Rhizomatous plants have a thick root which contains stores of nutrients for the plant to live on when needed, having this reserve of food makes propagating by division a reliable method of propagation.
It is usually best to divide Rhizomatous plants during winter in the sub topics as they are dormant or growing slowly. Firstly prepare the bed or pot that you will plant the divided clump into. The soil should be well drained to avoid rot and suitable for the selected plant type. See image 1
Then prune off dead foliage and and spent flower stems and cut back the foliage by half as this will reduce moisture loss from the plant and thereby improve survival rates. See image 2
Dig the plant out of the ground or remove it from it’s pot. Divide the clump, a spade a serrated knife or saw may be used as appropriate so you end up with many smaller clumps or individual plants each with a good rhizome as this will supply the strength to the plant until it starts to grow. Larger sections of rhizomes will have a better success rate the small ones. See image 3. WARNING cutting through soil will quickly blunten any sharp tool, don’t use your best wood working saw.
Plant the new plants in a bed or pot bury so that the soil covers all the roots and rhizomes, lightly press the soil and water with Seasol to reduce the effects of transplant shock. Only water moderately until the plant starts to grow. See image 4.