Rhododendron may appear beautiful – but one type of this impressive looking plant is posing a deadly threat to Scotland’s rainforest plants and lichens.

Rhododendron ponticum, a majestic shrub with its early summer explosions of bright pink blooms, is a delight to see in formal gardens across Scotland. It was introduced into the UK as an ornamental plant in 1763 and was widely planted for its flowers and to provide shelter and privacy. However, this super-shrub found the British climate very much to its liking and by 1849 there were already reports of rhododendron spreading out of formal gardens into the wider countryside.

Rhododendron ponticum ©Lorne Gill/SNH

There are hundreds of species, and thousands of cultivars, of rhododendron, most of which are no threat to Scottish wildlife.  However, Rhododendron ponticum is one of the greatest invasive non-native species threats to biodiversity in Scotland.  It has spread widely and become established over large areas of woodland and moorland, where its foliage casts dense shade, excluding all other plants from the ground beneath.   The leaf litter breaks down very slowly, and alters the chemical and biological properties of the soil.

Rhododendron ponticum growing on a blanket bog near Loch Torridon ©Lorne Gill/SNH

Nowhere is the impact of rhododendron on our native species starker than in our near unique temperate rainforests; found throughout western Scotland. In their pristine state these humid woodlands are ‘dripping’ with flowers, ferns, mosses and liverworts, and lichens. These species abound on boulders, wooded crags and as intricate miniature gardens on the rough trunks and boughs of ancient trees. Many species have their European headquarters in these elfin woods and some are globally rare. All of them need light, including the saplings that will secure our future rainforest habitat.

Mosses and ferns grow well in Scotland’s rainforests ©Dave Genney

As rhododendron has invaded our rainforest over the years, its dense shade has had a disastrous effect on many of these important native species. We have recorded these impacts over many years and it is likely that many species have become locally extinct before we’ve been able to document their existence. This pressure is compounded by others, such as over-grazing, climate change and historic losses of native woodland cover.

Once established, rhododendron is costly to remove. In 2010 it was estimated that it was costing £8.6 million each year to control the shrub across the UK; the total cost, estimated in 2013, of removing it from Scotland’s National Forest Estate would be £15 million. It is clearly much better to prevent future invasions rather than deal with the costly consequences of complacency.

Rhododendron control at Kippenrait Glen SSSI near Dunblane ©Lorne Gill/SNH

Recent research has shown that in areas where dense rhododendron has been cleared, the composition and diversity of species does not return to its natural state even thirty years after removal. Given time, some species recover better than others, so that the ‘community’ of mosses and liverworts can eventually be diverse but with different species dominating. Other species, such as woodland flowers, may require further management to re-colonise, such as re-seeding after the removal of some of the commoner fast-growing mosses that dominate the woodland floor once the rhododendron has been removed.

Yes, rhododendron is beautiful, but so are the numerous native species that can be lost when we let it get out of hand.  Please help our rainforest species into the future by managing invasive rhododendron carefully within gardens and replacing with non-invasive rhododendron species wherever possible.

Find out more about Rhododendron ponticum and invasive non-native plants.