I read with interest the post about the invasive Lantana in Australia. It made me think of Rhodendron ponticum and wonder whether this too is a case of a plant finding its perfect habitat and going bananas. As many will know, R. ponticum is in several areas of the UK considered a pest, covering whole hillsides in Snowdonia and other parts of the West Country. So I looked more into it.
What I found is that ponticum certainly has what it takes to succeed: ericaceous mycorrhizas that grow well on poor acid soils; plentiful, wind-dispersed seeds; toxic leaves; the fact it re-sprouts readily if cut back; and from underground buds if burnt back by fire. But then this is a fairly good description of Rhododendrons in general! Why is it that of the 800 or so species only one other, Rhododendron luteum, has naturalised in the UK? And why is luteum not a problem too?
The answer is in the history of ponticum's introduction to the UK. Ponticum's abundant seed and ease of propagation were discovered at a time in Victorian Britain when all the fad, led by William Robinson, was for wild and woodland gardens. At the same time, shooting game was growing hugely in popularity. On country estates, gamekeepers took over from foresters, and, as the fashionable gardener, J. C. Loudon wrote: "In Britain, it is planted as an ornamental shrub, not only in open situations but, on a large scale, to serve as undergrowth, and as a shelter for game".
An article in the Gardener's Chronicle of 1841 commented: "It is very easy to fill woods with them, by sowing the seeds broad-cast ... A man and boy can collect enough [seeds] to sow acres in hours." Thus, cheap and abundant, ponticum also become the rootstock species of choice for grafting hundreds of thousands of plants.
A weakness however, or strength (depending how you see it), was that ponticum tended to sucker and easily overcame the scions of these grafted plants. So you have the perfect storm for the ponticum invasion, created by us.
As Katharina Dehnen-Schmutz and Mark Williamson have put it: 'Ponticum was selected and hybridised for hardiness. Its spread and increase was from propagation by nurseries and estates. It was distributed over distances far greater than its seed could have travelled naturally. It was brought directly to habitats offering the most suitable conditions for its survival. Without all this the plant might perhaps still exist in the British Isles today just as specimens in botanical and horticultural collections like thousands of other introduced plants."
Ironically, in its native areas around the Black Sea and Spain, ponticum is now a rare and endangered species.
[Photos: rampant purple-flowered R. ponticum on a Welsh mountain side; and yellow-flowered R. Luteum, strategically planted for the scent it gives on a path at Stourhead]