At its best:
Jun, Jul, Aug
Push a spade deeply into the ground a few inches from a large, congested clump to loosen the soil.
Rock the spade gently to loosen the clump and break deep roots, then lift the clump out of the ground with your hands.
Select young growth from around the edges of the clump. Try to break off individual crowns by hand, each with roots and shoots.
Fork through the soil and add garden compost or well-rotted manure. Replant divisions in groups, firming round each plant.
Lightly fork through the soil surface before watering. The new divisions should produce a few blooms this summer.