When our horticulturist predecessors introduced Rhododendron ponticum they could not have anticipated the consequences. The science of ecology, which studies the inter-relationship of organisms within the environment, did not exist as a discipline. The potential for invasive spread of this particular species was not obvious. The Encyclopedia of Rhododendron Species, published 1993 by Cox and Cox, says that R. ponticum ‘spreads by seed and layers, and many gardens are now full of R. ponticum derived from suckers from grafting understocks…..outside the UK (it) is rarely troublesome as it is too tender for large parts of Europe and N. America’. Yet in 2008 it became a registered pest plant in New Zealand, banned from sale, propagation and distribution. And now, in much of Western Europe, R. ponticum is certainly regarded as problematic – has something changed?
Just to spell out some of these problems, the invasive spread of ponticum has dramatically affected our landscape, as well as impacting on native flora and fauna. Growing in full light or dense shade, it spreads by seed and vegetative growth, out-competing most other plants, either by shading or by the accumulation of toxic leaf litter, or both. Its poisonous nectar kills bees, as well as resulting in honey that is toxic to humans. And its role as reservoir and vector for Phytopthora ramorum has serious economic consequences for the forestry and horticultural sectors. Similar consequences arise from the necessary effort to eradicate it – 53,000 hectares of it in Scotland.
What those early ericaceous enthusiasts could not have dreamt of, in their worst nightmares, was the concept of anthropogenic climate change – which now entirely alters the context of conservation. One way in which it does this is by affecting the geographical range in which an organism can survive, thus giving rise to ‘species shift’, sometimes in a possibly benign way, e.g. Common Cranes nesting in Aberdeenshire, but also with dire consequences e.g. the northerly progression of Xylella fastidiosa ssp. multiplex.
Residents of the island of Colonsay, who have for many years been battling with ponticum, are convinced that a recent upsurge in the spread of this species is a function of climate change.
Climate change is here to stay, but the degree to which it will affect this planet depends entirely on how seriously we address the role of the human activities that fuel it. At the moment it would appear that governments are falling short of their obligations – some more than others – and that the 1.5 deg.C target agreed on at the Paris Conference will not be met. Climate scientists are now talking about the possibility of a 3 degree rise by the end of the century, with catastrophic consequences. How such a temperature rise will affect Scotland is unknown. Right now, the predictions made some thirty years ago seem fairly accurate: wetter summers, milder winters. But if the melting of Arctic ice shifts the position of the Gulf Stream away from our coasts then we may be in for very much colder conditions in the UK.
This uncertainty should not deter us from planting rhododendrons; but given the increased awareness that ponticum planters lacked, there is a valid argument for a careful environmental impact study of any new planting scheme, particularly in public spaces. After all, it is an offence under the Wildlife and Countryside Act 1981, as amended by the Wildlife and Natural Environment Act 2011, to plant or cause to grow in the wild any plant outwith its native range or to allow a non-native species that is not exempt or licensed to spread into the wild.
It is tempting for Rhododendron enthusiasts to insist that ponticum is the only badly-behaved member of the genus. But given that that species has adapted to UK conditions in an unpredicted way it is not unreasonable to assume that one or two of the other thousand species in the genus might do something similar.
As mentioned, climate change has the potential to render a particular environment more or less suitable for any given species. Also, rhododendrons are notoriously susceptible to hybridisation – one authority has even suggested that UK ponticum is itself a hybrid, which might explain its extraordinary vigour.
Gardeners have been aware of the ‘alien threat’ for long enough. Philip von Siebold brought the first hosta plants to Europe from Japan but he also brought Japanese knotweed, around the same time as R. ponticum was enthusiastically introduced as game cover on Scottish estates. Himalayan balsam and American skunk cabbage have since joined the unwanted list, to mention just a couple more. Defra estimates that invasive, non-native species as a whole cost the British economy £2bn. annually. So while we are naturally inclined to try out new subjects in our gardens we should be alert to the possibility of the ‘next ponticum’, particularly if we are to succeed in sharing our love of genus Rhododendron.