Rhododendron – Himalayan beauty or poisonous Mediterranean invader?
Late spring is just the time that country estates and gardens are opening their gates, to showcase their Rhododendrons and closely related Azaleas. They are stunning. The flowers are short lived, and you must be quick to catch them at their best.
Rhododendrons are also on the ‘most wanted’ list of invasive aliens, as one of greatest threats to our woodland habitats and heathland. There can be few wild plants in Britain that engender such a wide range of emotions as the Rhododendron. In answer to the question ‘Himalayan beauty or poisonous Mediterranean invader’? It is both.
There are over 800 species of Rhododendron, over half of which come from the Himalayas and western China, many brought back as seeds from plant collecting expeditions by Joseph Hooker, (son of William Hooker, Kew Gardens director) in the mid-nineteenth century. These are, in the main, non-invasive. The ‘troublemaker’ is a pink-purple flowered species - Rhododendron ponticum from the Mediterranean.
The etymology describes this plant well. Rhododendron, from the Greek rodon = rose, dendron = tree, and ponticum, describing its origin, ‘Pontus’, now part of modern-day north-east Turkey. It can also be found in Spain and Portugal, although it was once much more widespread in Europe. There is even pollen evidence dating back to pre-ice age and warmer interglacial times that indicate it was growing in Ireland but was eventually pushed back south of the Alps by the ice, never to return. So, R. ponticum is arguably not so much an alien in the British Isles, as a prodigal son.
Our ‘troublemaker’ arrived earlier than the Hookers period of Rhododendron fuelled fever. Ponticum arrived in 1761 via botanical gardens in the Netherlands. It soon became widely planted on country estates, as a garden ornamental, and was also used as a shelter belt and game cover. However, having escaped the gardens of the rich it became widespread. It favours wetter, acidic conditions, which is not surprising as a member of the Heather family (Ericaceae).
Ponticum is an aggressive competitor; it shades out native species and in time can even replace woodlands, because tree seedlings cannot establish among the throng of stems. Rhododendrons, like other ericaceous species, have specialised fungi (ericoid mycorrhiza), which allows improved nutrient extraction compared to other plants. This is why members of the Heather family are also good at growing on poor soils, and out-competing their neighbours, by depriving them of essen/al nutrients.
Rhododendrons do not invade quickly, but their spread is relentless. Each large shrub produces millions of extremely light seeds, distributed by wind. It can also spread vegetatively by process of suckering from its roots and by layering. When a branch touches the soil, new roots develop, and another plant is born.
Finally, yes, it is poisonous. The entire plant contains the neurotoxin, Grayanotoxin. This prevents grazing herbivores from chomping away at its leaves. Although animals generally leave it well alone, poisoning of sheep and deer can happen when they are starving, as in deep snow.
The nectar also contains the same neurotoxin, which can kill the British honeybee, but not the Turkish sub-species of bee. These bees produce consequently ‘Mad Honey’. This was successfully used by the King of Pontus in 65BCE as the first recorded use of a chemical weapon. Left on the roadside for the invading army from Pompei to eat, this resulted in their unwitting poisoning, stupor, and subsequent slaughter.
Fortunately for Rhododendrons, the toxin is tolerated by Bumblebees, who are the preferred pollinator, and unknowingly are encouraging the spread of this invasive species. Rhododendrons are one of the many invasive alien plants in the UK. Other well-known invaders include Himalayan Balsam, Giant Hogweed, Few-flowered garlic, New Zealand Pigmyweed, and Japanese Knotweed to name just a few.
Given the right condiitons, alien plants can destroy an entire ecosystem by upsetting the fragile natural balance that exists between plants, animals, and fungal inhabitants. Remember… “Be careful what you bring back from your travels! Unlike our botanical and horticultural ancestors.”