The rhododendron is a plant hated in the UK by almost anyone, who deals with nature conservation, and for a good reason. An aggressive invader, it damages habitats, poisons livestock and wild game, eradicates local flora and leave entire swathes of countryside barren.
The devastation brought about by Rhododendron ponticum has been touched upon in numerous studies, articles and news pieces, and there are few people left who do not recognize the need for action against this invasive species.
Rhododendron ponticum is a dangerous invasive plant for many reasons. It can grow very tall, up to 8m in some instances. It creates very thick canopy, taking away virtually all light away from any plants that may grow underneath it. It has very strong lateral growth, allowing it to spread from one spot onto a large area, which otherwise might be unsuitable- steep hills, river banks, patches of waterlogged soil etc. The plant itself is toxic, with high content of phenols and diterpens, which means that animals and insects will not graze on it, as it will lead to poisoning, especially if young leaves are consumed, as they have the highest concentration of these toxins. Furthermore, older leaves are tough and unpalatable, discouraging any herbivores even further.
The Rhododendron also has small seeds, which it produces in vast quantities, aiding its swift colonization of new areas- seeds are dispersed by wind, and as such can travel long distances from the parent plant, making it very difficult to control the spread of the species. It can also grow vegetatively, branches taking root whenever they touch ground, making for a notoriously difficult weed to kill.
Once the Rhododendron has invaded an area very few plants can survive- in most cases, only established trees, taller than the Rhododendron itself. But even these have finite lifespans, and eventually they die out, leaving only Rhododendron behind. With no food available, herbivores will disappear, and with them any predators that might prey on them. Thus an area taken over by Rhododendron ponticum will become essentially barren, leaving no room for any biodiversity.
The removal of Rhododendroon is a lengthy and costly process, involving physical cutting, root removal, seedling destruction and herbicide use. The plant is hardy and will grow back from a single stem, or a root, and the seedlings are difficult to spot, until they have grown larger. Not to mention, the size and amount of seeds produced by adult plants makes it almost impossible to stop the spread once it has started. Many resources are devoted to the control and regulation of Rhododendron, from simply keeping it in check (mostly in areas where it was planted on purpose, like parks and gardens, or areas where it is valued for its representative look), to actively eradicating it, in areas where nature conservation and biodiversity take precedence (nature reserves, protected areas, SSSI’s etc.). That’s the situation in the United Kingdom.
Allow me to introduce you to a different world now: a world, where the Rhododendron is not a threat. Where it is not only tolerated, but taken care of and encouraged. Where it does not become invasive at all, and any notion of it destroying natural habitats raises eyebrows and is met with disbelief.
Where I come from, in Poland- that is exactly the case. Just like it was for the Victorians, for the people of Poland, the Rhododendron is just another flower plant, greatly cherished for its beautiful form and flowers, impressive size and huge variety. All sorts of varieties and species are grown and cultivated, including the infamous Rhododendron ponticum, to no ill effect. Entire parks and gardens are devoted to showing off a multitude of Rhododendron varieties, and there is no effect on biodiversity, wildlife or in fact very little effect at all.
Why is this the case? This question has been on my mind ever since I have been introduced to the problem at University, and I have decided to investigate a little, in order to find out what exactly has made the species invasive in Britain, but not so in Poland- or any other country in Continental Europe for that matter. Now, I have already spoken about the dangers of invasive Rhododendrons earlier on- the toxicity, the unpalatable tissue, the lateral growth and great hight, the thick canopy and multiple ways of spreading over an area. But the Rhododendron found in Poland has the same traits, and yet it has not created a problem. In order to find out why, we must look closer at the plant itself.
Firstly- in Poland, a wide variety of Rhododendron is found, including the ponticum variety, but in much lesser proportions, than those found in England. This is a rather minor factor, but it is still important to remember, that the one species which did became invasive in th UK, is not planted that often in Poland, and therefore has less opportunity to spread.
Climate is obviously an important factor as well- we must remember, that Rhododendrons naturally occur in wet or humid conditions, in upland or mountainous regions and thrive on acidic soil. It is, in my opinion these climate traits, combined with temperate climate found in the UK that help the spread and growth of the Rhododendron. humid air, friendly temperature range and an abundance of uplands and mountains make for the kind of environment, where the Rhododendron is found naturally. Add to that the fact, that many areas colonized by the Rhododendron (like heathland) have an acidic soil Ph, making for an ideal place to live in. In contrast, Poland has a much more continental climate more extreme temperatures, much less rainfall and overall air humidity. Acidic soils are also much less abundant than in the UK, and a good proportion of soils are sandy and free-draining, meaning that there is little moisture retained in the soil. Other types of soil are rich and fertile loess soils and black soils, which support a wide variety of flora species, making for a bigger competition, especially for young seedlings.
In Poland, as oppose to England, there is a domestic species of Rhododendron as well-very rare and found in isolated sites only, but there is a good chance it was more common in the past. This would lead to the development of herbivores, especially the invertebrates, which could feed in the Rhododendron- and it would not be a big step for them to start feeding on different species and varieties as well. Same goes for certain diseases and fungi, which would evolve to attack the Rhododendron. There has been little research in this are however, and nothing can be said with certainty, however probable it may seem.
What is certain, is the Rhododendron has no natural enemies on the British Isles, giving it free rein anywhere it can colonize. No livestock, wild animal or invertebrates feed on the Rhododendron in Britain, making human intervention the only means of control. It is a well-known fact, that a species without natural enemies and living in favourable conditions will soon be out of control, and that seems to be precisely the case with Britain and the Rhododendron invasion.
Are the reasons I have qouted above enough to answer the question why did the Rhododendron become invasive in Britain and not in Poland? They answer the question in basic terms, but there are far more factors to be considered. Let us not forget, that Britain is an island, and as such much more susceptible for invasion by foreign species to start with. Add the climatic conditions, the soil, the lack of natural enemies, and the passion with which the Rhododendron was planted all over Britain by gardening and park enthusiasts. The is an answer in there, a situation, wherein the sum of all the factors gave rise to an invasive species- whereas in Poland one, or more, of the ingredients was missing. Unfortunately, all that is left to do, is to cut down the plants on sight, destroy seedlings and roots and keep sites populated by other plants, to encourage diversity and competition. It is difficult and expensive battle, but ont that must be fought, if the British countryside is to retain its beauty and diversity.
R.Milne and R.Abbott- Origin and evolution of invasive naturalised material of Rhododendron ponticum L. in the British Isles- Molecular Ecology, vol. 9 issue 5, 2000
http://www.countrysideinfo.co.uk/rhododen.htm- Rhododendron: A killer of the countryside
Colin Edwards- Managing and controlling invasive rhododendron- Forestry Commission Practice Guide, Edinburgh, 2006
http://www.eryri-npa.gov.uk/the-environment/invasive-species/rhododendron- Snowdonia National Park- Rhododendron
Maguire, C.M., Kelly, J. and Cosgrove, P.J. (2008). Best Practice Management
Guidelines Rhododendron Rhododendron ponticum and Cherry Laurel Prunus laurocerasus. Prepared for
NIEA and NPWS as part of Invasive Species Ireland.