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How And When To Prune Rhododendrons

Robert L. Furniss
Portland, OR

"How do I prune my rhododendrons?"  The usual answer to this frequently asked question is, "Very little.  Remove the dead and sickly branches and let the plants grow naturally."  Sometimes this is good advice.  It applies best to small, bushy-type rhododendrons and to rhododendrons in woodland and mass plantings, but it is not the whole story.  At times it is adequate, even misleading.

Definition. Pruning is the removal of parts of a plant to control growth.  More art than science, it is an adaptation of natural processes to achieve horticultural objectives.  Broadly, pruning includes the removal of any unwanted parts of a plant, including flowers, buds, soft wood, hard wood, basal sprouts, and sometimes roots.  Pruning is not a routine treatment applied cookbook style.  Nor is it a substitute for requirements for vigorous growth, such as fertilizing, watering, controlling pests, and planting properly.

Objectives. Pruning is for some cultural purpose.  Before plant surgery, the grower should decide what the pruning is intended to accomplish.  Is the grower trying to revitalize treasured old plants, to produce plants for sale, to stimulate maximum number of highest quality flowers, to enhance the year-around appearance of the plants, or to achieve some special landscape effect?  Has something gone awry that needs correcting?  The kind and amount of pruning depends upon the nature of the planting and the purpose of the grower.

Pruning can accomplish a lot.  It can start early in the life of a plant, as in the heading back of nursery stock to achieve compactness.  As the years roll by after planting, many fine rhododendrons decline, become leggy, or develop into brush heaps for lack of attention.  Such plants often can be revitalized and improved by judicious pruning and training.  Of course there are limits.  Medium-sized 'Elizabeth' cannot be forced to grow tall by pruning, and giant-sized 'Loderi King George' cannot be dwarfed.  Most rhododendrons respond well to pruning.  Some that do not sprout readily from old wood cannot be much improved.  Others that sprout abundantly should not be opened excessively to light.  If in doubt, proceed cautiously, or seek expert advice.

When to prune. Pruning of hardened wood can be done at any time except during periods of freezing weather.  Early spring generally is best because the new growth then has a full season in which to develop and mature.  Pruning immediately after the blooming period is standard practice.  However, some rhododendrons that bloom very heavily should be pruned prior to bloom to reduce the number of flowers and thus maintain vigor of the plant.  Thinning the flowers also can improve the quality and placement of the ones that remain.

Summer pruning often results in lush sprouts that are subject to aphid injury and may not harden sufficiently to withstand low winter temperatures.  Deadheading, which is the removal of spent flowers, should be done soon after the flowers fade, taking care not to injure the new growth.  This important job helps control insects and greatly improves the abundance and quality of the next year's bloom.  Soft wood pruning, or pinching back, is done during the growing season.  Removal of terminal leaf buds and shoots to promote branching should be done early in the season or from late summer on through fall and winter.

How to start. A good way to start to prune a rhododendron is to crawl under it, look up, and decide what structural changes are needed.  If the plant has been long-neglected, it likely will be necessary first to cut out a tangle of dead branches.  Then remove cross branches and weak wood.  Remove excess branches to give remaining ones room to grow.  Except when layering a plant, remove drooping branches that scrape the ground and provide handy stepladders for weevils to climb and feed upon the leaves.  Remove spindly shoots that sometimes develop along the bole.  Remove sprouts from under-stock on grafted plants.  These sprouts spring up from the base of a plant and produce flowers of a different color, often lavender.  Fortunately modern hybrids are mostly grown on their own roots, and so remain true to color.  In removing hardened wood, make clean cuts, prune flush with the bole or main branches; do not leave stubs.  Thinning the small outer branches is the final step in the pruning process. 

Pruning for compactness. The compact, profusely budded rhododendrons of the nursery trade usually are the objective of commercial growers and landscapers.  These plants are produced by good cultural methods, including de-budding and summer pruning of current growth to induce multiple branching and abundant flowers.  In the home garden, the day ultimately comes when the branches become too numerous and need to be thinned to restore high quality foliage and bloom.  When this occurs, insect damaged, sun-scorched, winter-injured, and scraggly foliage and branches should be among the first to be removed.  Before planting a rhododendron, keep in mind that it is best to select one that will not outgrow the allotted space.  A tall-growing variety just isn't suitable in front of a picture window.  No amount of pruning will make it fit there comfortably and attractively.

Pruning to a single trunk. In some kinds of landscaping, plants are pruned high and trained to a single trunk or a few main stems.  This treatment reveals the structure of the plant and texture of the bark, thus improving the year-around interest and beauty of a planting. An arched canopy over a woodland-type pathway can be achieved by high pruning of adjoining plants.  The openness of a high-pruned plant facilitates the placement of ladders for deadheading and grooming the top, and provides ready access for watering, fertilizing, and mulching.  If a single-trunk plant is the objective from the beginning, heading back can be delayed to encourage height growth.  Also, while a plant is young and flexible, its trunk can be shaped for character by bending.  If a plant has branched very low or has multiple stems, it will be necessary to cut away some lower branches and all except one or a few of the stems to achieve the desired tree-like effect.  The 'Loderi' and 'Naomi' hybrids and many other large varieties respond well to the single trunk treatment. Bushy varieties do not; for example, 'CIS,' 'Bric-a- brac,' R. racemosum, R. williamsianum, and others.

Pruning for special effects. Sometimes it is desirable to prune a group of rhododendrons so that the foliage on one side is allowed to cascade nearly to the ground and that on the other side is pruned high to reveal the beauty of the trunk and large branches.  Rhododendrons pruned in this way exhibit an unbroken bank of foliage or bloom when viewed from one side and a wooded-dell effect from the other.  The exposed trunks should face the north or east, or be protected from the sun by buildings or other plants.

In general the profile of rhododendron plants is regular.  Individually they are difficult subjects to train for asymmetrical or tiered effects.  These landscape effects can best be achieved by grouping rhododendrons of different sizes and textures, or by inter planting them with suitable companion plants.  However, some azaleas, such as R. calendulaceum, are exceptions to the rule in that they respond beautifully to pruning for irregular effects.

Pruning to rejuvenate. Rhododendrons that have outgrown their site or have become tall, ungainly, and sparse of bloom can be rejuvenated by judicious pruning, preferably in early spring.  Don't attempt to do it all at once.  The plant likely will survive one-shot surgery, even make a strong recovery, but it is no way to treat an old friend.  It is better to spread the rehabilitation over 2 or 3 years.  Each year cut back some of the heavy branches to latent buds.  Let the light in to encourage new shoots to form.  Plants that have deteriorated in the top should be cut back and rejuvenated with new growth originating low on the bole.  Prune with the dual objective of retaining the mature structure of the rhododendron and of improving its vigor and capacity to bloom.

Pruning to salvage. When catastrophe strikes and a large plant is broken or otherwise severly injured, don't despair.  It may be salvaged.  In the wild our native rhododendron, R. macrophyllum, often is killed back to the ground by fire, only to sprout again from the root crown and in a few years regain full vigor.  Cultivated rhododendrons that have to be cut back to a stump likewise frequently recover.

Pruning to facilitate moving. Sometimes large, long-established rhododendrons have to be moved.  This is a sizeable but relatively simple job.  For best results, it should be done in the fall or in early spring before new growth begins.  The roots are cut back (pruned) with a sharp shovel, leaving a wide but shallow pad of roots and soil.  Hauled or skidded to its new location, the plant should be set high in loose, well mulched soil.  To ease the shock of moving, some foliage should be pruned to compensate for the loss of roots.  In part this is accomplished by cutting off lower branches that hamper the moving and in part by pruning unneeded upper branches.  It is a good opportunity to shape a neglected plant.

Pruning azaleas. Most of this article concerns broadleaved, evergreen rhododendrons.  Azaleas require relatively less pruning, but some deciduous ones thrive better if the old shoots are periodically cut back to the ground to give new shoots growing room.  Some azaleas that sprout vigorously or send up suckers from the spreading roots need to be thinned occasionally at the ground to prevent excessive bushiness.  Azaleas can be made more compact by heading back the new growth a few inches in early summer.  Some evergreen azaleas will stand shearing, a practice that is common in Japanese landscaping and which produces very dense mounds of foliage.

Pruning for bonsai. The ultimate in controlling growth by pruning is the culture of bonsai.  Some small-leaved rhododendrons and evergreen azaleas are good material for bonsai.  This specialized aspect of pruning and growing rhododendrons is discussed in "Rhododendron Information," a book published by the American Rhododendron Society, and in standard texts on bonsai.

Treatment of pruned surfaces. It is often recommended that the cut surfaces resulting from pruning limbs an inch or more in diameter be treated with pruning compound. Probably more aesthetic than prophylactic in effect, this treatment appears to be optional with the grower.

Basic tools and references. Basic tools and procedures are discussed in standard references such as "The Pruning Manual" by L. H. Bailey and Sunset Magazine's "Pruning Handbook."  Detailed procedures for pruning rhododendrons are discussed and illustrated in "Rhododendrons of the World" by David Leach.

A final word. In summary, it is a myth that rhododendrons should not be pruned.  The essential thing in pruning is to decide upon the purpose.  Then don't be afraid to apply the saw and pruners to achieve the desired result.  The rhododendrons will appreciate the attention and respond to it.