Use a garden fork to lift the whole plant out of the ground gently, taking care not to damage its roots. Separate a section of roots and replant the parent plant. Wash the roots in water to remove any soil.
Using a sharp knife, cut off 5cm lengths of healthy roots from the main root ball.
Using a sharp knife, remove the outer layer from one end of the cutting. This will stimulate new root growth once the section of root is planted.
Fill pots with gritty compost and use a pencil or dibber to mke planting holes. Gently lower the cuttings, with the nicked end pointing downwards, into the compost. Water well.
Cover the compost with a thick layer of horticultural grit, to keep the compost moist. Label pots and place them in a cool spot indoors or in a cold frame.