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: 

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.

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.

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.

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 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 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.

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.

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.