Dividing Rhizomes


Why Divide?

After a perennial has been growing for several years, it tends to become an overcrowded clump with declining vigour and flowering. By dividing or splitting up the clump and only replanting the youngest, healthiest sections, it is possible to give the plant a new lease of life.

When to Divide

Whilst fibrous-rooted perennials may be divided during the dormant period, plants with rhizomes (fleshy, almost horizontal underground stems) such as rhizomatous lilies, irises and elephants ears (Bergenia) are generally divided in the late summer.

How to Divide

Step 1: Lifting

  1. Loosen the soil around the overcrowded plant and then lever it out of the ground using a fork, taking great care not to damage the rhizomes.
  2. Shake off the excess soil and remove any dead stems and leaves to make it easier to locate the best points for division.
  3. Wash the remaining soil away with water so that you can see which parts are healthy and which need to be discarded.

Step 2: Dividing

  1. Break up the clump roughly using your hands or a hand fork. You may need to use the back-to-back fork technique used on large perennials for large clumps.
  2. Use a clean sharp knife to cut away fist-sized sections of the healthy, firm parts of the rhizome. Each new piece should have buds or leaves above it and roots below it.
  3. Dust cut areas with a hormone rooting powder to help prevent disease and to encourage new growth.
  4. Discard the centre of the rhizome and any old or diseased sections.
  5. Trim back any long roots by one-third.
  6. Irises tend to be top-heavy, and so their top growth of leaves and stems should be trimmed to prevent wind-rock. Cut their leaves into a mitre-shape approximately 15 cm (6 in) long.

Step 3: Re-Planting

The rhizomes should be replanted shallowly so that their top part just shows above the soil. Leave a space of approximately 12 cm (5 in) between each plant, making sure that their leaves and buds are upright. Firm in well and water. Keep watering during dry weather until new growth is seen.

Perennials should be normally be divided in the semi-dormant seasons of early autumn or early to late spring, so that the roots can develop in reasonably warm soil. However, rhizomes such as irises should be divided in midsummer, straight after flowering, as they rot easily in winter.