Sandy’s Garden ... Rhododendron – Ally or Alien?

Sandy Simpson
Sandy Simpson

A few days ago I was driving east along the M876 as I returned from visiting friends in the west of Scotland.

My eye was caught by a blaze of colour on the hillside half-a-mile east of the junctions for Denny. There, on the south side of, and above, the motorway, a stand of rhododendron bushes was in full flower, a bank of vivid purple contrasting with the bright green of the surrounding vegetation, demanding to be looked at … and yes, I am aware that I should not have allowed my attention to have been diverted by this distraction. And the very next day, travelling through Laurieston, my eye was again caught by a splash of purple, this time on the pavement towards the east end of the village, where the petals discarded by a rhododendron at the foot of a garden had fallen to create the striking effect of a lacy carpet overlaying the walkway.

It was … and is … easy to understand our ancestors’ fascination with the rhododendron. To be botanically technical for a moment, Rhododendron is a family of evergreen shrubs that is native to parts of the Iberian Peninsula, Eastern Europe and west Asia. It was first introduced to England in 1763, probably from Spain and Portugal. The first rhododendron bushes to be introduced into England … and yes, I do mean England … were no more than half-hardy and were grown in large greenhouses by those who could afford such luxuries. However, plant breeders did not take many years to develop a frost-resistant hybrid; and a combination of hybrid vigour, natural and artificial selection and further cross-breeding helped the plants to adapt to colder temperatures, allowing them to be grown outdoors throughout the United Kingdom wherever there is suitably acidic soil.

We don’t actually know when the first rhododendron plants found their way to Scotland, but we do know that the Royal Botanic Garden in Edinburgh was home to a Rhododendron ponticum hybrid in 1814. Its willingness to thrive in Edinburgh, allied to its attractive early summer flowers and its ease of cultivation, led to it becoming a favourite with landscape gardeners and with the owners of Highland estates, where it was introduced to provide cover for game birds, encouraging them to breed successfully and so increase the ‘sporting’ value of the estate come the month of August. And, of course, the acid soils which made life difficult for many plants were the very soils which the rhododendron loved, allowing it to thrive.

But … and there’s often a but where exotic plants which have been introduced into the United Kingdom are concerned … Rhododendron ponticum has had a severe impact on the biodiversity of native woodlands and open ground habitats. Its prolificacy, its ability to survive the harshest of Scottish winters and its tolerance of complete exposure to sun and ability to thrive equally well in deep shade allow it to out-compete native plants. Native tree seedlings, moorland and grassland vegetation and understorey plants which like to live in the dappled shadow of taller native plants cannot survive under the oppressive leaf canopy of Rhododendron ponticum. A scientific study undertaken 35 years ago showed that even such robust plants as holly, yew and ivy, which are able to offer determined resistance to being overwhelmed by rhododendrons, find it very difficult to regenerate in the sunless shade of rhododendron foliage and eventually succumb. Nowadays, the Rhododendron ponticum whose striking flowers caught my attention as I drove along the M876 is the most damaging and most widespread alien plant in acid-soiled habitats in our islands; and, while I may admire its flowers, it poses a real threat to the biodiversity of large areas of our countryside.