More About Pruning Rhododendrons
Fred E. Knapp
Locust Valley, New York
Reprinted from the New York Chapter ARS Newsletter.
The would-be reader may well wonder how any article could propose to offer more about pruning when so much has already been written and rewritten by so many truly authoritative rhododendron pundits. Many growers have closed their minds to another repetition. The author shares this deadened interest, and does not propose to review the detailed procedures of pruning. This will not be a "how to prune" article, but an attempt to show that "how to prune" is the least important, least interesting part of pruning. There is more to it.
All growers prune their rhododendrons, often unaware that they are doing so, often without real conviction or attention to the individual plant, usually without any follow-up plan. It is the undifferentiated nature of most pruning, the lack of a learning process, that makes it interesting for the author to present one more article on this hoary burden of the rhododendron fancier. Instead of reciting in detail the collected wisdom of earlier writers and readers one more time, this article will try to show that pruning should be a learning process, an adaptable tool even in revision, repaying a little extra thought with disproportionate rewards. Therein lies the "more" of the title, for the basic information is certainly not new.
Pruning is an interactive process between plant and grower, in which the most conventional advice is never wrong but is often less than ideal. If the reader will consider the following paragraphs germinal to his own thinking, he should be able to develop a better approach to his own pruning problems, grow better specimens, and occasionally give sage advice to others. All of this is fun.
One new technique for pruning, which the author has never seen recommended, must be mentioned at once. Keep records. Record what was done, what characteristics of the plant controlled the decision to do it, and what was the plant's response. Speculate on any outside factors such as a severe drought, or a frost, or heavy fertilization, etc. which may have interacted with the pruning itself. (And please do not ask me about my records - I plead the Fifth - ) This is the only route to real understanding of how to prune a specific problem plant.
Reasons To Prune
Several articles have been printed in recent years showing the sensitivity plants have to pruning. These articles describe the reaction noted, via sensitive electrodes to the mere thought of pruning in the mind of a nearby gardener or scientist. Whether these be true observations is not entirely clear, but surely pruning must be regarded as an affront or a trauma to a plant, despite the potentially beneficial results from our point of view.
Clearly then, we need a reason to prune, and there are many apparent reasons. Seedlings and cuttings benefit from shaping from an early age, plants damaged by crowding or mechanical influences, (falling tree branches, careless kids at play, tipsy garden tourists, etc.) need reshaping, older plants can be rejuvenated, the thin plant can be thickened or the dense plant opened up, size control and reduction can be attempted, transplanting shock can be altered, and of course dead and unthrifty or diseased growth should be removed. Although one can improve plants damaged by cold, or borers, or limited die-back, one should never expect pruning to substitute for reasonable cultural practices and an appropriate hardiness level for the prevailing summer and winter weather. This is a substantial list of reasons for pruning, and the methods and requirements for them are extensive and variable.
If one were to generalize, to try to express them all as one overall idea, it could be said that in every case the grower removes some portion of the plant in order to focus or channel its energy and growth in a manner which produces a healthier plant in a form more compatible with the grower's own ideal for that plant. Sometimes that form may be unnatural for the plant, and the knack of pruning and plant culture is to satisfy our "healthier plant" clause despite that.
Kinds Of Pruning
The kinds of pruning, or perhaps degrees is more apt, which we need to consider are pinching, the removal of flower or apical vegetative buds, and cutting. Cutting may be to a rosette of leaves, or to hard brown wood, or to the ground. Deadheading should be considered as a special case of pruning, perhaps under pinching, and is the unconscious pruning act referred to at the beginning of the article.
One should realize that nature also prunes. Some falling branch damage and winter kill, if not too severe, can be helpful, and flowering itself in many clones will serve in a manner similar to pinching, stimulating multiple shoots. We will not get into esoterics like chemical pruning of azaleas, but sometimes a frost can do a perfect pinch job on a plant.
Probably the very best plant to prune is the best grown plant in your garden. It will respond predictably, and it can stand more abuse. Unfortunately, it may not be the one which needs pruning.
Just before laying impious hands on your plant, step back and evaluate why it needs (you believe) pruning. What is wrong with the plant, or what is contradictory to your own standards for it, and what are the likely reason for its condition? This should be a part of the records you are about to keep. Take note of such items as the apparent state of health of the plant, its previous growth characteristics including especially its response to any earlier pruning, the inter-nodal length between leaf rosettes and how many rosettes are held at one time, the number and location of visible latent or adventitious buds, and finally your own priority for that plant in that site. Note whether you will be cutting healthy or suspect dead or dying tissue, and adjust sanitation practices accordingly.
When all this is stirred together in your mind, you may have a revised requirement for the plant. Some should be fed and watered, not pruned. Some perhaps should be transplanted to another site. No amount of pruning can significantly thicken a plant which holds only one rosette of leaves, although it can encourage more branches holding those single rosettes. Perhaps the rhododendron could be helped more by pruning adjacent plants to let in light and air circulation than by cutting the plant itself. In any case, the pruning finally done should have been measured against a considered opinion of how the plant arrived at its present condition, and how it is expected to respond including appropriate aftercare.
Elepidote Vs. Lepidote Pruning
Before proceeding any further, we should clarify the term rhododendron as used here. We are really concentrating on elepidote or large leafed rhododendrons. Evergreen azaleas, with a few specific clones perhaps excepted, can be cut or sheared at will by an gardener, tyro or expert. In the author's experience, lepidotes are quite as tolerant - cut as hard as you like. Even little straggly specimens of pot culture plants like R. microleucum fairly leap into furry growth when pinched or cut. The deciduous azaleas also respond well to hard cutting. Among the classes of rhododendrons, then, only the elepidotes are a challenge for pruners. The main commentary in this article is for them, although the underlying principles apply to all the classes.
Within the bounds of the elepidote class, there are variations between species and hybrids, and many degrees of clonal variation as well. This is one of the reasons for keeping records, so that one can learn to recognize some of these and predict their effect on pruning.
All of our plants have adventitious buds, supposedly, but on some plants they are evident and plentiful. If there are little greenish knobs making minuscule volcanoes in the bark on your plant's branches, you can expect new breaks in any area where they are evident. Some clones show them right down to the main trunk. These are the safest plants for drastic - ground-level - pruning. If the plant does not have visible buds, they are (probably) still there, and you may still expect the desired response, but disappointment is more of a possibility.
Some plants respond promptly, most do, but in the author's garden, some large but poorly grown yak seedlings cut around May 1 are just expanding their new leaves now (early September). Winter hardiness on such new growth is questionable. It is a combination of cultural factors and clonal variations which lead to the uncertainty of response to recommended pruning techniques. Some plants, such as 'Prince Camille de Rohan', will deliver five or even seven new shoots if pinched or cut. Some others will energize only one bud when pinched - not much help. These will generally provide 2 - 3 shoots if cut after the typical single bud has begun to stretch, but some really malicious plants will stubbornly stick to their single shoot response even then. Cutting back to the next node has a chance of raising the number of shoots.
Typical advice for drastic pruning is to allot three seasons to the task, cutting a third of the plant back each time. Again, for a plant which has the right characteristics, this is effective.
Some clones will not respond to this, putting all their energy into the buds in the remaining structure while the cut portion remains bare, usually dying back. Such a plant may respond quite well to being cut back over its entire canopy. The latter method is quicker and surer, but frightening at first, and of course more damaging to the landscape effect of the plant for the first two seasons.
There seems to be no typical advice for truly drastic pruning, ground level or total removal of the plant structure. Most gardeners are afraid to do it except when dealing with liners, most writers afraid to recommend it. Not to be lightly regarded, it nevertheless will usually work quite well.
Aftercare is more of a problem than lack of new shoots. The new growth is dense and juvenile gigantism of the new leaves is common, causing poor air circulation. Damage from insects is likely, as the growth is succulent late in the season and close to the ground, and winter damage is also abetted by the lingering softness of late growth. Nevertheless it can do wonders for an old monster grown out of shape and site, or as a preparation for later (say 2 years) transplant of a very large specimen.
Timing of pruning activities has been thoroughly discussed. Take seriously the advice against fall pruning; local damage or dieback will follow. Similarly heed the strictures against late summer pruning which can stimulate growth that has no time to prepare for winter.
The more drastic the cut, the earlier it should ideally be made, but there is wide clonal sensitivity to timing, and there is the practical problem of managing one's own time. The preferred pruning "window" time is from early April, when first new life stirs in the plants - perhaps when R. mucronulatum or R. dauricum blooms in your area - until peak of bloom for the individual plant.
Despite the above advice coupling drastic with early pruning, the author dug a large plant on May 25th a few years ago for a pruning lecture to the local chapter. The plant was healthy, leggy, about 1¾" calibre (note that this traditionalist has not succumbed to "caliper" for this purpose), with a lost name-tag and a so-what nice pink flower. It was considered expendable. After demonstrating several options, the author lopped it to a 3" stub, and later replanted the ball and the stub. Against all reasonable advice, and with no special aftercare, the plant has a good rack today. If you wish to try the method, however, do it in April and do not transplant for at least two seasons.
Transplanting of plants in general is usually subjected to the balanced reduction plan, which logically suggests reducing the top to match the loss of roots. For rhododendrons, this is counterproductive. Soon you will be nursing along a plant with a reduced root ball trying to support a more extensive leaf canopy than the plant had prior to the transplanting, and tender growth at that!
Avoid stimulating added top growth when transplanting. Leave the top to its own devices. If you must do something, experiment with ideas borrowed from the propagation bench. Often one cuts the leaves in half to reduce transpiration from an un-rooted cutting. Perhaps this might be a useful trick with a full sized plant? (This was suggested to me by another party, but neither of us had tried it.) Normal pruning urges and techniques, however, should be avoided when transplanting.
Aftercare has been mentioned as part of the pruning process. If cultural problems existed, correct them. Be sure to mulch, to water, and to feed a bit - favoring phosphorus more than nitrogen. Spray for insects if they are a local problem and if the amount of new growth will be unusual, as in more drastic pruning. Just be sure the plant gets a little more TLC than usual for one season, or two if necessary.
Dissertations of this sort should always include a list of "don'ts" for those who absorb negative ideas more readily than positive ones. Don't hesitate to prune, but don't fail to record what was done for future reference, that is, don't do it mindlessly. Don't prune late in the season. Don't expect any guaranteed miracles, but don't forget that any plant can be replaced by a better one. Above all, don't fail to learn something each time you cut a plant.
This article has rambled around various facets of pruning at some length. The message intended is that pruning should never be considered as an isolated subject or process. There is vital interaction with the nature of the plant itself, as a clone, the state of cultural health the gardener has brought it to, and the treatment afforded it after the pruning is done. When the pruner thinks about these processes, and the visible signs of them which we can monitor and alter, pruning becomes more sensible and less a set of rules.
Records, for those of us with short memories, are indispensable to the interactive thinking that makes pruning most effective and also makes it fun to do. Experience and growing awareness will soon make pruning easy.