Plant of the Week – 23rd May 2022- Rhododendron ponticum

There can be few wild plants in Britain that engender such a wide range of emotions as “The Rhododendron”, Rhododendron ponticum.  To some it is the enemy, to others a beloved playmate or beautiful adornment of the landscape.  It feeds some creatures but starves or occasionally poisons others.  And it is a plant with a fascinating story to tell.

Rhododendron ponticum L. Photo: Richard Milne.

Let us start by clearing up a couple of misconceptions.  People often say of Rhododendrons that they’re invasive, and come from the Himalayas, but both are only partly true.  The only invasive species (at least in Britain) is R. ponticum itself, whereas the other ~1000 species behave themselves more properly if grown in gardens.  Furthermore, whereas well over half of Rhododendron species hail from the Himalayas and western China, in fact the genus is scattered around the northern hemisphere, and also occurs throughout the islands of southeast Asia, even reaching Australia.  Furthermore, the genus as a whole does not originate from the Himalaya, despite the concentration of Rhododendrons there, and the convictions or great plant-hunters like George Forrest.  We know this partly because of DNA-based family trees for the genus which show that the Himalayan species result from a fairly recent burst of explosive speciation, but also because the oldest Rhododendron fossils are 60 million years old, making the genus older than the Himalayas!

Sheep learn to avoid R. ponticum if they encounter it regularly, as in Scotland.  However they may eat it if starving or unfamiliar with it, for example if cut branches are thrown into a field.  If so, they suffer paralysis that will eventually be fatal, unless given the antidote, which is a large cup of tea! Photo: Richard Milne.

Rhododendron ponticum, as its name suggests, does not come from the Himalayas.  It occurs along the south and east coasts of the Black Sea, including the Pontus region of antiquity, forming large populations wherever the rainfall is high enough.  Then there are four disjunct patches of distribution well away from there, one in Lebanon, one in southern Spain and two in Portugal.  Fossil records show its presence around northern Italy during one of the interglacial epochs, indicating that once these disparate patches were connected.  More remarkably, it was also present in Ireland around 400 000 years ago, again during an interglacial period.  At that time, it would have formed a part of the Lusitanian flora, which exists because of the mild climate there, and includes so many other species with Mediterranean connections, including four from Ericaceae (Arbutus unedo, Daboecia cantabrica, Erica erigena and E. mackiana).  So, R. ponticum is arguably not so much an alien in the British Isles, as a prodigal son.

A huge mass of Rhododendrons loom over the road by the Royal estate of Sandringham.  The News of the World once claimed that the plants represented a lethal threat to the Queen Mum! Photo: Richard Milne.

When the most recent glacial epoch receded, R. ponticum was not among the Lusitanian elements that successfully migrated back to Ireland up the west coast of Europe. Maybe it moved too slowly to reach the Channel before it flooded – while the spread of the modern invading populations is often inexorable, it is seldom quick.  In contrast to Senecio squalidus which raced across Britain from a single introduction point, populations of R. ponticum probably always hail from relatively local plantings.  That is why, driving around western Scotland, one becomes aware of it having a very patchy distribution: abundant for some stretches of roadside and then absent for long stretches afterwards. Another possibility is that the climate at the time was not wet enough, preventing it from spreading upwards through western Europe.  Certainly in Spain and Portugal now, it is hanging on grimly in pockets where it is just about moist enough. 

Unable to do so unaided, R. ponticum was therefore returned to the British Isles by humans – specifically an enterprising Dutchman called Conrad Loddiges, in 1763.  At first it was sold at a premium price, but the cost of a plant soon plummeted as it began to reproduce.  A mature plant can produce a quarter of a million seeds per year, and those estates where adult plants of it were growing soon had seedlings for sale by the cartload.  By the middle of the 19^th century, it was being planted en masse as cover for game birds to aid in the shooting boom at the time.  One quote lifted from magazines at the time tells us that opinions differed about how effective R. ponticum was as game cover, not to mention revealing just how much indiscriminate slaughter the upper classes were engaging in at the time:

“the mere fact of his lordship having killed 1367 pheasants, 500 hares, besides rabbits, in one day, in covers abounding in Rhododendrons, is evidence that Rhododendrons are not disliked by pheasants and hares”

Craw, W. 1864. ʻRhododendrons as cover for gameʼ. The Gardenersʼ Chronicle and Agricultural Gazette: 54

So, at least in some places, R. ponticum did not exactly escape into the wild, rather it was put there and left to enjoy itself.  However, around the same time, it was beginning to seed itself outside of gardens and estates, especially in the wetter, more acidic habitats of western Britain.  Gradually, therefore, it achieved the broad distribution across the British Isles that it now enjoys, becoming a familiar part of our landscape, even if not always a welcome one. 

A large number of young plants of R. ponticum colonising a lochside in Scotland.  They will have been seeded from mother plants close by, because seedlings are very rarely seen far from a mother plant, perhaps because only when close to other ponticum plants are seedlings avoided by herbivores. Photo: Richard Milne.

Its villainy is well documented – an aggressive competitor, it shades out native species and in time can even replace woodlands, because tree seedlings cannot establish among the throng of Rhododendron stems.  Supposedly it also poisons the soil around it (allelopathy), although this has been called into question.  Although most land plants have mycorrhizae (fungal partners for their roots), species of Ericaceae have a special additional type of mycorrhizal association, employing different fungi to extract nutrients from the soil that other plants struggle to reach.  This is why the family is so good at growing on poor soils, and it may be that the twin effects of shading and nutrient extraction are more significant than allelopathy for the suppression of competitors by Rhododendrons.  What is certain is that given the chance, R. ponticum can form a monoculture in which few other plants can survive.  Remarkably, it can even invade peatlands and arrest the process of peat formation.

The effect on animals is more mixed.  Worm numbers decline significantly in the soil beneath is, and the nectar of ponticums is toxic to honeybees in Britain (but apparently not where it is native), so British honeybees generally avoid the plant.  Bumblebees however love the plant, with its copious flowers supplying a bonanza of food.  This in turn means theoretically fewer visits to other flowers in the vicinity, although one study found the impact on foxgloves to be relatively light.  Moreover, if the resources from the rhododendron allows pollinator numbers to increase, then this will in time ameliorate the effects of competition for other flowers.  Meanwhile wood mice numbers increase under the plant, which might be good news for their predators except that shielding them from bird attention may be part of what caused the increase in the first place.

A “tunnel” formed by ponticum plants in a wood in Richmond park, which formed a natural playground for children.  Not all ponticum populations are a threat, and these ones never spread, probably because any seedlings outside the wood were swiftly dealt with by the park’s resident deer. Photo: Richard Milne.

The most varied impacts of the plant are on human beings.  Banks of the plant in flower are a breath-taking sight, regardless of where they are and what they may be doing to local ecology.  Children absolutely love to climb and play in it.  The spindly, twisting branches make a natural climbing frame, and below this large populations form little tunnels.  It is a natural softplay centre.  I grew up playing among Rhododendrons in one wood in Richmond Park, and was greatly saddened when it was removed from there a few years ago.  Though the plant is poisonous, this represents almost no threat to humans.  I say ‘almost’, because a few years back two hikers became trapped in a massive thicket of the plants in the Knockmealdown mountains of southern Ireland.  They had taken a shortcut and began to encounter a few plants of ponticum, then a few more, until soon they were ploughing through a tight thicket.  Eventually, exhausted and with no end in sight, they called mountain rescue.  There followed a strangely comical rescue in which the stranded hikers had to periodically shake the rhododendrons to reveal their location to the rescue team fighting through the plants to reach them.  Eventually the rescuers brought them to a lakeside and they were taken to safety by boat.  The plant has also been claimed to represent a mortal threat to the Queen Mother, but as this was by the now defunct Sunday Sport, the claim might perhaps have been exaggerated.  It happens that the Queen Mother herself was something of a Rhododendron afficianado, having been involved in plantings at Windsor Great Park.  Had she seen the article, she would probably have laughed her royal head off.

Another potential threat to humans comes from the nectar, which contains grayanotoxins.  While not fatal, ingesting even a tiny amount of these will cause temporary sickness, hallucinations and unconsciousness.  There are two historical tales of troops of soldiers being poisoned by “mad honey” made from Rhododendron nectar in parts of Northeast Turkey where R. ponticum grows.  A company of Greeks survived the experience, but 1000 romans were slaughtered by cunning local warriors who had left jars of raw honey lying around for them to find.  I suspect that the main culprit here might be the yellow azalea R. luteum, which grows naturally in the same area.  Cases of “mad honey disease” are reported in Turkey and elsewhere (from other species) from time to time, but only raw honey from bees that foraged Rhododendron flowers carries this risk.

It does occasionally poison livestock, especially sheep, but by and large they know to avoid it.  Poisonings only happen when the animals are starved (e.g. by deep snow) or come across it having never met it before.  Rhododendron poisoning causes paralysis and eventual death in ungulates, but remarkably it can be cured by giving them large amounts of strong black tea.

Dark red flecks on the corollas indicate genetic influence from other species, most likely R. arboreum.  Hairy ovaries suggest R. catawbiense, and long calyx lobes R. maximum, but plants lacking any of these might still be hybridised, it is impossible to tell without DNA evidence.  Also, further species might be involved locally.  It is hence impractical to try and subdivide the UK material, and the name “R. x superponticum” should not be used as it simply causes confusion. Photo: Richard Milne.

In many places it is being actively attacked.  Rhodie-bashing weekends are a staple outing for physically active conservation groups up and down the country, and large sums have been spent on trying to control it in Snowdonia, where it is rampant.  In Scotland, removal of populations that are invading the rare and precious temperate rainforest habitats are essential for their conservation.  Nearby, in Inverewe Garden, shredded ponticums are being put to use as biochar!  (see https://www.nts.org.uk/stories/inverewe-gardens-biochar-project). Around Torridon, where huge populations used to adorn the roadsides, the plant has been massacred, leaving relatively few individuals clinging on here and there.  Some locals miss the colourful displays they used to provide.

Not content with causing discord and controversy in the countryside, R. ponticum has managed to do the same in the scientific community with the thorny issue of naming.  This stems from the fact that not all our plants are genetically pure R. ponticum.  After being introduced to British gardens, it soon came into contact with two species introduced from America, R. maximum and R. catawbiense, followed after 1812 by R. arboreum, shipped from India as seeds stored among grains of sugar.  Both by design and by accident, it hybridised with all of these species, and probably many others.  Wild British material of R. ponticum contains both molecular (i.e. DNA markers) and morphological (hairy ovaries, long calyx lobes, deep red corolla flecks) of genetic material from other species.  However, a common misconception is that ALL British Isles material of ponticum is hybridised.  This has certainly not been proved and the likely truth is that our plants vary from pure ponticum to perhaps 25% other species, with a lot of them in the middle.  About half our plants show no morphological signature of hybridisation, and among these some will be hybridised, while others won’t, and you’d need to sequence large parts of the DNA genome to be sure.  Moreover, because each population derives from a local introduction, some may have hybridised with related species in the garden where they were planted, while others did not. Put simply, our ponticum material is a complicated mix of pure and hybridised material, and any attempt to subdivide or reclassify it is doomed to failure, though it hasn’t stopped people trying.  There is no better name for it than “Rhododendron ponticum”, with the acknowledgement that some of it has been altered by hybridisation.

The ability of R. ponticum to hybridise may have had consequences for its evolution.  Both R. maximum and R. catawbiense are known for their extreme cold tolerance, and some of this may have passed to the less hardy R. ponticum during hybridisation.  When the UK endured an exceptionally cold winter in 1895, it was noted that ponticum-catawbiense hybrids survived whereas pure ponticums “were an ugly brown, and dead to the ground”.  This raises the possibility that natural selection acted on our plants to favour those containing pieces of catawbiense DNA, equipping a species introduced from Spain for survival on mountain slopes in Scotland.  But this has yet to be proved!

Reference

Milne R (2017) Rhododendron. Reaktion Books, London.

©Richard Milne