Rhododendron is one of the most striking shrubs with beautiful flowers and lush foliage. For popular shrubs in many landscapes, the question is how to prune a rhododendron, and this is a common question. Although rhododendrons often require little pruning, especially in semi-natural settings, these shrubs respond well to occasional pruning. In fact, severe pruning may be necessary if growth is excessive. Pruning rhododendrons is usually done for maintenance, shaping and rejuvenation – as is the case with overgrown plants.
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Best time to prune the shrub
When to prune rhododendron? According to most professional landscapers, the ideal time to prune rhododendrons is late winter, when the plant is dormant. However, any time between the first frost in the fall and the last frost in the spring (as long as the sap level is low) is suitable. Immediately after lush spring growth, when the new foliage is still hardening off, is one of the worst times to prune rhododendrons. This will likely prevent flowering.
If you are considering pruning, fertilize the shrub in late fall of the previous year . Pruning later can result in floppy growth. Since buds form on next year’s flowers, they will be well advanced by the time flowering stops. Therefore, you should cut your rhododendron down no more than 38-50 inches after it has finished blooming. Cut the plant back to expose the inner branches. Follow the branch to the last wreath of leaves you want to preserve, and cut just above those leaves.
Pruning Rhododendrons: Which pruning methods to use
There are three common reasons for pruning rhododendrons – maintenance, topiary and rejuvenation – and the pruning method for each is easy to learn. The result is a shrub with dense branching, lush foliage and abundant blooms.
Method 1: Maintenance pruning removes old flowers and dead wood.
Prune at the base of old flower clusters to focus the plant’s energy on producing growth rather than seeds. Also remove dead or diseased parts of the shrub – you need to follow the branch back to the healthy wood. You will prune the rhododendron there. Maintenance pruning consists of removing the spent flower clusters, called panicles, and dead or diseased wood. The panicles are not only unsightly, but eventually form seeds that consume valuable energy that would otherwise be available to the plant for vegetative growth. Maintenance pruning is the simplest type of pruning and the only one you need to do every year.
To remove old flower clusters on rhododendrons, use pruning shears to cut off the flower clusters at the base, about an inch above the new shoot. Some people simply grab the stem with their thumb and forefinger and tear the cluster from the plant. Most of the time this works well, but occasionally the bundle breaks off and takes some of the new growth with it. Using pruning shears can prevent such accidents.
Maintenance pruning is best done when the flowers have faded and before the new growth under each shoot is more than an inch high. Many rhododendron species can benefit from maintenance pruning. If your flowers and flower stalks simply wilt and essentially disappear in the weeks following bloom, then your variety will not set seed and will not need to be topped.
Method 2: Topiary encourages the plant to grow naturally.
Topiary and other formal pruning techniques shape a plant into a form that it would not naturally take. As the name implies, topiary is about changing the shape of the plant, and it can be done for many reasons, from promoting denser branching to controlling plant width or height.
Topiary improves the appearance of a plant by encouraging more branching at growing points. Because many evergreen rhododendrons retain their leaves for about three years, a branch will have a series of leaf whorls, each of which grows for one year. The leaf whorls are separated by leafless stem segments called internodes. In general, broad-leaved rhododendrons have much longer internodes than small-leaved and deciduous species and benefit most from shaping.
To shape a rhododendron, follow the branch from the end to the last leaf crown you want to keep. Cut the branch about a half inch above the top leaf in that clump. Repeat as needed. Topiary is easiest to do in late winter when the plant is dormant. By doing this, you sacrifice some flower buds, but the newly sprouting shoots have a full growing season ahead of them.
Method 3: Rejuvenation pruning requires drastic cuts to the old wood.
Rejuvenation pruning involves carefully cutting back each main shoot of the plant’s framework. Rhododendrons often have three or more main branches protruding from the plant’s crown. These branches, called main branches, form the basic framework of any shrub. Each branch is pruned at a different height to achieve a staggered arrangement that makes the shrub look natural as the new shoots mature.
Rejuvenation pruning is best for restoring shrubs that have become droopy, overgrown or otherwise unsightly. Many rhododendron species and hybrids can be cut back severely and will be like new again. Rejuvenation pruning involves removing most of the plant’s branches, allowing vigorous new growth to emerge from previously leafless old stems. The new growth matures into a new framework of branches that can then be shaped into a magnificent shrub over the years. This type of pruning is best done in winter when the plant is dormant.