Growing Concerns: Root systems have a role to play in plant division
When digging herbaceous perennials for dividing, observe the root system. There are five different types of root systems; offsets, stolons, rhizomes, taproots, and woody roots. It is the root system that guides division and separation.
For roots that form clumps or offsets, the roots can be either teased apart or cut apart to produce divisions. Each divided section should include plenty of roots and three or more growing points, which are often referred to as eyes. You might have to grind a sharp edge on a spading shovel to divide dense clumps.
Plants that form offsets include aster (symphyotrichum species), purple coneflower (echinacea purpurea), hosta (hosta species), and tickseed (coreopsis species).
Corms and bulbs are modified stems that produce offsets that can be teased apart and separated. Crocus (crocus vernus) is an example of a corm, and daffodil (narcissus species) is an example of a bulb.
Perennials with stolons or modified stems that run at or on the soil surface will form new plants at the nodes. These new crowns with roots can be cut apart, creating a new plant. Plants with stolons include strawberry (fragaria species), woodland phlox (phlox stolonifera) and bugleweed (ajuga reptans).
Plants with rhizomes or underground modified stems develop suckers beyond the mother plant. The suckers can be dug and transplanted elsewhere in the garden. The rhizome also can be dug and cut into pieces, making sure that each piece has a node, or growing point.
Plants with rhizomes include bearded iris (iris species), lily-of-the-valley (convallaria majalis) and pigsqueak (bergenia cordifolia).
It is not recommended to divide plants with taproots, but if you must, do it carefully. The root can be sliced down the length of the taproot with a sharp knife. Each piece must contain at least one eye bud and a portion of the taproot with a few side roots.
Plants with taproots include balloon flower (platycodon grandiflorus), butterfly weed (asclepias tuberosa), cushion spurge (euphorbia polychroma) and Oriental poppy (papaver orientale).
Semi-woody perennials often form roots on stems that rest on the ground or are buried by accumulating organic mulches. Simply cut the rooted stem away from the mother plant and transplant.
Plants with woody roots include candytuft (iberis sempervirens), moss phlox (phlox subulata), lavender (lavender angustifolia) and Russian sage (perovskia atriplicifolia). Plants can purposely be propagated by bending a stem to the ground and covering the stem with soil. This is referred to as layering.
Many perennials can be divided in either spring or fall with a few picky species. The optimal time to divide bleeding heart (dicentra species), primrose (primula species), monkshood (aconitum napellus), baneberry (actaea pachypoda), and wild ginger (asarum europaeum) is in the spring. Moss phlox, sweet woodruff (galium odoratum), poppy (papaver species), iris and peony (paeonia species) should be divided in the fall.
Divided transplants should be kept moist for best root development. Mulch new divisions after the ground freezes to prevent heaving during the winter freeze-thaw-freeze cycle.
Some of the ugliest plants can be rejuvenated with proper timing and technique. Dividing is just another way to enjoy the bounty of the garden.