Rhododendron: Blacklisted?

Dr Trevor Dines
Plantlife Botanical Specialist





Sitting on a hillside above the village of Beddgelert and surrounded by the peaks of Snowdonia, I feel like I should actually be in the wilds of northern China. All around me are billowing mounds of Rhododendron, their fat buds ready to explode and paint the landscape purple. This is proper Rhododendron forest; thick, dense, dark and completely silent. This is how Rhododendron kills – the canopy of evergreen leaves blocks sunlight so that nothing can grow beneath it and chemicals from its fallen leaves inhibit the germination of seeds. Once Rhododendron has taken hold, eradication is costly and time-consuming.

Only it’s not actually Rhododendron ponticum, a rather weak and tender species from southern Europe, we’re talking about, but an invader of our own making.

The view in question. © Dr Trevor Dines

All invasive Rhododendron found in the wild throughout Britain is a hybrid created by Victorian gardeners. By crossing R. ponticum with more robust North American species they created a highly invasive newcomer. Perhaps in allusion to its power to spread, it has now been named R. x superponticum.

But what’s in a name? The European Parliament is currently drawing up plans to blacklist the most invasive non-native species. This new legislation will ban the possession, transport, selling or growing of species deemed to be of "Union Concern". This is a major step forward in co-ordinated efforts to tackle the spread of the worst offenders, but alarm has been raised in some quarters around the lack of clarity over which species will be included. It’s been suggested, for example, that 300 garden Rhododendrons could banned as they can trace their parentage back to Rhododendron ponticum.

It is highly unlikely that R. ponticum will be included. Since R. ponticum is native to Spain and Portugal it can’t be listed as a concern for the whole Union; only the most invasive non-native species across Europe can be included. Instead, it can only be placed on a Regional list for those countries where it’s a problem and this limits what can be done about controlling it under the new legislation.

We are a long way from deciding what goes onto the list. The regulations being voted on now won’t come into force until 1st January 2015 and there will be a further 1.5 years to put the measures in place. Until then, we have time to develop selection criteria and propose which species should be listed. Any species proposed will have to be put through formal Risk Assessment before making it onto the list, a process we’ll all be involved with.

As for the hybrid R. x superponticum, research published by the RHS allows us to clearly identify this taxon from others. This one R. ponticum hybrid is invasive and it’s in our gift to determine how it’s treated under the new legislation. If any other Rhododendron hybrids are invasive, and I’m not aware of any that are, they should be put through Risk Assessment and treated accordingly. As one Spanish colleague put it, raising concerns now is rather like “putting on a plaster before having an injury”.

Just as these species have leapt over the garden wall and escaped, we sometimes need to look over the garden wall ourselves and acknowledge the damage they’re causing in the wild. The last few sites for Starfruit, a beautiful water-plantain, are threatened with New Zealand Pigmyweed and the species might now be extinct in Britain. On Gower in south Wales the only British sites for Yellow Whitlowgrass are threatened by seven different species of Cotoneaster. And on the coast of south west Ireland, farmers are having to abandon fields that are being taken over by Giant-rhubarb.

Back on my Welsh hillside I’m surrounded by Rhododendron on all sides. In the valley below a magnificent Celtic Rainforest - home to countless rare mosses, liverworts and lichens – has been infested. Although the oaks still stand proud the ground beneath them and all the life it supported has been obliterated. Even where I’m standing, high in the hills, an advancing front of young Rhododendron saplings can be seen marching across the heathland. The Junipers won’t last for much longer. In a few years they’ll be skeletons under the Rhododendron forest.