JARS v64n2 - Tips for Beginners: Pruning Rhododendrons: How and When

Tips for Beginners: Pruning Rhododendrons: How and When
Don S. Wallace
McKinleyville, California
Reprinted from the Williamette Chapter Newsletter, September 2009

Are your rhododendrons lush and full with dense branching so the stems are rarely seen? If not and you would like this look, then you need to prune. Pruning rhododendrons will improve the plant's structure, making full shrubs with good branching and more flowers. It is also true that some rhododendrons, because of their genetic makeup, will never need pruning, as they hold their older leaves for many years and grow slowly. But, many of the available rhododendron hybrids today will benefit from yearly pruning to improve their growing habit. In this article I will tell you when to prune and some different techniques that work well.

When to Prune

It is generally agreed that the best time to prune rhododendrons is right after they have finished blooming. New growth begins to flush at this time and energy is flowing in the plant, so any pruning will elicit a response in the plant. Another school of thought is to prune the plant in mid winter so that dormant buds can begin to develop as the plant comes out of dormancy. This works well if you live in milder climates like coastal areas. Also, this type of pruning eliminates the flowers for your spring display. If you do not care about the flowers, and live in a mild climate, then this might be a good option.

How to Prune

To develop a good structure on young rhododendrons, we tip prune them for their first five years. You can continue this practice and will be rewarded with plants that have increased branching, and thus more flowers. After the plant is ten years old, we would not recommend this type of pruning any more.

When your plants have started to flush out new growth, pinch or break any "singles" (only one shoot is emerging). If there are two or more shoots emerging, leave them alone. Within a month's time you will see multiple shoots forming where only one had started.

Another pruning technique that is used to shape a rhododendron and keep it fully clothed with leaves is called "selective pruning." Here we cut back any overly vigorous branches that extend beyond the desired size of the shrub. This type of pruning will cause the plant to "break" out new shoots along the main stems, thus adding foliage where it is needed. If this is done yearly, your plant will become very handsome.

When cutting back using this selective technique, there are no rules as to where to cut. Just cut anywhere and new shoots will emerge along the stem from the hidden "eyes" just beneath the bark.

As with all of these types of pruning, we suggest that the plant be fertilized at least one month prior to the cutting. This will increase the chances that lots of dormant buds will break out to fill in the shrub.

The last type of pruning that will be discussed here is "hard pruning." This technique is employed when a plant has become tall and lanky...let's not beat around the bush…ugly! Or, you just want to bring the plants down in size to enjoy the flowers more. But, a word of warning, once a rhododendron is down-sized, that doesn't mean that it will stay that size. Most of these large shrubs are just that, "tree" types of rhododendrons, and will, over time, become the size they were before cutting. If you want a rhododendron planted under your living room window, then it is best to choose one that will only grow to about one m (three ft) tall. Rhodies that have been chopped to fit under a window will always need to be pruned, and will not flower.

Hard pruning is done with loppers or a pruning saw, as you will be cutting branches too thick for pruning shears. Our rule of thumb is to cut out one-third of the plant per year, so over three years you will have reduced the size of your plant and it will have "bushed out." So, let's say your plant is 2.5 m (8 ft) tall. You will be pruning one third of the branches down to 1.35 m (4ft) from the ground. Cut anywhere along one of the main stems or trunks and new shoots will emerge from the dormant and hidden "eyes" along this branch. If you do this over three years time, you will have accomplished your job, and be rewarded with a lush, bushy shrub that will have many more flowers!

Don Wallace is a member of the Eureka Chapter and owner of Singing Tree Gardens.