||The plant is responsible for the destruction of many
native habitats and the abandonment of land throughout the British Isles. The reason for
this is simple. Where conditions are suitable, Rhododendron will out compete most native
plants. It will grow to many times the height of a person, allowing very little light to
penetrate through its thick leaf canopy. This effectively eliminates other competing
native plant species which are unable to grow due to insufficient light. This in turn
leads to the consequent loss of the associated native animals.
Introduction to Britain
Rhododendron ponticum is native to countries in the western
and eastern Mediterranean such as Spain, Portugal and Turkey and also occurs eastwards
through Asia into China. It is not native to Britain, but was first introduced in the late
18th Century. It became especially popular on country estates in Victorian times,
providing ornamental value, as well as cover for game birds.
The origins of British Rhododendron are uncertain. There is some
suggestion that it came from Asia. However, recent
research indicates that it probably originates from Spanish and Portuguese
Rhododendron thrives in milder, wet climatic conditions, where there
are poor, acidic soils. Unless established stands are constantly kept in check, they will
expand into adjacent areas, rapidly eliminating the majority of native plant species. The
twentieth century demise of many former country estates and the consequent lack of control
of remaining Rhododendron has allowed this species to invade large areas of the British
Rhododendron invades areas both vegetatively and via seed.
Established plants spread by lateral horizontal growth of the branches. A single plant may
eventually end up covering many metres of ground with thickly interlaced, impenetrable
branches. Where the horizontal branches touch the ground, they will root, continually
extending the area of Rhododendron cover.
||It is worth
noting that because of its extremely lateral growth form, Rhododendron plants are capable
of extending well into areas which otherwise would not be suitable for their growth. For
example Rhododendron is capable of dominating large areas of wetland with its canopy,
while the main stem and roots of the plant are well back on suitably dry land.
For the same reason streams can become completely
overgrown and shaded out by Rhododendron growing on the banks. This severely affects
animal life in the stream. Fish such as trout depend upon invertebrates which fall off
native bankside vegetation for 80% of their food.
Rhododendron seeds are tiny and hence wind dispersed. Each
flower head can produce between three and seven thousand seeds, so that a large bush can
produce several million seeds per year. Although not all the seeds will grow
successfully, but given the right conditions, a good many will germinate.
Seedlings have difficulty becoming established in areas where there
is already continuous ground cover from native plants. Establishment is best in disturbed
areas where the native vegetation has been in some way disrupted, providing an opening in
the plant cover. Specific mycorrhizal associations
with the roots of Rhododendron plants provide great competitive advantage and allow the
plants to flourish in nutrient-poor soils.
Toxicity of Rhododendron
Potentially toxic chemicals, particularly 'free' phenols, and
diterpenes, occur in significant quantities in the tissues of plants of Rhododendron
species. Diterpenes, known as grayanotoxins, occur in the leaves, flowers and nectar of
Rhododendrons. These differ from species to species. Not all species produce them,
although Rhododendron ponticum does.
These toxins make Rhododendron unpalatable to most herbivores.
Phenols are most concentrated in the young tissues, such as young emergent leaves and
buds. This provides a primary defense against herbivores, before the tissues have acquired
the added deterrent of physical toughness found in older tissues. Young emergent leaf buds
have the additional protection of a sticky exudate which also contains phenols. This
physically discourages small invertebrates from eating the buds, because they get stuck in
the exudate. Its poisonous nature must act as a further discouragement.
Grazing animals are discouraged from eating Rhododendron foliage
because of its toughness and unpalatability. The unpalatability is learned and cases of poisoning may result in
animals such as sheep and cattle if they ingest sufficient quantities because of extreme
hunger or inexperience. The general toxicity of Rhododendron to herbivores means that it
cannot generally be controlled by grazing.
Cases of human poisoning are also known. Most are caused by the
consumption of honey produced from Rhododendron flowers. This is known as 'Mad Honey Disease', or 'Honey
Intoxication'. Cases of this have been recorded from as far back as 400 BC. It results in
relatively short-lived intestinal and cardiac problems and is rarely fatal. The severity
of symptoms depends on the amount of contaminated honey consumed. It is worth thinking
carefully about the siting of bee hives if Rhododendron is a prominent feature of the
Effects of Rhododendron
There is some evidence for allelopathic interactions (the
production of adverse effects on other species) between Rhododendron and other plants.
This may include the inhibition of germination, or of establishment of the seedlings of
competing species. Direct poisoning is a possibility. As noted above, the
tissues of Rhododendron contain significant quantities of phenols and other potentially
toxic chemicals. There is also evidence for the prevention of mycorrhizal development
in roots of the seedlings of competing plant species. Research and debate in this field is
Once Rhododendron has invaded an area few native plants survive. In
woodlands only those trees which manage to grow above the level of the Rhododendron canopy
will persist. These of course have a finite life span and on their death, there is no
replacement because seedlings cannot become established under the lightless canopy. At
this point, the Rhododendron completely dominates the area. This is particularly
noticeable in Silver Birch woodlands as this species is short lived.
Once the native plants have disappeared, the animals which rely upon
them either directly or indirectly for food cannot survive. Thus Rhododendron areas are
essentially barren. Even where trees exist above the Rhododedron canopy, species such as
woodland butterflies disappear. This is because the caterpillars of most woodland
butterflies can only feed on the wildflowers and grasses which are found in the glades and
rides of well managed woodland.
||The Dormouse, one of
Britain's most endangered species of mammal, has been severely affected by habitat loss.
Some of this has been because Rhododendron has out-competed native plants and shrubs which
are essential for the dormouse to survive.
The toxicity of Rhododendron and its alien status means that there
are few animal species associated with it in Britain. However, a number of herbivorous
invertebrate species have been linked to it.
Most of these are not restricted or specific to Rhododendron.
It is important to bear in mind that just because a species has been found on
Rhododendron, it does not mean that it habitually lives there. It may have fallen or been
blown into the Rhododendron from neighbouring native plant species. The canopies of trees
can contain vast numbers of individuals of different invertebrate species. It is common
for these to fall or be blown off to locations other than their chosen habitat. Indeed the
air can contain an astonishing variety and number of small organisms, including
invertebrates. Spiders are even able to balloon hundreds of miles in air currents. Biomass (the total weight of individuals) of the species is also
important. One individual of a species does not imply an association, whereas a plant
covered in a great many individuals would tend to indicate some sort of relationship.
Occasionally a few Rhododendron leaves can be found which have
obviously been eaten by an insect. However the toxicity of the leaves means that this may
well have resulted in the subsequent death of the individual(s) concerned. Rhododendron
growth is in no way controlled by herbivores feeding on it in great numbers. Even if the
odd, isolated herbivore can occasionally be found on Rhododendron, there is certainly
little in the way of biomass to feed carnivores such as other invertebrates and birds.
There is also the added likelihood that any herbivore feeding on Rhododendron would also
contain toxins derived from the foliage.
The flowers of Rhododendron are very attractive to insects,
particularly Bumble Bees. In the main flowering period of May/June, the exotic showy
blooms monopolize the attentions of pollinating insects, virtually to the exclusion of all
others. This means that the flowers of native plant species in the vicinity suffer from a
lack of pollinating insects. As a result they may not successfully set seed. This is yet
another way that Rhododendron may be detrimental to competing native vegetation.
All of this means that areas dominated by Rhododendron have an
exceedingly impoverished fauna in comparison to native habitats, both in terms of species
and of biomass. If there is little eating the Rhododendron, then it follows that there are
few or no carnivores eating these herbivores and so also, few top carnivores. Song birds
which feed on either seeds or invertebrates are reduced to trying to survive in smaller
numbers by feeding in areas above or adjacent to the Rhododendron. Once the song bird
populations decline so do species such as sparrowhawks which predate upon them.
Many broadleaf woodlands are capable of supporting a complex and
interlinked rich diversity of plant and animal life. This is because there can be as many
as four distinct layers of vegetation. These are:
- The climax vegetation which consists of mature trees such as Oak or
- An understorey of shorter, shade tolerant woody species such as Hazel
- A field layer which is typically of grasses and wild flowers.
- A ground layer of mosses and lichens.
Crucially, each layer supports a whole range of invertebrates which
in turn support the larger and larger animals which make up the food chain. To put the
value of native plants in perspective, a single species such as Willow, can support over
400 different species of insect and mite. There may well be hundreds or even thousands of
individuals of each of the species present representing a large biomass.
However it is not just woodlands which have suffered. Heathlands,
which have declined by 75% in just the latter half of this century, have also been
seriously affected. Once again, Rhododendron dominates many such areas and as a
consequence the heathland plants and animals originally present have disappeared. It is
hardly surprising therefore to find that habitat destruction, including that caused by
Rhododendron, is responsible for the disappearance of 150 species from Britain in 100
Restoring areas which have been colonized by Rhododendron is not
just a matter of cutting the vegetation. The plant is notoriously difficult and expensive
to actually kill. The leaves are waxy and herbicide treatment must include a chemical
additive to help break this surface down. Even then, where Rhododendron is well
established with a large root system, such herbicide treatment usually has to be done over
several years. This is because herbicides do not translocate well through the plant.
Techniques such as mist spraying or tractor application are usually
out of the question in woodlands because of the risk to trees.
There are a variety of techniques for the mechanical removal of
Rhododendron. These usually employ a tractor or tracked swing shovel with a rotary flail
mounted on a moving hydraulic arm. In sensitive conservation areas such techniques may
well not be appropriate. Such mechanical devices often leave a thick layer of smashed
Rhododendron on the ground which may have to be removed using expensive manual labour.
Once Rhododendron has been removed and eradicated the toxic humus
layer still remains. Where this has built up over a number of years the resulting thick
mat prevents natural regeneration. The only solution is to remove the layer. This often
cannot be done by mechanical means and thus may require several hand operations to effect
conditions which favour regeneration. However, even then, reinfestation can easily occur
from the millions of seeds which will have been produced over the years. Thus areas where
Rhododendron has been present require careful monitoring over a number of years. Prompt
action must be taken at a relatively early stage to prevent re-establishment.
Further problems exist because any serious infestation is likely to
affect neighbouring areas which may not be in the same ownership. Unless these areas are
cleared the seeds as well as the limbs of the plant will encroach. Given time, a single
plant can cover 100 metres² and grow to more than 10 metres high. This should be taken
into account when making out a management plan. Each site is different but where there has
been a significant presence of this plant and where it is present in adjacent areas, it is
prudent to budget for a reinfestation figure of 15% per year after initial clearance
In summary: Rhododendron
is an introduced species. It is highly invasive. It destroys habitats and thus whole
colonies of native plants and animals disappear. Because it is so expensive to control and
physically prevents access, land has been abandoned. However such areas can be restored
but reinfestation must be prevented.