Perennials: dividing

Most perennials benefit from division every two to three years to maintain health and vigour. If you want to increase the number of plants you have by dividing them, the task can be done more regularly.

These are just a few examples of plants that can be divided: Agapanthus, Anemone, Aster, Bergenia (elephant’s ears), Convallaria (lily-of-the-valley) Crocosmia, Dierama, Delphinium, Epimedium, Eryngium (sea holly), Euphorbia, Gentiana (gentian) Geranium, Helianthus, Hemerocallis (daylily), Hosta, Iris, Lychnis, Lysichiton, Lysimachia, ornamental grasses, Primula (primrose) Ranunculus (buttercup), Salvia, Sedum, Verbena, Zantedeschia (arum lily).

Plants can be divided successfully at almost any time if they are kept well-watered afterwards. However, division is most successful when the plants are not in active growth.

• Divide summer-flowering plants in spring (Mar-May) or autumn (Sep-Nov) when the soil is dry enough to work. In wet autumns, delay until spring. Spring is also better suited to plants that are a touch tender
• Many spring-flowering plants, such as irises, are best divided in summer (Jun-Aug) after flowering when they produce new roots

Here are our simple tips for dividing perennials:

• Lift plants gently with a garden fork, working outwards from the crown’s centre to limit root damage. Shake off excess soil so that roots are clearly visible
• Some plants, such as Ajuga (bugle), produce individual plantlets which can simply be teased out and replanted
• Small, fibrous-rooted plants such as Heuchera, Hosta and Epimedium can be lifted and pulled apart gently. This should produce small clumps for replanting
• Large, fibrous-rooted perennials, such as Hemerocallis (daylily), require two garden forks inserted into the crown back-to-back. Use these as levers to loosen and break the root mass into two sections. Further division can then take place
• In some cases, a sharp knife, axe or lawn edging iron may be needed to cleave the clump in two
• Plants with woody crowns (e.g. Helleborus) or fleshy roots (e.g. Delphinium) require cutting with a spade or knife. Aim to produce clumps containing three to five healthy shoots

Aftercare

Plant divisions as soon as possible and water them in well. They can either be replanted in the same spot,  taking the opportunity to mix in a little garden compost or other soil improver first, or moved to a new part of the garden. Alternatively, pot up individually to build up size, overwintering pots in a frost-free environment.

Here are three plants that benefit from using slight variations on the basic techniques.

Crocosmia and Dierama

• Divide Crocosmia and Dierama in spring
• To remove the corms without damage, dig down 30cm (1ft) to avoid and gently lift
• The roots of both perennials form ‘chains’ of corms, which can be replanted intact or individually separated. There is evidence that maintaining the ‘chains’ intact may be the better option
• Discard wizened or diseased corms and trim old leaves
• Replant sections at their original depth

Some crocosmias, such as Crocosmia ‘Lucifer’ and C. × crocosmiiflora ‘Jackanapes’, produce underground stems (stolons) which can be detached, along with fibrous roots, to produce new plants. However, it is worth noting that crocosmias flower profusely when crowded, so do not divide clumps too often – every two or three years should be enough.

Likewise, dieramas resent disturbance and will take time to flower again after division.

Hostas

• Lift and divide congested clumps of hostas in spring or late autumn
• Hostas with tough, fibrous roots can be divided with a sharp spade, slicing the clump in two
• Large clumps can be split further to leave sections containing five or six shoots
• Cultivars with loose, fleshy roots are best teased apart by hand or with two hand tools by placing them back-to-back to lever them in two
• Plant the divided sections at their original depth, with the shoots above the soil surface

There are few specific problems associated with dividing, especially if carried out between autumn and spring. However, ensure that plants don’t dry out while they do re-establish. It is also worth carrying out slug and snails control as these are often problematic pests for perennials.

Borders: revitalising an over-mature bed
Deadheading plants
Irises: dividing
Ornamental grasses
Penstemons
Perennials
Perennials: cutting back
Perennials: planting
Perennials: staking
RHS Find a Plant
RHS Video: Dividing perennials