As the blooms fade on your perennial plants, the opportunity arrives to propagate your favored specimens by through root division.
The best candidates for this process will have been growing for at least two years, and preferably a bit longer. The ideal time for propagation by root division is after a healthy plant has had time to develop a substantial root system, and before it has become crowded and less productive of blossoms. When divisions are planted, they should be watered lightly and shaded temporarily to limit loss of moisture.
The preferred time of the year to divide perennials is early spring or early fall, rather than the mid-spring to late summer period, when perennial plants are growing and producing blossoms. With this schedule in mind, right now is a good time to consider which plants to divide. This timing gives the divided plants the fall and winter months to develop roots and prepare to burst into bloom next spring.
The general guidelines mentioned above apply to all kinds of perennial plants, but the process differs somewhat with broad categories of these plants.
Rhizomatous and Tuberous Plants
Plants that grow from rhizomes or tubers can be dug up carefully with a garden fork, separating the rhizomes or tubers by hand or knife, and replanted at the same depth as the original plant. Tuberous plants include Arum, Calla (Zantedeschia), Canna, Dahlia, Spurge, and others.
Examples of rhizomatous plants are Iris, Canna and Bergenia. Other in this category: ginger (Zingiber officinale), the related White Ginger Lily (Hedychium coronarium), and the bamboos, which are members of the grass family (Poaceae) and manage propagation quite well on their own.
Once dug and divided, these plants could be replanted immediately, or should be kept in a cool and dark place until a convenient time for replanting in the fall.
Another category of perennial plants is the “clumpers,” which have fibrous root systems and clumping growth habits. These plants’ root balls can be dug up with a garden fork or spade, and then either pried apart by hand or split with the spade. The gardener might need to use two garden forks to divide really large root balls.
The number of divisions to be made from a given plant will depend upon the size of the root ball. Often, dividing the root ball into four quarters will be appropriate. While a larger number of smaller divisions might be desirable, they could require more time to become ready to bloom.
The roots of divided plants in this category should not be allowed to dry out. Ideally, they should be dug during an overcast day, replanted promptly, and watered in. Trimming the foliage to reduce transpiration also will help the plant to bounce back from the process.
The clumpers comprise a large group of perennial plants. Examples: coral bells (Heuchera), cranesbills (Geranmium), columbines (Aquilegia), daylilies (Hemerocallis), plantain lily (Hosta), primroses (Primulus), lamb’s ears (Stachys), bugleweeds (Ajuga), Japanese anemone (Anemone hupehensis, which propagates readily on its own), stonecrop (Sedum), yarrow (Achillea), and several larger grasses.
These plants have shallow stolons or thin rhizomes, and spread across the ground. They can be divided in the same as way as clumpers. Examples include bee balm (Monarda), goldenrod (Solidgo), and aster (Symphyotrichum).
Plants with Woody Crowns
Plants that have woody crowns can be divided with somewhat more effort than other categories. The basic process is the same as for fibrous-rooted clumpers but typically require cutting the root structure apart with garden shears or saw, e.g., a pruning saw.
Examples of these plants include Astilbes, bear’s breeches (Acanthus), foxtail lilies (Eremurus), goatsbeard (Aruncus), lilyturf (Liriope), peonies (Paeonia), and wild indogo (Baptisia).
Plants Best Not Divided
Some plants have root structures that do not divide well: they might have single taproots or single woody roots, and are best propagated by seed. These plants can be recognized easily when they have been dug up. Examples include lavender (Lavendula), Russian sage (Perovskia), Allysum, carnations (Dianthus), Euphorbias, foxgloves (Digitalis), butterfly weed (Asclepias), and others.
Dividing perennials can be a satisfying project for avid gardeners, and the most inexpensive way to multiply favored plants to the landscape. Tour your garden in a search for division candidates.