Rhododendrons don't require annual pruning in the way that a rose does, for instance. It is just a tool that can be used from time to time to improve the size or shape of your plants.
Many of the alpine and dwarf varieties such as yak hybrids need little or no pruning to maintain a good shape, as long as they are grown in plenty of light. As soon as rhododendrons start to flower freely, most growth comes from lateral shoots from below the flower buds and therefore nature does its own pinching. When plants are young and before they start to flower freely however, the simplest way to ensure a plant of good habit is to pinch out all the single, terminal buds as they elongate in the spring. Wait until this becomes easy to do: if you have to dig out the bud, then it is too early. I recommend this for vigorous dwarfs, subsection Triflora species and most larger hybrids. With species, many of the largest ones are, in essence, trees, and they should be left to form a single trunk until they attain a substantial height. Others such as the very vigorous R. decorum are better pinched as a small plant to encourage bushiness. Spring frosts do a lot of natural pinching, with the central bud being frosted, leaving the later growth to come from below as multiple shoots.
Rhododendron Expert Kenneth Cox describes how to prune overgrown rhododendrons and camellias.
Pinching is a preventative measure while pruning is a cure.
Pinching means removing the terminal or central growth bud of a rhododendron as it elongates to encourage multiple branching from lateral buds which form in the leaf axils of each whorl (circle at the branch end) of leaves. Pruning involves cutting a mature shoot or branch so that buds break and grow further down, hopefully improving the shape. The photo below shows a pinched central bud removed to encourage branching.
There are several reasons why you may want to prune a bush. It may have been crowded out by other plants and become straggly. It may have been severely damaged by cold or wind, or had its first flush of growth frosted, causing unsightly, brown, distorted leaves. You may want to reduce it in size because it has got too big or because you want to move it. Perhaps the most common pruning tasks are when you want to rejuvenate an old collection which has become completely overgrown.
Scaly-leaved rhododendrons and all azaleas can be pruned to any point on a branch or shoot and new growth will come from buds lower down. Maddenia and vireyas grown indoors tend to get straggly if not hard pruned every few years. With larger-leaved, non-scaly varieties, you should cut back to a whorl of leaves (see photo). Although it is possible to prune at any time of year, perhaps the most satisfactory time is straight after flowering. This gives time for production of new growth and for formation of flower buds the following year. Alternatively prune in early spring, just as new growth is starting to elongate. When pruning, it is best to leave a substantial quantity of leaves: cutting back into too much old wood is risky. Among larger species and hybrids, there is considerable variation in how well they respond to pruning. If you can cut back to a healthy whorl of leaves, one or some of the buds above each leaf stalk will almost certainly grow. It is when a more severe pruning is required, cutting back to a bare trunk, that results vary considerably.
As a rough rule, varieties with a smooth bark such as R. thomsonii, R. barbatum as well as most of the large-leaved species such as R. sinogrande are reluctant to respond to being cut back to the trunk. One exception is the large-leaved R. macabeanum which produces plenty of shoots from the base if cut back deliberately or by severe weather.
Amongst the best responders to being severely cut back are R. arboreum and the ubiquitous R. ponticum. The latter’s ability to regenerate is one of the reasons it is so hard to eradicate in areas where it has become a pest. This species was traditionally used as a rootstock for grafting cultivars and its extreme propensity to throw suckers has commonly overwhelmed whatever was grafted on top of it in old collections, incorrectly referred to as ‘reverting’. Many R. ponticum-lined drives of stately homes in the U.K. were formerly plantings of hardy hybrids of various colours. This is important to bear in mind when trying to rejuvenate plantings which date back to the 1950’s or earlier in the U.K. The characteristic purple flowers and dark green, shiny, narrow leaves of R. ponticum should make it possible to identify which the suckers are. These should be wrenched off at the base of the stem. If you prune them off, they will just produce a whole cluster of shoots from the below the cut. Since the 1950’s grafting has fallen from favour as cuttings and more recently micropropagation have become the major production techniques for rhododendrons, (except in northern Europe where most cultivars are grafted on ‘Cunningham’s White’-fortunately far less prone to suckering).
