Invasive Rhododendron – what’s in a name?

14d5da19ea8-98dbced4df5ccbfc2462b56473d9542c.1400Rhododendron ponticum is the name commonly used for the invasive purple rhododendron frequently found growing wild in the British countryside. In itself, a nice shrub in its season, but it has the ability to spread by seeds or layers and can regrow from roots or small parts of the stem so it is difficult to eradicate. On acidic soils in areas with high rainfall, it spreads rapidly, and can quickly form a ground-covering monoculture, smothering native species. It has also recently become a susceptible to a number of toxic fungi, which then spread with it to infect woodlands, leading to greater efforts to eradicate it.

As a result of this, Rhododendron ponticum appears in Schedule 9 of the Wildlife and Countryside Act 1981, making it an offense to cause it to grow in the wild – an obvious precaution against further spread. But WACA 1981 also makes it an offense to offer the plant for sale, or to possess viable plant material (including seeds) for the purposes of sale – a major problem for garden centres up and down the country who all offer R.ponticum varieties as garden plants.

As a result effort has been made to try to differentiate the invasive plant from the garden varieties, with Cullen in 2011 suggesting that the invasive plant was a “hybrid swarm” to which he ascribed the name R. x superponticum. However DNA testing has revealed that the common UK invasive plant is derived from the wild native populations of R. ponticum in the Iberian Peninsula (which shares its propensity to spread) and distinct from the native  R. ponticum population in the Caucasus (which does not spread as freely) and from which many of our non-spreading garden varieties have been developed.

On the basis of this finding, it has been suggested that the invasive Rhododendron common in the UK (and also found in the Iberian Peninsula)  is named Rhododendron ponticum spp. baeticum, making it distinct from the garden varieties bred from Rhododendron ponticum spp. ponticum (the Caucasus species), neatly solving the legal tangle of the species being sold in the garden centres. Hopefully, if this distinction becomes widely recognised, WACA 1981 Schedule 9 will be updated with the specific name, and the issue of it’s sale in garden centres will finally go away!

Just as a footnote, it’s worth remembering that before the last Ice Age, R. ponticum was widespread as a native plant in the UK and much of northern Europe, and it’s only with the changing climate as the ice advanced that it was eradicated from our shores. Had not the changing sea levels submerged the land bridge as the ice retreated, we might now be considering this population to be a “troublesome native species” rather than the “invasive non-native species” it is now branded!

 

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