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
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.|