Gay Wilhelm, Master Gardeners of Placer County
From The Curious Gardener, Fall 2008
Overgrown perennials tend to have fewer blooms. In general, it is best to divide spring and summer blooming perennials in the fall, and fall bloomers in spring. By dividing the plant when it is not flowering, all the plant’s energy can go to root and leaf growth. Fall division should take place between September and late October. Allow at least four to six weeks before the ground freezes for the plants to become established. And it is best to divide them on a cooler or cloudy day.
Water plants to be divided thoroughly a day or two before you plan to divide them. Prune the stems and foliage to 6 inches from the ground in order to ease division and to cut down on moisture loss.
Once the division is made with a sharp shovel or digging fork, shake or hose off loose soil and remove dead leaves and stems. Be sure that divisions have attached roots.
Some fall division perennials are plants with spreading root systems. As they mature, these plants often crowd each other out and compete for nutrients. By dividing and adding fresh amendments, you will give your plants a fresh start and increase their blooming potential. Aster, bee balm, lamb’s ear, and purple coneflower are examples. They can usually can be pulled apart by hand, or cut apart with shears or knife. Each new plant should have three to five vigorous shoots.
Another perennial group to divide in the fall has clumping root systems that originate from a central clump with multiple growing points. This group includes astilbes, hostas, daylilies, and many ornamental grasses. Keep at least one developing eye or bud with each division. If larger plants are wanted, keep several eyes.
A spreading rhizome like iris should be divided without breaking the rhizomes off the clump. Discard any rhizome sections that are older than one year, damaged or diseased. The divisions should retain a few inches of rhizome and one fan of leaves, trimmed back halfway. Replant with the top of the rhizome just showing above soil level.
Most perennials benefit from division every three to four years. You will be surprised at the plant material that can be easily propagated from a clump of overgrown perennials. Enjoy!
References
California Master Gardener Handbook, UCANR publication # 3382.