Garden perennials can be divided for three reasons:
1. To control and maintain the size of the mother plants.
2. To help rejuvenate the plant.
3. To increase the number of plants.
Dividing and replanting keeps rapidly spreading perennials under control, and allows you to maintain your original garden design. Dividing over-grown garden perennials will rejuvenate old plants, keeping them vigorous and blooming freely. Finally, dividing your perennials is an easy and inexpensive way to gain additional plants for your own garden or to share with gardening friends.
As a general rule, you can divide spring and summer blooming perennials in the fall, and fall blooming perennials in early spring. By dividing the plant when it is not flowering, all the plant’s energy can go to new root and leaf growth.
In most cold winter growing zones, hardy perennial division should take place from early September through mid-October. Allow at least four to six weeks before the ground freezes for the plants to become established.
If you divide in the spring, allow enough time for roots to settle in before hot weather. Spring division is ideally done in the early spring as soon as the ground has thawed and the growing tips of the plant have emerged. Spring divided perennials often bloom a little later than usual. You can disbud the plants during the first season to prevent blooming and encourage robust new root growth
Try not to divide perennials on hot, sunny days. Wait until a cloudy day, ideally with several days of light rain in the forecast. In clear weather, dig plants for division late in the afternoon and divide and replant in the cool of the evening.
Most perennials should be divided every three to five years. Some perennials such as German Iris, Chrysanthemums, Asters and Yarrows may need to be divided every one or two years or they will crowd themselves into poorly performing clumps of leaves and roots. Long-lived garden perennials like Bleeding Hearts (Dicentra) and peonies may never need to be divided unless you want to increase your stock.
There are a few sure signs that perennials need dividing: Flowers that are smaller than normal, centers of the plant clumps that are hollow and dead, or when the bottom foliage is sparse and poor. Plants that are growing and blooming well should be left alone unless more plants are wanted.
Plan ahead when dividing perennials:
Water plants to be divided thoroughly a day or two before you plan to divide them. Prepare the area that you plan to put your new divisions in before you lift the parent plant.
Prune the stems and foliage to 6 inches from the ground in order to ease division and to cut down on moisture loss.
Digging the mother plant:
Use a sharp pointed shovel or spading fork to dig down deep on all four sides of the plant, about 4 to 6 inches away from the plant. Pry underneath with your tool and lift the whole clump to be divided. If the plant is very large and heavy, you may need to cut it into several pieces in place with your shovel before lifting it.
Dividing the mother plant:
Shake or hose off loose soil and remove dead leaves and stems. This will help loosen tangled root balls and make it easier to see what you are doing.
Garden perennials have several different types of root systems. Each of these root types needs to be treated a bit differently when dividing.
Spreading root systems:
Spreading root systems have many slender matted roots that originate from many locations with no distinct pattern. Plants with spreading root systems include asters, bee balm ( Monarda ) lamb’s ear (Syachys), Achillea, and many other common perennials. These plants can crowd themselves and other plants around them to the point that none are performing well. Some can be downright invasive unless divided frequently. Their divisions can usually can be pulled apart by hand, or cut apart with shears or knife.
Large, vigorous plants with thickly intertwined roots may need forceful separation with digging forks. Put two forks back to back in the center of the plant and use them to pry the pieces apart.
Divide the plants into clumps of three to five vigorous shoots each. Small or weak and woody divisions should be discarded. Discard the center of the clump if it is dry and woody.
Clumping Root Systems:
Clumping-type perennials originate from a central clump with multiple growing points. Many have thick fleshy roots. This group includes Astilbes, Hostas, daylilies ( Hemerocallis ) and many of the larger ornamental grasses.
It is often necessary to cut through the thick fleshy crowns
(the central growing area between the roots
Keep at least one developing eye or bud with each division. If larger, quickly-maturing plants are wanted, keep several eyes.
Rhizomes are stems that grow horizontally at or above the soil level. Bearded Iris ( Iris germanica )are the most common perennial with this type of root system. You can divide irises any time between a month after flowering until early fall.
Cut and discard the rhizome sections that are one year or older. Also, inspect rhizomes for disease and insect damage. Damaged rhizomes should be trimmed and treated with powdered elemental sulfur, or discarded if too badly damaged.
Iris divisions should retain a few inches of rhizome and at least one “fan” of leaves, trimmed back to 2 or 3”. Replant with the top of the rhizome just showing above soil level. If you replant Irises too deeply, they may take several years to begin blooming again!
Tuberous Root Divisions:
Dahlias are an example of perennials (Although not cold hardy, Dahlias are perennials in their native range) with tuberous roots. The tubers should be cut apart with a sharp knife. Every division must have a piece of the original stem and a growth bud attached. After division they can either be replanted or packed in peat moss and stored for spring planting.
Dividing Large, Tough Root Systems:
If the root mass is very large, or tight and tangled, you can raise the clump 1 to 2 feet off the ground and drop it. This should loosen the root mass, and you can pull the individual plants apart. This is not a good method for plants with brittle roots such as peonies.
Plants that have very tough, vigorous root systems Aagapanthus, red-hot pokers (Kniphofia and ornamental grasses) may have to be divided with a shovel, saw or ax. You can also vigorously hose off soil to make the root system easier to work with.
Some Garden Perennials Resent Being Divided!
Some plants resent being divided and it should be avoided if possible. These include butterfly weed (Asclepias), euphorbias, oriental poppies (Papaver), baby’s breath (Gypsophila), gas plant (Dictamnus albus), Japanese Anemones, false indigo (Baptisia) and columbines (Aquilegia). They can be divided, but use care and go slowly until you learn each plant’s particular requirements.
Lenten and Christmas roses (Helleborus) are very difficult to move when more than a few years old. Usually you can find tiny seedlings around the base. These are relatively easy to move.
Lavender cotton (Santolina chamaecyparrus) and several other garden perennials are actually small woody shrubs and should not be divided. These include perennial candytuft (Iberis sempervirens), lavender (Lavandula), rosemary (Rosmarinus), and several of the Artemesias. These plants often have rooted layers (branches that have developed roots while touching the soil). These layers can be cut off the parent plant, dug up and replanted as though they were divisions. However, the best way to propagate these types of woody perennials is by cutting.
Replant New Divisions As Soon As Possible:
Never allow divisions to dry out. Keep a pail of water nearby to moisten divisions until they are planted. We often wrap them in damp newspaper until we are ready to work with them. Trim all broken roots with a sharp knife or pruners before replanting.
Plant the divided sections immediately in the garden or in containers. Replant divisions at the same depth they were originally. Gently tamp the soil around the roots to eliminate air pockets. Always water new plants in well with a gentle water nozzle immediately after planting.
Fall-divided perennials should be mulched the first winter to prevent heaving caused by alternating shallow freezing and thawing of the soil. The best winter mulch is loose and open such as yard leaves, bark, or pine needles. Be sure to carefully pull the mulch away from the new growth buds in the spring.