inexpensive way to get additional plants for your garden or to share.
However, remember that not all plants appreciate division. Butterfly weeds, euphorbia, baby’s breath, false indigo, columbine, rosemary, cotton candy, lavender, and rosemary are some things that are best left alone.
The work you do this fall will pay off next spring.
P. Andrew Rideout is the UK Horticultural Extension Agent and can be reached at firstname.lastname@example.org.