Rhododendron ponticum is one of those examples where a plant species has been introduced to serve a practical purpose and has turned into a liability instead.
Grown to provide for game birds, it has become what we class as a weed, or more precisely an invasive species. R. ponticum is listed in the Wildlife and Countryside Act 1981, where Schedule 9 of the Act and Section 14 of the Act state that it is an offence to plant or otherwise cause the named species to grow in the wild.
But is it the Rhododendron that was in the garden when you moved in? Don’t panic, we’re going to work out if it is – or not. And the latter is, on balance, more likely for most of us.
Firstly, there’s a brief history and an explanation why this now a problematic shrub in parks, woods and some gardens.
Rhododendron ponticum – a Brief History
The evergreen shrub Rhododendron ponticum is native to south west Europe and south west Asia.
It was introduced into Britain in the eighteenth century, as seed by Conrad Loddiges, probably from Spain via Gibraltar. Although this predates his plant nursery in Hackney, Loddiges nevertheless grew on the seed and sold the young plants as an ornamental flowering shrub for gardens, parks and estates.
The dense growth offered protection for game species and, as rhododendron thrives in acid soils, it was widely planted in Victorian hunting Estates in both woodland and heathland areas to provide this shelter.
By 1893 it was being sold on London markets as a flowering pot plant. Rhododendron ponticum has also been used as a rootstock for less hardy but more colourful rhododendrons from China and the Himalayas.
Most Rhododendron species are not found in the wild in the UK. If you find a wild shrub it is almost certainly ponticum as it now covers approximately 3.3% of our total woodland. And is often found growing near Cherry Laurel, Prunus laurocerasus, another problematic evergreen. At certain seasons, it may be easy to mistake one for the other, which is why taking photos across the year is important.
Rhododendron ponticum – Attributes and Eradication
Many of the attributes that made it so perfect for game cover have made the ponticum such a thriving and invasive plant.
Rhododendron ponticum is highly shade tolerant. It can thrive in dense shade with only 2% light; most plants need around 12% or more. If you remember your biology lessons from school, light (sunlight or artificial) is one of a plant’s requirements in order to photosynthesise and thereby produce its food (in the form of sugars). To be able to feed yourself in 2% light is therefore a very useful trait.
Not only does it form dense thickets and smother other species that way, but Rhododendron ponticum also sends out phenolic chemicals into the soil which suppress the ability of other plant species to grow nearby. It also reduces the population of earthworms, local birds and insects. The regenerative capacity of a cleared site is also compromised for some years.
Another problem is that this Rhododendron is a carrier for phytophthora, which can infect and kill many other plants.
Eradication is a long-term process for Rhododendron ponticum, due in part to the dense growth limiting access. To date there are no proven biological methods of control. Mechanical methods, literally cutting down and digging out the shrubs and thickets, is labour intensive. Rhododendrons are great survivors and the plants rejuvenate quickly from the cut stems. Chemicals, herbicides including glyphosate, can be injected into cut stems. This method has the advantage of more quickly killing the plants, often within six months following injection.
Now, back to that Rhododendron in your Garden
Having read the above, you may want to head straight for an axe! But before you start chopping your garden Rhododendron to pieces thinking its public enemy number two, take a long hard look at the plant. Three key questions: –
- Is it evergreen?
- Are the leaves leathery in texture?
- Does it have lilac coloured flowers in spring?
If you have any ‘No’ answers, relax.
(Although if the shrub is causing you problems, do get in touch as we can help with identification and suggestions to resolve the issue, including replacement planting suggestions).
If all your answers were ‘Yes’, you may have a Rhododendron ponticum, but equally you may not. The jury is still out, so what else can you do? The answer is to become your own horticultural detective before you call in professional help. This is a task you can easily do yourself, and, although it may take a bit of time, I’ve made it be enjoyable (see cake below).
Firstly, take some photos of your garden shrub, including close-ups of leaf and underside of leaf, stems, seed pods and flowers. Also take a shot of the whole bush to get an idea of the shape and size. Getting a 6ft garden cane or person whose height you know next to it in the photo may be helpful. Some of these photos may have to wait until the correct time of year. That’s fine, just make yourself a note to do it.
Now, working on the basis that it’s the right time of year and you’ve taken photos, the next thing to do is to go for a walk. And because Rhododendron ponticum is predominantly found in the parks and woodland of large estates, that’s where you’ll be walking. Find a shrub that looks like yours and compare with the photos you took of your garden shrub. I suggest you take the same shots of the one you’ve found. You may like to repeat this walk and photography on different occasions to get all the information for comparison.
Next, head for a café with friend / partner / children / dog, so you can compare the photos at leisure whilst having coffee and cake. This may require repeat visits to the café so you have the full photo gallery for comparison.
Rhododendron ponticum in Your Garden (or not)
Most Rhododendrons pose less of a problem in garden situations, unless your garden was part of an estate or adjoins woodland where the invasive shrubs are known to be.
What you may well have, is a variety that has been grafted on to the vigorous rootstock of R. Ponticum. This was common practice, so if it’s a well-established older shrub, over the years, shoots may have grown which, if not removed, will overwhelm the grafted Rhododendron. This is a situation I have seen many times and the following recommendation is generally the one I give.
Check where the lower branches are shooting from. If they’re below the graft, then simply cut them off. If you’ve caught the problem early enough, the top growth, ie the variety not the ponticum, should recover. You will need to check on a regular basis, as its not a one-off solution, I’m afraid.
But most of the time it is other aspects of Rhododendrons that are causing an issue, not that they’re a Rhododendron ponticum. Many of these shrubs can be a bit thuggish when they’ve been planted in the wrong place. Restorative pruning for older shrubs often resolves that problem.
If you’re looking to plant Rhododendrons, there are many ornamental and highly desirable ones to choose from. For example: –
Rhododendron ‘Irene Koster’ has scented flowers in late spring and early summer and only grows to 1.5m tall & wide.
Rhododendron caucasicum x Ponticum album Cunninghams White, 2.5m tall & 4m wide, more lime tolerant than most
Two things to note here – Firstly many shrubs are tree-tall and wide too, so check the mature size. Secondly, ‘Irene’ is a deciduous azalea, but botanically these are a sub-species not a separate species to Rhododendrons.
and finally
It is an absolute pleasure to one of the 25 National Plant Collections of Rhododendrons between March and June and see them towering above you bursting into bloom and then another is down by your knees, just as showy or perhaps delicate and scented. The collections cover a variety of Rhododendrons and Azaleas, species, hybrids and cultivars. Check out Plant Heritage’s page for the list and which ones are to be found where, as they’re found across the UK. A couple are included in teh blog links below as ganeral garden visits. (And hopefully, not a Rhododendron ponticum hiding anywhere!)
Whatever you do, enjoy your garden and if Plews can help you with anything, please do get in touch. For example –
Our Garden Consultancy & Advice Visits may help with some of your issues. Or does your planting need a rethink? Do you have a birthday coming up? Plews bespoke Gardening Lessons, where your classroom is actually your own garden make a wonderful and practical gift. You could learn how to grow an orchard of citrus fruits in your conservatory! We help you with both gardening basics and more ‘expert tasks’, depending on what you need. Have a read through this pdf download for info.
For further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below. You could come and find us on Instagram Pinterest and Facebook too.
Feeling nosey? You can have a peek at the progress of my garden renovation, Spitfires and Slow Worms, on Instagram and Facebook
You’ll get to see it in person if you come to the Plews Gardening Workshops Current ones on Eventbrite for you to book into. Follow for updates on topics and dates on Plews website, Instagram, Eventbrite, Facebook
Plus get your local garden club, allotment, WI, U3A or other group to ask me along to give a talk (and bring plants 😉)
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Gardening with Shrubs – extending the interest
Scented Shrubs for Christmas Morning
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Flowering Shrubs for Late Summer Colour
5 Scented Flowering Shrubs for a Midsummer Garden
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