Sunday 13 October 2013

Garden myths discussed. A modern meme

Rhododendron ponticum is allelopathic

When I read the popular press I learn that Scotland  and Wales has been taken over by Rhododendron ponticum and Japanese knotweed, not to mention bracken. In my previous post Peter Williams eloquently described how a hybrid swarm (I love that phrase) called R. x super-ponticum is extremely invasive. It smothers adjacent plants and on wet Welsh hillsides and pastures starts to take over. I know that Peter considers this rampant vigorous plant when in its optimum growing conditions has the botanical resources to be a complete thug. On a related theme my botanist friend, Mike Ashford tells me that in his lecture on ‘the functions of leaves’, if he is feeling provocative he declares that leaves are a weapon! They enable a plant to overgrow and swamp their neighbours. As gardeners we know that it is sometimes appropriate to snip off a few leaves to restore a balance of power between competing neigbouring plants.

Rhododendron x superponticum  A real threat to the environment but perhaps overhyped?

Varieties of true Rhododendron ponticum growing in a parkland are not a threat to nature
I recently read in a popular garden magazine that Rhododendron ponticum is allelopathic. That means that it exudes toxins into the soil which enables it to suppress and outgrow other plants. I complacently filed this information away at the back of my head. The idea is completely plausible and indeed those other two thugs, bracken and Japanese knotweed really are allelopathic. I mentioned this to Peter who is both an expert on rhododendrons and allelopathy. He was not aware that ponticum was allelopathic and as a good scientist made no further comment.

Memes are information that pass from one person to another and like Chinese whispers a story becomes somewhat garbled in transit. It only needs someone to suggest that ponticum is so invasive that “it must be allelopathic” that it quickly becomes “ponticum is allelopathic”. If the next person in the chain is horticulturally authoriative, is even titled, or worse, mentions it at a horticultural conference and it gets into the popular press it becomes a horticultural fact. It must be true.

Now personally I have no idea whether or not we are dealing with a myth. Rhododendron x super-ponticum is an amazingly aggressive plant which does suppress and kill surrounding  herbaceous and scrubby vegetation and can survive under very dense tree cover such as in an oak wood. The story is all the more plausible because ‘super-ponticum’ does produce toxins that makes its leaves poisonous and unpalatable to animals.
Rhododendron has amazing powers of regeneration 

In an oak wood 
James Merryweather is an ecologist and botanist and coincidentally is a former scientific colleague of Peter Williams. He has investigated the notion that Rhododendron x super-ponticum is allelopathic and can find no evidence whatsoever that it is so successful because it exudes phytotoxins with which it kills neighbouring plants.
James Merryweather is actively involved in practical considerations of rhododendron control and measures that will help restore local ecologies in Scotland. A true return to original conditions is difficult for all manor of reasons. Not least is the loss of mycorrhiza and other beneficial soil living organisms and there will also be local loss of propagules of formerly colonising plants. James promotes a simple manual method of initial restoration. It’s called the lever and mulch method. This involves a fairly straight forward dismantling of the rhododendron and leaving cut branches on the surface as mulch.This is Infinitely better, cheaper and kinder to nature than scraping them out of the ground with great ugly bulldozers as sometimes happens!
Please read James’s beautiful prose as he elucidates his passion. I love his turn of phrase when he debunks the allelopathic story. He refers to a cautious speculation that has lead to a dogmatic factoid.

These rhododendrons were originally planted on graves in my cemetery garden in Barnsley a very long time ago.

They were cut back hard to the ground fifteen years ago - you can safely do this with all garden rhododendrons.
To find other gardening myths on this blog insert ‘myths’ in my search box


  1. Martyn cit back our camellia in the same way and it too regenerated,

    Couldn't it be that other plants can't compete with the rhododendron for light when trying to grow under the dense leaf covering (you sort of hinted at this when you said they use their leaves as weapons). Also being tough evergreen leaves they are not likely to produce the leaf litter that is beneficial to the soil. Also the ground beneath the shrubs does tend to be very dry. What I am trying to suggest is that there could be a whole raft of explanations as to why other plants don't thrive around rhododendrons

    1. Spot on Sue, there is no doubt that rhododendrons are very well equipped to out compete other plants. My point is that allelopathy is not the cause of this as you acknowledge with your comment about leaves as a weapon. Those rhododendrons in Worsbrough cemetery although they are not the super aggressive super x ponticum out grow competitors but their spread is very slow and they are not going to escape into the countryside. Thank you as ever for helping me to make something clearer with your incisive comment.
      An interesting point is that although R. ponticum probably needs acid soil to establish itself many plants by their presence can change the soil pH and no doubt the tough acid leaves of the rhododendron will do so.

  2. We have lots of purple Rhododendrons growing along the roadsides near here, which regularly get a savage trim by the Highways Agency / local council, and they always come back with renewed vigour the next year. I wonder which type they are... Beech trees inhibit the growth of smaller plants too - but I expect it's simply because they block out the light. Or are they allelopathic?

    1. I googled this and apparently there is some evidence for allelpathy in beech. The more you look into this phenomenon the more interesting it is, apparently some trees leach toxins into the soil from their leaves, some can produce volatile toxins and some exude toxins from their roots. My own experience with beech is that plants do not grow well under it, but I would have generally put this down to heavy shade and drought. I have grown early Spring bulbs such as snowdrops and winter aconite under beech- there is light in winter and the soil has benefitted from winter wetting.

    2. Roger, you remind me of my grandfather, who used to say he never had trouble from bracken, wherever it grew he'd plant beech trees to get rid of it. Pulling my leg I expect, but he did have a beautiful old beech wood carpeted with white Cushion Moss and nothing else.
      On further thought, don't we know how beech leaf-mould makes a most fertile and gentle compost for raising tender seedlings? If there's actuall allelopathy, can that all point to it being of root origin, rather like someone showed in the case of the black walnut?

    3. I have looked into this a little further and cannot find any sound information that beech is allelopathic. Peter Williams was telling me last night how well his cyclamen and other woodland plants grow under it. My own beech is the Dawick beech which is tall and thin and plants under it get water and light.
      The heavy shade and drought in a dense beech wood on the other hand suppresses most things as you suggest
      As you mention beech leaf mould is an excellent ingredient in compost

  3. I moved to a ponticum infested area of Sussex a couple of years ago, including plenty of huuuuge ones in my 0.75 acre garden. There are some in a hedge, and I wouldn't say they poison the competing shrubs, they just try to compete with them. And because they get clipped, they don't win.

    But left to their own devices they certainly win. Nothing will grow under them, and it's not just the lack of light. Even after removing all lower branches the weeds still don't even try. It is partly the felt like layer of leaf mould and fine red roots, but that doesn't appear to be the only cause. That dry mulch doesn't exist around the drip line of the plant, but still no weeds have grown there over the last six month, not one. There has been a weedfest elsewhere in the garden this summer, but not there. So my guess would be the ponticum are doing something similar to a beech tree.

    It seems to be very superficial though, as soon as you dig out any roots, the weed race is on. That's why I now raise the crown so I can see the space, but leave them where they are until I'm ready to tackle it. It works well.

    1. You have given this a great deal of thought Sarah and no doubt some heartache. Your analysis of the problem is very interesting and helpful and might very well suggest the roots are allopathic - I am quite prepared to be wrong as I indicate in my piece. The fact that the weeds appear as soon as the roots are chopped does not perhaps indicate toxins in the soil, but of course any might wash away quickly.
      Although my ponticums in Worsbro cemetery will not be super-ponticum (and of course we do not know whether yours are - and personally I could not tell the difference by looking) I do find that bramble, self sown hollys and yew seem to fight their corner from the middle of the clump!

    2. You are right, brambles, holly and yew do crop up underneath them. And fir trees. I was meaning little weed seedlings. The brambles might not be from seed, so if they are sending in shoots they can fight a bit harder. And the yew and holly....well that does back the theory that it's light levels, not toxins, as they both probably grow in very low light. I've also found heather hiding under the outer edges of the canopy....but only the outer edges, which in itself is interesting!

      It will give me something to think about this winter, while I'm chopping up a few more of them :-)

    3. Interesting that we have similar experiences. Just as an aside when I originally eliminated brambles from Worsbrough cemetery by spraying and thought they were gone, for a few following years blackberry seed germinated everywhere - perhaps under the rhodo they had escaped my spray or just come from seed.

  4. This is the first I've heard that a variety of rhododendron is invasive in some parts of the world. They do not grow well in my area because of our alkaline soil.

    1. Yes its ironic that most gardeners do not have a suitable soil. I cannot grow them at Bolton Percy where the soil is slightly alkaline. I would not be without my rhododendrons here, especially the fantastic less vigorous species that Peter has given me

    2. What a fascinating subject, for forty years I have believed and passed on the knowledge that rhododendrons are allopathic by virtue of a substance secreted from the leaves. Prompted by your post I have examined a bank of rhododendrons, including two ponticum, which are growing against a wall covered in a large leaved ivy which in turn grows under the rhododendrons where the leaves become small, sparse and lacking colour and pops out the other side where the leaves flourish. As the plant is not killed I can only think that the change in foliage is due to lack of light and nutrients (there are no roots put down in this section) and not down to toxins, unless of course ivy is so tough that it is resistant to everything the rhododendron can throw at it?

    3. Very interesting evidence Rick. I think your question mark says it all, it does not necessarily deny allelopathy.
      Fascinating that the story has been around for forty years
      After your and Sarah's observations I keep examining my rhodos at Worsbrough. I note Sarah's 'little weeds' come right to the base of the clump but not within it.

    4. Hi,just found this.As a Rhododendron/Azalea grower and hybridizer ,thought I'd better say something.When moving to Ireland from Sussex,17 years ago,I thought it would be nice to have a Ponticum hedge along the boundary of my garden.So I scattered pounds of seed. Nothing took.The next year I scattered pounds of seeds,and also a couple of seed trays Nothing took on the boundary but the seed trays were full.I thought this was strange as there was already ponticum and laurel growing on the boundary.Present day,I have two acres of rhododendrons and azaleas surrounded by leaf mould and moss,very little self seeding occurs.What I am trying to illustrate,is that for rhododendrons to become invasive,conditions need to be more than perfect and usually man-made .A forest nearby, is always the subject of rhododendron hysteria..Ponticum has become very invasive and there is very little natural woodland rejuvenation.
      The farmers let their sheep graze freely through the woods,devouring all the natural seedlings and leaving the unpalatable rhododendrons.There is no competition for the it spreads.In addition the ponticum is chopped to the ground,making it shoot from it's roots it spreads.The same happens at Killarney National Park where the sheep are joined by the deer....Ponticum is usually left alone until it is a problem....this is bad land management,whether it's a national park or a vicarage in Surrey.I personally would love an acre of ponticum....unlimited turning wood,great firewood and the best weed suppressing leaf mould you can get.
      As I have already said, I have two acres of rhododendron hybrids ,plants from China and species from the Himalayas.....,but I still can't grow my ponticum hedge.
      As for rhododendrons being poisonous,my big problem, are the bees that crash into my garden every May smashing the flowers to bits that really is an invasion.

    5. Hi Rhoda
      Just found your fascinating contribution! I think we were off line when it came in.
      Your own experience is really interesting. Amazing your ponticum seed does not grow!
      The fact that the leaves are unpalatable to grazing animals must be a significant reason for its success in man made habitats

  5. James Merryweather20 January 2014 at 11:21


    Thank you for this. Please contact me: huntsup(at)

    James Merryweather.

    1. Sorry I did not thank you for your comment which somehow I missed. Internet problems at the time!

    2. Have just had a look at your fascinating website and have e mailed the link to our friend Peter Williams

  6. I live in Newfoundland, Canada, where we struggle to grow many things, due to our rocky soil. Where there is soil it is highly acidic. I have a rhododendron which was a self seeded plant I transplanted from my home in St. Mary's Bay to my property in St. Philip's, near Conception Bay. It has flourished and has had four babies. I am so excited by this and plan to move these to other parts of my property which are forested with spruce, fir, and birch. It will be beautiful.

  7. Congratulations and best of luck with the babies


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