Do not plant with the rhizome exposed - this will not be deep enough for the plant to be properly anchored. The plant will push itself to the surface when it has established new roots. This leads to soft lush growth and few flowers - in extreme cases it can promote rotting by impairing drainage. Avoid using high nitrogen fertilizers. Instead use a high phosphate fertilizer at half rate when planting and in spring to encourage extra flowers.
Divide clumps regularly.
Congested clumps perform badly, flower infrequently and encourage rot. Lift and divide your clumps at least every third season, this usually being carried out in August or September. Replant only the youngest growth from the outside of the clump, discarding old woody rhizomes from the centre of the clump. Finally - be sure to make good labels. The labels provided with your plants are not intended for long term garden use.