When and how to prune rhododendrons
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Rhododendrons don’t need a lot of pruning; even as a professional gardener, I’ve not pruned rhododendrons too often, with the exception of cutting back overgrown or dead sections. You can prune rhododendrons to keep them in shape, remove spent flowers to serve energy, rejuvenate the plant or remove diseased or dead bits. Most pruning is best down bwteen late winter after the first frost and right up to the last frost in spring. This is when they are technically dormant for winter and less likely to suffer.
Here’s how to do all this and more.
Deadheading spent flowers to reserve energy
Deadheading isn’t pruning. It’s removing the faded blooms once they are past there best. Cutting off the old flowers lets the plant spend its energy on producing new flowers and fresh growth rather than making seeds on the old flowers.
How to deadhead rhododendrons
The first step in any cutting process is to sterilise your tools. You don’t want to transport disease from one part of the plant to another. Then snip at the base of the old flower. Look for a place that’s 1cm to 2cm above any new emerging growth and cut there.
Don’t be tempted to just grab the stem with your hands and break it off. You may damage the growing part of the plant and accidentally take away some emerging growth. It also leave them more respectable to diseases.
Deadheading keeps your plant tidy, stops lots of seeds from forming and flying off into the wind, and leaves more room in the plant for the air to circulate, which also helps prevent mildew another problem rhododendrons can get.
Removing diseased or dead wood
I’ve moved this up in the list as it’s something you can do when you deadhead your rhododendron. Ideally, you should inspect your plants on a regular schedule to catch any diseased or deformed flowers, leaves or stems as soon as they occur.
The article Rhododendron pests and diseases is very informative in telling you what symptoms to look out for that indicate disease or pest infestation.
How to remove dead or diseased rhododendron parts
Once again, sterilise your cutting tools. Here, especially, it’s best if you sterilise them between every cut although I usually do it between plants. Look at the dead or diseased branch and follow it down towards the trunk until you come to a healthy part. Make your cut just into the healthy part so that you get all the diseased branch or stem. Look out for dormant buds as you want to cut above them.
You may want to burn any dead or diseased parts that you cut off. Many plant diseases are notoriously resilient. They can spread from your compost heap, if you put them there, to other parts of your garden.
Rejuvenating your rhododendron with some hard pruning
Rejuvenating your rhododendron is a more extreme pruning technique. You’d use this if you (or someone else) has let the plant run wild for a while, and it’s not been touched for years. It may be overgrown with leggy stems and branches, all over the place. You need to take an aggressive approach to pruning here, but rhododendrons respond well to that.
This hard pruning involves removing most of the branches. The rhododendron then has to develop new growth on the otherwise leafy stems left. In the growing seasons after the rejuvenation pruning, you can use lighter pruning to control the new shape of your plant.
How to prune your rhododendron for rejuvenation
Most rhododendrons have three or more primary branches that shoot out from the base. These create the main structure of the plant. Rejuvenating your plant is the only time that you would cut back all these branches. Cut each branch back to a different height to give a staggered look to the bush. Follow the branch down to just above an emerging latent bud, and cut 1cm above that. It’s even better if you cut above a cluster of buds.
Pay attention to any small pink dots you see on the main branches. These are where new branches will eventually emerge. Leave a good number of these behind.
While you’re there, cut out some of the smaller branches from these in the interior of the plant. This gives more space for the air to circulate and that’s good for the health of the plant.
Pruning rhododendrons for rejuvenation is best done at the end of winter when the plant’s still dormant.
Shaping your rhododendron
If your plant is starting to look a bit messy but isn’t out of control yet, consider a session of pruning for shape. This is where you cut some branches to encourage growth in a particular direction. This is when you prune your rhododendrons purely for the visual appearance.
How to shape your rhododendron
Select the branch that’s not contributing to the pleasing shape of your plant. Follow it down towards the base, stopping at the last group of leaves that you want to keep. Cut just above these with a sterilised cutting tool.
Shaping is best done when the plant’s dormant in winter. You will get rid of some of the flower buds that were due to emerge. But the plant will have a complete growing season to recover and for new stems to emerge.