Dividing perennial herbs and houseplants helps to enlarge and revitalize often old unproductive plants.
By David Squire
Propagation (Fox Chapel, 2016), by David Squire, is the essential guide to raising new plants for the home and garden for both novice and experienced gardeners. Squire contributes his lifetime experience with cultivated and native plants with an interest in historical medicinal roles, folklore, and customs of plants. This excerpt is from “Division and Layering”section.
This is one of the simplest ways to increase herbaceous perennials, and the technique needs little equipment other than a couple of garden forks. Herbaceous perennials survive in cold regions because, during the cold winter months, they die down, leaving only old, dead stems showing above ground; each spring, they develop fresh shoots. In late autumn or early winter, this growth dies down and the plants survive by means of their dormant roots.
You can lift and divide herbaceous plants at any time between early autumn and mid-spring, whenever the soil and weather are suitable. Usually, this means autumn in areas where the weather is mild; but spring is better in areas where cold winters are regularly experienced.
Sometimes gardeners leave the old stems and do not cut them down until early spring, so that during winter they can protect the roots from severe frost. Old stems, leaves and flower heads covered in frost also create an attractive winter feature, especially when caught by low rays from the sun.
Use sharp pruning shears to cut away any old stems still remaining on the plant, especially when dividing a clump in late winter or early spring.
Use a garden fork to lift the clump. Then, insert two garden forks, back to back, into the clump and draw the handles together to lever it apart.
Use your hands, if necessary, to finish pulling the clump apart. Do not create very small pieces, as they will not produce dominant plants.
Plant the divided pieces into borders. Use a trowel to form holes and draw friable soil around the roots. Firm the soil and water the entire area.
The root type of individual herbaceous perennials influences the way in which you should increase them. There are three different basic types of roots:
Fibrous Roots: The majority of herbaceous perennials have spreading and fibrous roots; you can lift and divide these in the way shown on the left. They include:
• Achillea spp.
• Anaphalis spp.
• Artemisia spp. (herbaceous types)
• Aster spp. (both summer- and autumn-flowering Michaelmas Daisies)
• Astilbe spp.
• Astrantia spp.
• Campanula spp. (herbaceous types)
• Coreopsis spp. (herbaceous types)
• Filipendula spp.
• Geranium spp. (not to be confused with pelargoniums)
• Helenium spp. (herbaceous types)
• Helianthus spp. (herbaceous types)
• Leucanthemum maximum (syn. Chrysanthemum maximum; Shasta Daisy)
• Lysimachia spp. (herbaceous types)
• Lythrum spp. (herbaceous types)
• Monarda didyma
• Phlox spp. (herbaceous types)
• Rudbeckia spp. (herbaceous types)
• Solidago spp.
Fleshy Roots: A few herbaceous perennials have woody, fleshy roots; lift the plant and use a sharp knife to separate crowns into several pieces. They include:
• Delphinium spp.
• Lupinus polyphyllus (also increased from seeds and cuttings)
Rhizomatous Roots: These are frequently grown in herbaceous borders; lift and divide the roots, and replant the divisions. They include
• Iris germanica
This is the easiest way to increase, and revitalize, any fibrous-rooted houseplants that have become congested, their pots packed with roots, and that are producing little fresh growth. These have usually developed from a plant originally planted in the center of a pot. Therefore, when dividing the plant, select young pieces from around the outside; often, the center part is old and unproductive, and is best discarded. The new plants will be fresh and vigorous.
Houseplants that are mainly grown for their beautiful leaves are best divided in spring. Flowering houseplants, however, should be divided after their flowers fade – the earlier the better. Leave those houseplants that flower in autumn until spring before dividing them.
Dividing houseplants in spring (or as early as possible) gives young plants a chance to become fully established in summer, while growing strongly.
Before the newly divided parts are fully established, place them in gentle warmth and light shade.
To check if a clump-forming houseplant needs repotting, remove the pot. A mat of roots indicates that division is needed.
Use your fingers to pull the rootball apart into several substantially sized pieces. It may be necessary to cut some roots.
Repot each new plant into a clean pot; position at the same depth as before, then trickle and firm soil around the roots. Then, water the plant.
The yellow-edged form Sansevieria trifasciata var. laurentii is best increased by division. Pull the rootball apart into several pieces.
Add potting compost to a pot and put a new plant in place. Adjust the soil’s height, so the plant is at the same depth as before.
Hold a plant and trickle and firm soil around its roots. Leave a 1 ⁄ 2 in (12 mm) gap between the compost and the pot’s rim. Then water the plant.
Saintpaulia ionantha (African Violet) can be increased from leaf-petiole cuttings, as well as by severing a healthy leaf and suspending the leaf-stalk in clean water, but congested plants can also be divided. In spring or early summer, remove a congested plant from its pot and gently tease it into separate pieces. Ensure that each new plant is a respectable size. Then repot them into individual pots. Firm the soil and water.
Many houseplants are good candidates for division, including the following:• Aspidistra elatior (Cast Iron Plant)