Pruning Rhododendrons:
The above comments associated with generating new growth on evergreen azaleas are also generally applicable to dwarf rhododendrons, so let’s begin by discussing the medium-sized hybrid plants. Usually the aims of the nursery trade for these plants is a compact, well budded-up plant that is the result of de-budding and summer pruning to induce multiple branches and plenty of flowers during the first Spring that they are put on sale in the garden centre.
In reality few cultivars retain their original compact shape and, much to the disappointment of many gardeners, over the years a plant will grow to its ‘natural’ shape that is in many ways dependent on the species of rhododendron in its parentage. In my experience there are very few plants that actual fit in the categories of dwarf or medium-sized rhododendrons as their ultimate size often depends on the growing conditions in the garden where they are planted.
This brings me to another question often asked at Gardening Scotland, ‘How do I go about pruning a rhododendron that is blocking out the view from our picture [or patio] window?’ It is surprising how often the flowers of the plant in question turn out to be red. A tall-growing variety is clearly not suitable for planting in front of a picture window and no amount of pruning will make it fit its location and still retain its attractiveness. It is better to grasp the mettle and move the plant before it becomes any larger and even more difficult to handle. Once again this raises the problem of how does a member of the public determine the ultimate size of a plant at the time of purchase. But the questioner is not the only one with a problem as there are not many readily available, attractive, low-growing, large-flowered red plants to recommend as an alternative!
Another frequently asked question relates to the refurbishment of older plants. Often this question is raised by someone who has moved home relatively recently, has acquired an old garden containing a number of unlabelled, mature plants that have outgrown their location. They are unable to identify the plants and they wish to cut them back severely to gain more space and improve the light. Handling a question of this sort without being able to see the garden itself is very much a case of the blind leading the blind! There is a distinct danger of making the assumption that an old garden of the type outlined above is likely to contain a selection of old hardy hybrids.
I am constantly amazed at the number of old gardens that come to my notice that turns out to contain an interesting collection of rhododendrons. My visits to old gardens of any significant size, on both sides of the Atlantic, suggests that the owners of gardens, planted in the years prior to WWII, were far more discerning than many writers give them credit for and many such gardens contain a selection of species, “classic” English hybrids, or both. In cases where it is planned to carry out remedial work in an old garden it can be counter-productive to instigate a programme of clearance work and severe pruning without first having a clear idea of the names of the cultivars involved in the work.
Many plants do not flower properly when they have been located in a competitive environment for many years and some cultivars do not flower at all. Often their flower size is smaller than normal and the colours, blotches and tonal differences are washed out. It is preferable to begin by removing overhanging branches, saplings, and climbers such as ivy, with the aim of opening-up the woodland canopy to let the light reach the rhododendrons and azaleas. This course of action will amply repay the time and patience involved as the plants will begin to flower properly over the following couple of seasons and the naming of previously unidentified cultivars will be a more straightforward task.
So, there are a number of aspects to be considered in regard to the original question and some of these cannot be addressed ‘at arm’s length’. There are some generalities that can be explained to provide food for thought and these will help determine if specialist on-site advice should be sought.
Whilst most rhododendrons can be cut back successfully there are a number of exceptions. I well recall the advice of the late-John Basford on this subject, ‘If the plants are rough-barked they have a good chance of re-generation, smooth-barked plants are not so hopeful.’ Add to this comment that many large-leafed species, and their related hybrids, tend not to produce new growth from old wood, so cutting these plants back severely should be avoided.
Plants with smooth or peeling bark in the Subsections Barbata and Thomsonia tend to be lanky growers and rarely regenerate from old wood after severe pruning. This may go some way towards explaining why many of the large-flowered red hybrids are inclined to have a sparse branching habit and become ungainly garden plants despite appearing to be well-branched plants when distributed by the nursery. Many R.griersionianum hybrids can also have an ungainly habit. Removal of the growth buds on the lanky limbs over a two-season period will provide some corrective action for this problem. Most old hybrids prune easily, especially those with R.ponticum parentage, however, there are some hardy-hybrids such as ‘Nobleanum’, ‘J.H.Agnew’, ‘C.B. Van Nes’, ‘Mrs. C.B. Van Nes’, ‘Alice’ and ‘Bagshot Ruby’ that resent being pruned.
No attempt should be made to severely prune plants that are failing or in a poor condition. They will not respond to such treatment and the shock may be fatal. These plants need to be examined closely and the necessary remedial action taken to correct the problems. This can sometimes become more of a salvage operation in old gardens where the plants have been neglected for a considerable period. In many instances the problems have their origins in some form of competition from adjacent plants and trees; competition at the top of the plant for light or at the roots for nutrient and moisture. Whilst the remedy might also involve cutting out dead or damaged branches from an ailing plant it may be many weeks before new buds begin to develop and in the first season there may be very little new growth.
Another worthwhile check is to see if the plants are sprouting at the base and, if so, does the characteristics of the leaves, flowers and the “stance” of the new growth look different than the main plant itself? A large number of ‘classic’ English hybrids were grafted plants that utilised R.ponticum, or a hybrid of R.ponticum, for the rootstocks and many old gardens contain a number of these old hybrids. Some species were also propagated in this way. Whilst this methodology was gradually discontinued over the latter part of the 20th Century by nurserymen in Great Britain the practice still persists in mainland Europe and many imported plants are grafted. Opening-up the canopy of an old garden can also result in high light levels reaching the base of a grafted plant and this may encourage the R.ponticum rootstock to sprout. This aspect needs to be monitored and the necessary action taken.
A more general problem is that the severe pruning of grafted plants often results in the generation of suckers from the more vigorous rootstocks and these suckers need to be removed as soon as possible or they will take over the plant by starving the grafted-on cultivar. The foliage of R.ponticum is rather distinctive, but if you are still unsure whether the sprouting at the base is R.ponticum then look for the purple flowers in early-June. All suckers must be completely wrenched-off or knocked-off at the base, a spade is very handy for this job. Do not saw off the suckers, as if a stump or any residue of the sucker is left it will produce further vigorous new sucker growth the following year.
Despite all the good advice offered over the years there remains extant a persistent myth that severe pruning of rhododendrons can result in a smaller growing plant. In general terms, whilst severe pruning often results in a more bushy plant it is not practicable to reduce the actual size of large, mature rhododendrons by pruning as they often regenerate and fill out relatively quickly. Severe pruning will only defer the problem for a while as the more vigorous hybrids will have returned to their original size in well under ten years. Faced with this reality most gardeners then take steps to remove the plants and this can be quite a major undertaking as the roots can be extremely large. A more satisfactory solution from all points of view is often overlooked. In many cases it is a relatively simple task to slightly re-align the path or drive.
Now for another exception to the rule ! It is well worth noting that many plants in the large-leafed category do regenerate from the base after an unusually severe frost has decimated the main branches of the plant. For this reason it is recommended that all plants are left in-situ after a particularly hard winter as it may not be practicable to assess the overall damage until the plants have had an opportunity to instigate new growth. Plants damaged in this way often regenerate and grow very quickly as they have a large mature root system. Drastic action in the form of digging up apparently dead plants at an early date can lead to the inadvertent loss of plants that can be difficult to replace.