Perennial plants which form spreading clumps should be divided every few years to perpetuate the existing stocks and prevent overcrowding.Small clumps can be divided by hand or with a trowel. Larger clumps or very old clumps, however, have to be prised apart using garden forks.
Lift the entire plant out of the border and divide it in half. If it has a large matted root system, stick two garden forks back to back into the centre of the clump and force it apart.
Remove the young healthy shoots from the outer side of the clump by breaking or cutting them off.
Cut all the leaves right back to within 2.5cm of the roots and replant immediately, or pot up in potting compost and put the pots in a shady area until you can replant them.
Bibliography: Geoff Hamilton - Successful Organic Gardening