How To Divide Perennial Plants
First, not all perennials can or should be divided. Those that are more shrubby; having only a single stem that supports the branches cannot be divided without destroying the plant. Perennials that can be divided are those that spread by underground rhizomes or roots, such as Daylilies, Hosta lilies and Iris. 

Of the perennials that can be divided, there are a few reasons for doing so: 

  • To control the size of the plants and to help rejuvenate old plants. 

  • Dividing and replanting keeps rapidly spreading perennials under control. 

  • Dividing perennials is also an easy and inexpensive way to gain additional plants for your garden or to share.

How Often to Divide Perennials

Some perennial plants need dividing more often than others. A sure-fire indicator that a perennial plant needs dividing is when the plants have crowded themselves to a point where they are nothing but leaves and roots with no blooms. While some perennials might only need dividing every 3 to 5 years, others, such as Chrysanthemums, might require more frequent dividing. Bleeding hearts and peonies may never need to be divided unless more plants are wanted.

Signs that a perennial needs to be divided are smaller than normal flowers, centers of clumps are empty of foliage or dead, or when there is sparse or no bottom foliage. Plants that appear healthy and are growing and blooming well can be left alone, unless you want more plants to spread around the garden or to give to gardening friends.

When to Divide Perennials

In general, it's best to divide spring and summer blooming perennials in the fall, and fall bloomers in early spring. By dividing the plant when it is not flowering, all the plant’s energy can go to root and leaf growth. 

When dividing perennial plants in the spring, allow enough time for roots to settle in after planting before hot weather arrives. I usually divide about the time I see new growth emerging from dormancy. Keep in mind that the perennials you divide in spring might flower a little later than they usually do. Sometimes they might not flower until the next year.

It is best to divide perennials on a cloudy day rather than a hot and sunny day. Too, if you'll be transplanting the divisions, try to schedule the dividing to be done when several days of rain are forecast afterwards. Otherwise, you can provide water to keep the soil moist.

How to Divide Perennial Plants

Pre-Dividing Preparation

Make sure to deep soak the soil around plants you intend on dividing a day or so before you divide them. 

If you intend on transplanting some or all of your divisions to other spots in your gardens, prior to digging and dividing, it's a good idea to prepare the soil in the area(s) where new divisions will be planted. 

Before dividing, 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 Plant

Using a sharp pointed shovel or spading fork dig down deep all the way around and underneath the plant, at least 6 inches away from the plant. Pry underneath with your tool and lift the whole clump to be divided. For plants that have grown very wide you might have to use a hatchet or other chopping tool to cut these into several smaller sections for easier lifting and moving.

Separating the Plant

Shake the plant or use a hose to remove loose soil and remove all dead leaves and stems. Removing this loose soil helps to see what your doing and loosen tangled roots.

You'll need to identify the types of roots before diving them as each different type requires a different method for dividing. 

Basically speaking, there are three different types of roots systems: spreading, clumping, rhizomes, and tubers. 

Spreading Root Systems
Spreading root systems have many slender matted roots that originate from many locations with no distinct pattern. Many perennials, such as asters, bee balm, and lamb’s ear, have spreading root systems. Perennials with spreading root systems often crowd out their own center. Some can be invasive and take over a small garden or part of a large garden unless divided frequently.

Spreading root systems can often be pulled apart by hand, or cut apart with pruners or a sharp knife. Large, vigorous perennial plants that have thick, tangled roots may need to be separated with the use of greater force using digging forks. You'll want to divide the plants into clumps of three to five vigorous shoots each. You can discard small or weak divisions or the center of the clump if it is weak.

Clumping Root Systems
Clumping root systems originate from a central clump that expands over time. Daylilies, hosta, liriope, and many ornamental grasses are good examples of clump-forming perennials. You can dig these as you would spreading perennials; lifting the entire plant.

When dividing the clumps it may be necessary to cut through the thick crowns with a heavy, sharp knife. You can also pry apart these roots with digging forks. You'll want to keep at least one eye/bud/shoot with each division. If larger plants are wanted, keep several eyes.

Rhizome Division
Rhizomes are swollen roots that grow horizontally at or above the soil level. Bearded irises are the most common perennial with this type of root system. Divide irises after they've flowered or in early fall. Iris divisions should retain a few inches of rhizome and one fan of leaves, trimmed back halfway.

Dig up a clump at a time and use a knife to cut away the younger rhizomes (swollen roots) from the older ones. Make sure to inspect rhizomes for disease and insect damage. Damaged rhizomes should be trimmed and treated, or discarded if too badly damaged. Toss the old piece and replant the young pieces about 2 feet apart at the same depth. It is very important to make sure the new home for your irises is in a well drained spot. Replant with the top of the rhizome just showing above soil level.

Tuberous Roots
Dahlias are an example of perennials with tuberous roots. These can be dug in the fall. After digging the plant, 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 stored for spring planting.

Other Helpful Tips

  • Plants that have very tough, vigorous root systems, such as ornamental grasses, may have to be divided with a shovel, saw or ax. You can blast soil off the root mass with a jet stream from the hose to make the root system easier to work with.

  • Never allow divisions to dry out. Keep them misted or dunk them in water occasionally.

  • Trim all broken roots with a sharp knife or pruners before replanting.
  • Plant divided sections immediately in the garden or in containers. Replant divisions at the same depth they were growing in their previous location. Firm soil around the roots to eliminate air pockets.
  • In colder regions where the ground freezes, 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 pine straw or leaves.