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Ever wondered how to prune rhododendron? These seemingly common shrubs can sometimes be a mystery to gardeners. They are often overlooked as large plants that can swamp garden spaces, but rhododendrons are border stalwarts and when in flower demand attention.
Rhododendron is, on the whole, a low maintenance woodland shrub and due to its reliable nature often used in ornamental landscaping. Preferring a pH between 4.5-5.5 rhododendrons favor acidic soil, they make are excellent choices for flower bed ideas in shadier spots, and with the right care grow well in pots.
This fascinating diverse genus of plants has over 1,000 colorful species of evergreen shrubs and deciduous rhododendron and azalea, with palettes ranging across the spectrum from white, yellow, orange and red through to pinks and purples. These prolific bloomers are also a big hit with pollinators.
Rhododendron macrophyllum is the US state flower of Washington and commonly found in the Pacific Northwest; Rhododendron maximum is prevalent throughout the Appalachian Mountains in North America, featuring in the West Virginia flag and is the state flower of Virginia.
Whether you grow dwarf Rhododendron impeditum ideal for USDA zones 5-8, to giant Rhododendron 'Broughtonii' trees, hardy down to -5°F ( -21°C), that can live for over 130 years, there is a variety apt for most gardens. It is important to know how to prune rhododendron – as well as when to prune them – to keep them looking their best and blooming year on year.
How to prune rhododendron – evergreen varieties
Our advice will focus on how to prune evergreen rhododendron, which will differ from how to prune azaleas.
The fundamental basis of pruning is to reduce height, maintain plant health and invigorate new growth and blooms. Methods will often be similar for other flowering shrubs.
The first step is to take a good look around your shrub. Note its shape, overall health and condition. Remove any dead, diseased or damaged material and check if there are any visible pests or problems.
Pruning helps avoid shrubs becoming completely overgrown but how do you prune rhododendron to maintain a natural look?
Pruning young rhododendron
Even young rhododendron can sometimes need a bit of pruning to encourage new, stocky growth.
‘On young rhododendrons, or those which are looking a bit sparse, we recommend pinching out single growth buds as they start to swell to encourage multiple branching. This will ensure that your plant retains a nice bushy shape,’ says David Millais, owner of rhododendron specialists Millais Nursery, experts in the art of rhododendron pruning.
‘Simply snap out the leading growth bud at each whorl of leaves as they start to expand. Instead of one lanky shoot, you should gain three to four bushy shoots from the buds above the leaf petioles, each of which can support flowers the next season,’ he adds.
How to trim and shape a rhododendron
An important part of pruning a rhododendron is shaping and crown lifting.
SHAPING – ‘I avoid "snipping" away small bits as you can end up with a "hedged" look – flat on one face rather than a nice rounded shrub. Look carefully at the misplaced branches, find a node, where the branches are coming out in a whorl, and remove the central extension growth. This will allow those lateral branches to quickly hide the pruning scars,' explains Bethan Pettitt, head gardener at The Brantwood Trust.
CROWN LIFTING – Another way to enhance the look of your rhododendron is to lift the crown or as Bethan Pettitt likes to call it, ‘stem beautifying.’
‘In thick, tangled areas of large, mature rhododendron where you don’t want to remove the plants or hard prune, as they might be an important screen – or privacy hedge – clear spindly growth from the base and main branches showcasing the wonderfully twisted, contorted shapes of the multi-stemmed trunks,’ she advises
'This sometimes requires a little follow up the next season. Simply run your hands over the stems the next year removing any soft sappy regrowth, it’s a satisfying task,’ she adds.
Pruning can often feel like sculpting. As gardeners we are essentially helping nature to create a piece of living art.
‘If shafts of light make it through the canopy, highlighting the stems and playing over the fallen blossom on the woodland floor, then all the better,’ says Bethan.
How much can you cut back a rhododendron?
Established old rhododendron eventually may need a heavy prune to rejuvenate the plant. Prune on a frost-free day when the shrub is dormant during late winter to very early spring.
‘Sometimes radical coppicing is required, but this will open up a large hole in the landscape. Unfortunately, there are no half measures, and it is best to cut hard back almost to stumps, so that growth starts low down to create a bushy plant from ground level,’ David Millais advises.
‘If possible, leave a few green shoots to help draw the sap up through the plant. New growth starts from dormant buds below the cut, forcing their way through the old trunks during May and early June. Species with smooth bark typically do not respond well to coppicing,’ he adds.
How do you prune an old leggy rhododendron?
If left unpruned, over time rhododendron can become leggy and unsightly, pruning is then essential to regaining a stocky shape.
‘When pruning rhododendron that are getting a bit large for their space, I select a range of the oldest, thickest or most badly placed stems and remove them right to the base of the plant,’ Bethan Pettitt advises.
How to care for a rhododendron after pruning
After pruning, mulch your shrub with peat-free ericaceous compost or leaf mulch. If you are growing rhododendrons in pots as part of your container gardening ideas, then remove the first few inches of soil and top dress with fresh compost. Water with rainwater and ericaceous feed to give plants a boost while they recover from their prune.
Rhododendron have shallow roots so be careful not to over or underwater them as this can affect your plant's health.
Pruning tools to use
An important part of learning how to prune rhododendron, is having the correct tools for the job.
Pruning can be achieved with a sharp pair of secateurs but for larger branches you may need loppers or even a saw.
Whatever tools you use, always ensure the blades are sharp to make clean cuts without leaving jagged edges, which could encourage fungal diseases to enter wounds.
‘Be sure to clean any equipment used carefully to prevent the spreading of diseases, such as Phytophthora,’ says Bethan Pettitt.
Always clean tools with a disinfectant before pruning different plants to prevent cross transfer of diseases.
What happens if you don't prune rhododendrons?
While pruning will maintain a good shrub shape, there is often no need to prune rhododendrons at all – simply deadhead after your plant has finished blooming at the end of spring to ensure that all the plant’s energy is concentrated into producing foliage and not seed.
If your garden plot allows, leave rhododendron to grow naturally with minimal intervention. Of course, if it out grows its space you are now armed with all the expert information needed on how to prune rhododendron successfully to maintain plant health and maximize blooms.
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Debi Holland runs her own garden business in the south of west of England, gardening in beautiful country homes, and also writes for numerous online and print publications and presents online talks advocating ‘Gardening for Wellbeing.’
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