THAT RHODODENDRON WEED
Rhododendrons are, along with Primulas, Dodecatheons, Mecanopsis and Lilies, one of my favourite genera in the garden. As I write in May, I have some 20 or more different Rhododendron species in flower in my garden. They vary from small growing species, such as R. keleticum, R.pemakoense, and R. keiskei, mostly no larger than a dinner plate in diameter to R. williamsianum, R. yakushimanum and among the biggest R. yunnanense and R. oreotrephes at around 2 metres in height. None of them are the 6 or 7 metres high giants more commonly associated in popular imagination with the genus; all are appropriate in scale to an average domestic garden.
Rhododendrons belong to the Order Ericales and the Family Ericaceae. Precisely where the Ericales fit into the great order of things depends on which of the eight different systems of classifying plants you wish to adopt. Taxonomic botanists have great fun amongst themselves debating the finer points of these different systems of classification and are not much nearer coming to any agreement now than they were when I studied Botany at university over fifty years ago. To add further to the confusion, the Royal Botanic Gardens at Kew go their own way - to quote from the book 'Vascular Plant Families and Genera' compiled by R.K. Brummitt and others and published by the Royal Botanic Gardens, Kew in 1992 - 'Kew publications generally adopt the families used in the arrangement of the collections in the Kew Herbarium, which have never been defined in any publication and which may change with time anyway.'
it to say that the Order Ericales includes some seven
Families, including the Ericaceae. The family Ericaceae
in turn comprises some 116 genera according to
classification of the genus Rhododendron has again not been clarified by the
fact that the Rhododendron specialists at the Royal Botanic Gardens at
situation is a little better once you get down to the level of individual
genera. The Genus Rhododendron
comprises about 600+ species. I say
about because new species are being found frequently, particularly in the
mountains of the various islands of the
spread west has been less generous in giving rise to new species. Five species are found in the Caucasus
and four have penetrated into Central and
The Botanical Society of the British Isles book 'Alien Plants of the British Isles' by E.J.Clement and M.C. Foster (1994) reports six species of Rhododendron, which have escaped from gardens in the British Isles and established themselves in the wild, but only R.ponticum has naturalised well and competes with our native vegetation so aggressively as to be considered a serious weed. The other species are:-
R. arboreum introduced
for game cover in
R. luteum (The
Yellow Azalea) found as an established garden escape in acid woodlands
particularly around Burnham Beeches in Buckinghamshire. This is the most widespread of the minor
species and is now found scattered throughout 91 out of the 2823 10 km2
R. minus introduced for game cover and now self sown in
R. sutchuense reported
as a garden escape in woods at Whorlton in
In contrast R. ponticum is found in 1787 out of the 2823 10 km2 in Britain and 461 of the 1007 10 km2 squares in Ireland according to the New Atlas of the British and Irish Flora published in 2002.
has Rhododendron ponticum
been so successful in establishing itself as a flourishing weed in the
British countryside? We are all
aware that in Rivington this species can compete very well and oust native
species, creating a near 100% thicket.
All this since its first recorded introduction from
Rhododendrons form copses, which give shelter to birds such as Pheasants - a major reason for the extensive plantings in Victorian times was to improve the shooting for the local landlords. Apart from his shelter function they do not provide a good habitat for birds or mammals.
The problem remains - what can we do with this invasive weed? It is not easy to control with herbicides to which it shows a degree of physical and chemical resistance. For starters the leaves are not easy to wet with a herbicide spray which readily runs off and little is retained. Even if you can get some herbicide to stick to the surface, absorption through the leaf surface is poor. The most effective herbicides, which I helped to develop nearly 50 years ago when working as a field Botanist for Pest Control of Cambridge, are now banned as potentially carcinogenic.
'Rhodi bashing' - a mass onslaught with axes and saws - gives the impression of doing a good job but in fact is not very helpful if roots and stumps are left in the ground from which Rhododendrons can regenerate. Moreover, if the ground is suitable for Rhododendron growth, which it will be if it has supported a healthy growth in the past, the regeneration occurs quickly from seed unless measures are taken to make the land unsuitable. Liming to raise the soil pH to near neutral or alkaline levels is one possibility and on a small scale, such as a garden, is feasible but on a large scale cost implications soon arise.
It would perhaps be advisable to concentrate on preventing the colonisation of new habitats - attacking young seedlings when they are first seen during the initial colonisation of native woodland or scrub vegetation, when uprooting them is easier than attacking mature established thickets, which is going to demand substantial inputs of labour with no guarantee of success. If mature Rhododendron thickets are to be eliminated, it demands a long term commitment of repeated onslaughts to keep Rhododendron from re-establishing itself from seed and from regeneration from stumps in the ground.