As a Master Gardener, I am often asked about pruning rhododendrons. There are several reasons for pruning with different timing and methods. The main reasons are; 1.) removal of spent flowers (deadheading), 2.) removal of dead, diseased, or damaged branches, 3.) shaping of the plant, and 4.) plant rejuvenation.
Removing of the flower after it starts to fade not only improves the look of the shrub, but focuses the energy to making new growth versus producing seeds. On most rhododendrons deadheading is accomplished by pinching the truss at the base with a slight twist. It can also be accomplished with a small pruning shears such as a long nosed grape shear. There will be new buds emerging just below the truss, so be careful not to damage them. It will be easiest to deadhead early before the new growth has started to grow. If the plant doesn't set seeds, it is not necessary to dead head. Small-leafed rhododendrons rarely need deadheading.
Removal of dead, diseased or damaged branches can be done at any time, the sooner the better after it is recognized. Depending on the size of the branch, it may require the use of pruning shear, loppers or saw. Be sure to make the cut below the diseased part and burn or send it away in the garbage. Broken or dead branches should be cut just above a dormant bud. There may be inside branches with buds that do not see the sun that can also be removed because they will eventually die.
Pruning for shape enhances the rhododendrons form and habit. Depending on the landscape, the rhododendrons shape is important. For instance, tall rhododendrons should not be placed where they will obstruct the view or other landscape features. All rhododendrons can be pruned to maintain their natural habit, but to try to keep a tall growing rhododendron small or low growing is not practical. Pruning for shape should be accomplished in early spring to early summer, but not after July 1. Later pruning will encourage new growth which may not be hardened off prior to first frost and subject to damage. I prefer to prune for shape before the new growth emerges, in February or March. Early pruning will most likely remove some flowers the first year, but will assure more blooms the following year. Pruning after the flowers fade is another alternative, but reduces the flowers the next season.
As rhododendrons age they lose vigor and often become open and ungainly. As a consequence, they lose much of their value in the landscape. Rejuvenation pruning, sometimes major, can solve the problem. The larger-leafed rhododendrons have many dormant buds that can be forced into new growth. However, on very old wood, the dormant buds may have atrophied and are not viable. So there is some risk that drastic pruning may not be successful. But, by pruning just above viable dormant buds, new healthy growth can be directed to obtain the desired shape.
Some nurserymen recommend cutting back one third of the branches over a three year period. I prefer cutting the entire plant back to the height desired early in the season before new growth emerges. That may result in a loss of blooms the first year, but will result in a better shaped plant in less time and will have added blooms the next year. A word of caution however, the plant will have a large root system and may produce excessive new growth. To counteract this, I use a spade to chop off 50% of the roots at the drip line. This will balance the uptake of nutrients to the now reduced branch and leaf structure.
It may be difficult to see the dormant buds of some varieties. For these plants prune the branches back to the desired level and after the new growth emerges, prune off any stubs that remain.