Getting the Pruning Time Right:
Hardened wood can be pruned at anytime, except during freezing weather or when periods of frost are forecast, however, the best time to prune rhododendrons and azaleas is in the Spring, or earlier if the pruning is to be severe. Light pruning should be carried out immediately after flowering. Not only has the plant been dormant over the Winter and is ready to put out new growth, there is also the advantage of the majority of the growing season ahead to allow the plant to fill out and to enable the new growth to mature. But, perhaps more important in the eyes of the gardener, pruning at this time of the season minimises the loss of buds that will flower the following Spring. Some authorities recommend that severe pruning should be carried out in February or March in order to maximise the full growth potential of the new season; however, the likelihood of severe late-frosts occurring in Great Britain and causing damage to the open wounds and new growth is a continual threat. It seems prudent to wait until the end of March until the risk of severe late frosts is somewhat diminished before starting work. At all costs avoid late-Summer or Autumn pruning, particularly if you live in an area that experiences cold Winters with heavy frosts as these conditions can lead to injury to open wounds and damage to any new growth that may occur late in the season.