Should You Cut Back Rhododendron?

Last updated on October 23rd, 2023 at 08:24 pm

More than 1000 different species of rhododendron grow all over the world – in Southeast Asia, North America, Australia and even in the harsh climate of the Himalayas. Although the stately flowering shrub is mainly known by its botanical name, some may also be familiar with the European name ‘Alpenrose’. Thanks to its naturally picturesque growth habit, there is really no need to prune your rhododendron. In certain cases, however, pruning can still be useful, for example, in the case of bald specimens with dwindling foliage. You can read about how to do this professionally here.

Cutting rhododendron: When does it make sense?

Mostly evergreen shrubs grow slowly, but also dense and compact. You don’t have to prune a rhododendron, but you can. If left to flourish unhindered, they usually develop a balanced, rounded shape all by themselves. Moreover, even without stimulating pruning, they bloom vigorously every year.

Nevertheless, there are some pruning measures that make perfect sense. Although rhododendrons can reach a ripe old age, their vigor will eventually wane. Then a rejuvenation pruning can provide newfound vitality. Restrained maintenance pruning after flowering also allows you to make minor shape corrections and prevent the spread of disease.

Possible pruning measures for rhododendrons

Light maintenance pruning

Time: directly after flowering (depending on variety)
Purpose: minor shape corrections, removal of faded buds, diseased shoots and dead wood
Tools: rose shears or pruning shears

Rejuvenation pruning

Time: autumn or spring
Purpose: new vitality for old shrubs, stimulates branching and formation of new leaves on old shoots
Tools: pruning shears and pruning saw if necessary

Light maintenance pruning

Since many gardeners will want to remove the faded rhododendron buds in the summer anyway, a low-key maintenance pruning that allows for minor corrections is recommended for this time. Particular care should be taken with larger trees, as native birds like to breed in them during the summer. Therefore, no radical pruning measures should be carried out between March and September. On the other hand, caution is required because the flower buds on the rhododendron for the next year are already laid out under this year’s bloom.

3 steps for pruning:

1. To avoid injuring flowering plants, you must cut the rhododendron directly under the old inflorescence. Set your tool at a slight angle to prevent water from collecting on the cut surface. This way, you minimize the risk of fungal pathogens becoming established in the shoots.
2. Obviously diseased or injured shoots can be cut back to the healthy wood.
3. If individual, widely protruding branches interfere with the rounded growth habit, you may cut them back above a pair of leaves.

Important: Rhododendron is poisonous, so you should wear gloves when working on the plant.

Pruning and rejuvenating old rhododendrons

Many a woody plant needs heavy pruning on a regular basis to prevent it from becoming bare on the lower branches. With rhododendrons, this can only become a problem when they reach a certain age. The solution to the problem can be a rejuvenation pruning in frost-free autumn or spring. However, this should only be done when the shrub is sufficiently firmly rooted in the garden soil. Grafted varieties have generally not proved suitable for heavy pruning.

Before you decide to cut back your rhododendron heavily, take another close look at it. Has the shrub grown a good bit each year – or has it looked seemingly the same for years and not grown significantly? A specimen that has never rooted properly can also give the false impression that it has become bare due to old age. In fact, this is due to unsuitable soil conditions. A rhododendron that is firmly rooted in the soil should literally not be able to be shaken much.

Rejuvenation step by step:

Dead wood and diseased branches are generously removed and cut back to healthy wood.
Cut back all healthy shoots of the rhododendron to about 40 to 60 centimeters above the ground – you should not go much lower. Always cut diagonally above a pair of leaves.
If some shoots are too close together, you can thin them out a little and remove a few. This will allow more light to penetrate the interior in the future and the rhododendron will form more foliage again.

Pruning is followed by extensive watering.

Tip: To make the procedure gentler on your rhododendron, you can spread the rejuvenation pruning over two years by cutting back only half of the branches in the first year and not pruning the remaining ones until the next fall or spring.

To avoid opening the door to disease and pests through pruning, clean, sharp tools are key. Although trees and shrubs can reclose their pruning wounds very quickly, this takes longer with frayed cuts, and pathogens can be transferred from other plants if tools are not cleaned.

Although rejuvenation offers the advantages of breathing new vigor into old rhododendrons or bringing shrubs that have been improperly pruned for years back into shape, severe pruning should be well considered. For example, if you place more emphasis on height rather than dense growth, rejuvenation pruning may not be the right thing to do. To return to their original size, leisurely growing rhododendrons often require many years. Also to be considered is that after cutting the rhododendron in the next year or two, for the time being, do not expect the beautiful bloom.

Variety differences

Because in the meantime almost exclusively rhododendron hybrids are offered in the trade, one has divided the many crossings into variety groups. The groups differ not only in growth type and flower shape, but also in pruning tolerance. Smirnovii, viscidifolium or caucasicum hybrids belong to the more pruning-sensitive varieties, whereas the hybrid varieties of the species yakushimanum, maximum or wardii, for example, tolerate heavy pruning well.