If your garden has a vigorous perennial that has been in the garden for more than a few years, or it has started to choke out other plants and no longer looks healthy (dead centers, floppy and unable to support healthy foliage and flowers), then it may be time to divide. Dividing is also a great way for a gardener to get more of their favorite plants into their beds.
Some perennials like to be divided yearly (chrysanthemum), but most can go three-to-five years without division. Some can go much longer, and some even do not require division at all, like butterfly weed or baby’s breath.
While spring is the ideal time to divide most perennials, fall is the next best choice, and some plants even prefer a fall division. Dividing in September and October gives the roots enough time to establish before winter. As a rule of thumb in fall, gardeners will want to focus on dividing spring- and summer-blooming perennials, and leave the fall bloomers for spring division.
Follow these steps for successful transplants
1. Make sure the ground is moist to make digging easier and reduce stress on the plant.
2. Dig out the entire clump. Use a spade to dig down at least 7 inches to get most of the root ball.
3. Discard weak or dead sections.
4. Separate the clump into three-to-five pieces with a sharp knife or spade edge. It is best to leave three-to-five growing points in every clump. If separating peony, they need at least three eyes to ensure flowering the next season.
5. Immediately plant one clump back into existing spot, if desired. Clumps planted elsewhere should go into holes twice as wide as the clump. Add organic matter as needed.
6. Trim back any foliage to 6 inches from the ground to reduce transpiration and transplant shock.
7. Mulch with two-to-four inches of organic wood mulch.
Examples of perennials that can be divided in the fall include Asiatic lily, spiderwort, veronica, amsonia, Jacob’s ladder, oriental poppy, peony (leaving three-to-five eyes per division), astilbe, iris (should be divided in early September and have at least one fan of leaves when planted), yarrow, lady’s mantle, European ginger, wild ginger, aster, astilbe, bee balm, daylily, hosta, coreopsis, false rock cress, basket of gold, bellflower, perennial bachelor’s button, veronica, red valerian, campanula, cardinal flower, sundrops, yellow corydalis, turtlehead, and foxglove.
Ornamental grasses should only be divided in the spring.
Source: Kelly Allsup, Extension Educator, Horticulture, firstname.lastname@example.org