As should be apparent to any visitor to the garden at Airdlin Croft, we like Rhododendrons. In particular, we are attracted to the large-leaved species which need shelter from Aberdeenshire’s relentless winds and which may take years to flower. We also grow a number of the generally hardier and more floriferous hybrids. In fact we have equal numbers of species and hybrids, around a hundred of each. Or do we?
The more I get to know genus Rhododendron, the more I wonder about the validity of the species taxon when it comes to naming them. The capacity to hybridise makes identification from morphology alone well nigh impossible. And yet many ‘experts’, who from experience should know better, attach names to them with 100% certainty. Is this just mildly annoying or could it have serious consequences?
Many of our garden visitors express surprise at our fondness for Rhododendrons. For many people in the UK, ‘Rhododendron’ equates with Rhododendron ponticum (sic), that invasive, poisonous plant that has usurped 100,000 hectares of our countryside since its introduction in the middle of the 18th century.
‘But that’s ponticum’, we say; ‘it’s an exception’ - both assertions now seen as being untrue.
Two hundred and fifty years after its introduction, genetic analysis of UK ponticum shows it to be a hybrid, possessing genes of one or more North American species which were intentionally used by plant breeders to increase the hardiness of these non-native introductions (non-native since the last ice age, that is).
Occasionally taxonomists who are unsure about exact identity attach ‘aff.’ to the specific moniker, indicating an affinity with that species while recognising that it may not conform entirely. Is there a case for a more general use of the term?
Well, you may answer, if you want to be sure of obtaining a Rhododendron that matches the description of the species in every detail, then only purchase it from the most reputable nursery.
Really? Let me provide three examples (there are more) from our own collection which detract from that conclusion:
1/. Rhododendron sutchuenense, conforming in every detail with the description of the species, except that the corolla has a central blotch.
2/. Rhododendron calophytum, conforming in every detail with the description of the species, except that the corolla lacks a central blotch.
3/. A batch of seedlings in the polytunnel, grown from seed described as hand-pollinated Rhododendron macabeanum, showing the same degree of variability as an adjacent batch of open-pollinated Rhododendron arboreum, in which such variability would be expected.
And all of this from one of the country’s best-known Rhododendron specialists.
Now of course, these morphological differences from the species type do not necessarily render the plant unworthy of inclusion in the garden. It is just ‘mildly annoying’ that you don’t have the plant you thought you had.
Much more important is the possibility that your plant differs fom the species in ways that are not apparent to the naked eye – differences which can only be discerned by genetic analysis.
One of the fundamental principles I absorbed as a biology student is that changes to the genotype are not always reflected in the phenotype. In other words, genetic changes that arise from hybridisation or mutation are not always visible in the morphology of the plant – it’s visible form - but may have changed it in hidden ways that give the plant a competitive advantage over its neighbours – for example, in genus Rhododendron, a different location of toxins within the plant.
In a certain West Coast Scottish glen, a Rhododendron ‘species’ appears to be behaving rather like UK ponticum. No say the experts, that species isn’t invasive, it isn’t even hardy. Exactly the same defence was made of UK ponticum once upon a time.
The second greatest cause of biodiversity loss to the planet arises from the introduction of invasive, non-native species. How can rhodophiles avoid responsibility for making the same mistake again?
First, let’s be affing you, rhodies. Why not employ a more general use of the aff. suffix which acknowledges that the plant you are buying, or attempting to identify, has affinities with the species but which may differ in some respect that might be important (aff off, I hear the experts cry)?
And, to be on the safe side, avoid planting any Rhododendron in a situation where invasive tendencies cannot be adequately monitored or controlled.
We are gardeners. We care for the planet, don’t we?