Hi, when it says “ cut back by one third” is that of new growth or from the outside tip to the central stem, please ? Thanks. John
Most flowering shrubs benefit from annual removal of 1/3 of the stems/branches, starting with the oldest ones. Old wood is less productive of both leaves and flowers so removing it rejuvenates the shrub. If you want to reduce the shrub's height, light tip pruning is OK but it often results in crowded, unattractive branch tip growth. If the spot isn't large enough to accommodate the natural height, it is better to move the shrub than battle its natural instincts.