Perennials are the healthiest and bloom the best when they are young and have room to spread. After a number of years, you may notice fewer blooms, overall reduced vigor and the center of the plant has died out leaving a circle of foliage around it. The good news is you can rejuvenate them by dividing them. The best times to divide perennials are in the spring when new growth is still low to the ground, or in early fall when the nights become cool.
Begin digging a trench around the drip line, then cut at an angle down and under the clump from various points around the outer edge until you can lever the plant out of the hole. With a sharp knife or spade, cut the plant into half, and then be sure to cut out and discard the central dead portion. For large, heavy, plants, you may have to first dig the trench, and then slice straight down through the center as if it were a pie. Halving or quartering the clump while still in the ground, then undercut and lift each section out. Keep the divisions cool and moist until you are ready to plant them by putting them in a bucket or box with moistened newspaper draped over them. This will help prevent the roots from drying out. Before replanting one of the divisions back in the original hole, take the time to work compost or Bumper Crop into the existing soil to renew the soil and maintain fertility. Replant divisions at the same depth they grew originally. Backfill with soil and firm well. Finally, soak the soil thoroughly with a solution of Esbenshade’s Root Stimulator diluted in water to help reduce transplant stress and encourage root growth.
Rhizomes are stems that grow horizontally at or above the soil level. Bearded irises are the most common perennial with this type of root system. Divide irises anytime between a month after flowering until early fall.
Cut and discard the old rhizome sections that are in the center of the clump with no leaf fans. Also, inspect rhizomes for disease and insect damage. Damaged rhizomes should be trimmed and treated, or discarded if too badly damaged.
Iris division should retain a few inches of rhizome and two fans of leaves, trimmed back halfway. Replant with the top of the rhizome just showing above soil level.
Don't Divide these Perennials
Some plants resent being divided and it should be avoided if possible. These include butterfly weed (Asclepias), euphorbias, oriental poppies, baby’s breath (Gypsophila), gas plant (Dictamnus albus), Japanese anemones, false indigo (Baptisia) and columbines (Aquilegia).
Lenten and Christmas roses (Heleborus) are very difficult to move when more than a few years old. Usually you can find tiny seedlings around the base. These are easy to move. Lavender contton (Santolina chamaecyparrus) and several other perennials are actually small woody shrubs and should not be divided. These include perennial candytuft (Iberis sempervirens), lavender, rosemary, southernwood (Artemesia abrotaum), and several other artemesias. These plants often have rooted layers (branches that have developed roots while touching the soil). The layers can be cut off the parent plant, dug up and replanted as though they were divisions