C. armandii and C. cirrhosa are both vigourous plants that need regular pruning to keep under control. They flower on growth made the previous year so wait until they have finished flowering before doing any pruning. Cut back growth to about 1m below the area that you want the plant to cover. After three or four years both plants can develop dense, woody ‘bird nest’ growth. Cut back about one third of the plant to the base and repeat the process on the other two thirds in subsequent years.
C. ‘Early Sensation’, ‘Avalanche’ and the x cartmanii hybrids are so floriferous that they often exhaust themselves and produce little in the way of new growth. To keep the pants vigourous and healthy, prune them in two stages. Trim them immediately the flowers finish to remove the dying flower heads and prevent any energy being lost in seed production. Then at the end of May cut the whole plant back to about 50cm from the soil. This will encourage new growth and also prevent the plant from becoming bare and woody at the base.
C. paniculata tends to be a spindly plant so for the first few years after buying a new plant, cut it back to about 60cm from the ground. Once it has become bushy allow it to grow into the size you want and then trim it back each year after it has finished flowering.