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Mishra, A. (2014). An introduction to Invasive Species and its? control with case study of ?Rhododendron- An invasive Species?. PHILICA.COM Article number 414.

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An introduction to Invasive Species and its? control with case study of ?Rhododendron- An invasive Species?

Ashish Kumar Mishraunconfirmed user (Sustainable Resource Management, University of Limerick)

Published in enviro.philica.com

Invasive species harms our ecology and its balance in different ways. Understanding how invasive species are harmful and controlling invasive plant species is highly important in order to protect disturbance in ecosystem. In this paper, I have picked up one of the plant species, ?Rhododendron? which is an invasive species in Ireland, and UK, causing severe damage to the ecosystem and native plant species. This paper discusses brief introduction of these species and methods of controlling them. This particular case study can also be used as a medium to understand method of eradication of invasive studies.

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Invasive species are basically those introduced species which harms or disturbs the habitat adversely after being introduced. The adverse effect could be environmental, or ecological or economical. However, Invasive species can also be termed as widespread non-indigenous species (Colautti and MacIsaac, 2004).

There are few common traits those invasive species posses and through which they can be identified (Kolar and Lodge 2001). Understanding these common traits can help in identification of an invasive species at early stage of their spreading. Few of the common traits are mentioned as follows:

·         Rapid and Fast growth

·         Rapid reproduction and quick spread

·         High dispersal ability, and open to maximum medium for dispersal.

·         Ability to change and reframe growth form to fit appropriately to the given conditions. This ability is also called phenotypic plasticity.

·         Possess Ecological Competence, which in other words means, endurance capacity to endure, and survive in a wide range of environmental conditions.

·         Can survive by feeding on variety of food kinds.

·         Association with humans (Williams and Meffe, 1998)

·         Prior successful invasions (Ewell et. Al., 1999)

However, there are study results that states that many non-invasive species also possess similar traits (Kolar and Lodge 2001). Another important study about invasive species states that an introduced species must survive in a low density population area in order to be invasive (Tilman D., 2004).

Harmful effects:

Invasive species causes a lot of harmful effects to our ecosystem. They do disturb the ecological balance by their abnormal growth, and at the same time brings out other effects. Few of the harmful effects are as follows:

·         Damages Ecosystems: Invasive species have huge impact on our ecosystem. They can alter and change our ecological functions. Invasive plants can change the nutrient cycling, as smooth cordgrass (Spartina alterniflora) does, or like Tamarix, they can change the hydrology of the native ecosystem hydrology (Mack et. al., 2000).

·         Damages Forestry: Invasive species can damage forestry to a greater extent, causing not only loss of native trees and habitat destruction, but can also lead to financial losses. Asian long horned beetle (Anoplophora glabripennis) caused similar damage to American forest by infecting and damaging hardwood trees (Pimentel, Zuniga and Morrison, 2005).

·         Threat to Biodiversity: Invasive species are one of the top 5 reasons of biodiversity loss all over the world. It is one of the biggest and growing threats to existing biodiversity and habitats (Millennium Ecosystem Assessment, 2005).

·         Genetic pollution: Through unintentional, accidental, or intentional hybridization of the invasive species, a genetic pollution occurs which may lead to extinction of native species (Mooney and Cleland, 2001).

·         Damage to agriculture: Invasive species also threatens the agricultural system and yield. Agricultural loss from invasive plants in America in 2005 was I billion dollar (Pimentel, Zuniga and Morrison, 2005).

Understanding these harmful effects, it becomes important for us to think of suitable methods of understanding and identifying invasive species and suitable measures of controlling them. In order to reflect more on these two aspects, case study of Rhododendron can be useful. Rhododendron is an invasive species in Ireland, and UK causing severe damage to ecosystem.

Case study- Rhododendron in Ireland:

Rhododendron was introduced in Ireland in 18th century from Asia and North-West China as an ornamental plant (Parrott and Mackenzie, 2013). However, its vigorous growth forming a laurel kind of shape and restricting other plants to grow nearby made it an invasive species (Barron C., 2009).


Rhododendron is an evergreen shrub. The plant really looks beautiful with slightly purplish flowers. The combination of green and purple looks attractive and that was why Rhododendron was planted in Victorian hunting estates for the beautification of the gardens. It later was found that they are invasive in nature and acquire the place completely. The plant basically succeeds in autumn season. It gives out a cherry like fruit which has the seeds (Barron, C. 2009).

As per GB Non Native Species Secretariat (NNSS) report on Rhododendron, few common features of Rhododendron that can be useful in its identification are “A large evergreen shrub with leathery leaves, attractive purple to pink flowers and solid stems forming into a trunk when mature. Relatively easy to identify, but can be confused with cherry laurel or horticultural varieties of rhododendron. However, horticultural varieties of rhododendron are relatively rarely found in the wild. Spreads by suckers and seed, which are small and carried long distances by wind.

Favourable condition and status of naturalization:

Rhododendron is not native to Ireland, rather was introduced from Asia. Thus, it was supposed to have different favourable climatic conditions and was thus assumed to be growing under control. However, the actual scenario was different. Rhododendron likes peaty, sandy and acidic soils (Parrott and Mackenzie, 2013) which are common in Ireland. Rhododendron is also said to be adaptable to humid and wet type of climate which is a common scenario of western Ireland (Parrott and Mackenzie, 2013).

As far as its status of naturalization is concerned, it was a very popular garden ornamental plant, and thus was planted vigorously. It was highly successful in Ireland with its multiple methods of dispersion and reproduction. Apart from that, Rhododendron is also shade tolerant and can grow in absence of sun (Barron C., 2009). All these features together shows that Rhododendron is very much adaptable and has well naturalised in Ireland.


Rhododendron is an important invasive species with significant impacts on the environment and biodiversity. The humid and wet weather in the western part of Ireland instigates its growth. Rhododendron grows uncontrollably making it difficult for other plants to compete (EDWARDS C., 2006). The leaves of Rhododendron have free phenols which are toxic for other plants. Apart from that, their leaves are very thick which makes light impenetrable through them (Barron C., 2009).

As per a study of 248 rhododendron sites (31,905 ha) reported that the main consequences of rhododendron invasion were displacement of native species (45%), habitat change (45%) and hydrological damage (10%). Other impacts included visual effects, shading of watercourses, impeding access, concealing “muggers”, damage to fencing, loss of grazing and poisoning of livestock. ( Dehnen-Schmutz et al., 2004) 

Their toxic leaves are even avoided by mammals and thus they easily over power other plants nearby (Barron C., 2009). Apart from that, their leaves litter on the ground along with deep shadow created by them makes soil sterile and provides them an advantage over native species (Becker, 1988; Cross, 1975).

Rhododendron is also reported to have high impacts on the number of biodiversity in the region, which is very much obvious, seeing its traits and characteristics. Natural plant species play a very important role in maintenance of biodiversity, and any impact on the natural species will definitely affect the biodiversity in the region (EDWARDS C., 2006).

Important Incidents:

In Ireland, Rhododendron has invaded three of the important habitats. These were internationally significant habitat under European Commission Habitat directives: Upland Oak woods, bogs and Health (Maguire, Kelly, and Cosgrove, 2008).

It has widely spread over the 650 acres of Killarney national park, which is again an important and protected habitat (Maguire, Kelly, and Cosgrove, 2008).

Rhododendron also impacts on the number of birds. Reports suggest that number of birds have decreased widely in Oakland woods which are dominated by Rhododendron (Cross, 1981; Barron, C. 2009).

Other important things to notice:

Rhododendron is also said to be the main cause of the problem of sudden Death of Oaks (Maguire, Kelly, and Cosgrove, 2008). Actually, Rhododendron carries a serious plant pathogen, which is a fungus and attacks woodlands. This fungus not only attacks oaks, but almost any kind of wood. The name of the pathogen is Phytophthora ramorum. They turn the twig brownish black and gradually destroys them (Parrott and Mackenzie, 2013). Reportedly, Rhododendron is one of the major issues that woodland conservation organizations are facing.


Rhododendrons have also caused hazards to human health. Toxic honey from Rhododendron causes human poising. The severity of the case depends on the amount of honey digested and pre-health conditions of the individual (Maguire, Kelly, and Cosgrove, 2008).


The above information was enough to justify why Rhododendron is termed invasive and why measures should be taken to control it. It is affecting environment, biodiversity and health in many ways, and anything that has such a high impact should be eradicated(EDWARDS C., 2006).

Controlling Rhododendron:

Controlling Rhododendron involves 4 (four) phases in the procedure. The first and foremost phase will involve surveys of the area, where the age and the duration of the growth of Rhododendron are to be studied. The second phase will be eradicating them, and the third phase will be monitoring its growth further (EDWARDS C., 2006).


Phase I basically involves the surveying methods before the treatment basically starts. This will cover stage 1 and stage 2 respectively as mentioned above. It will involve various steps:

Step 1. Survey the whole area that is covered by Rhododendron, and understand its impact on nearby biodiversity resources and areas. This will also include understanding whether the site is a protected site or not, because then the controlling measures will be as per the regulations

Step 2. Mapping of the area will be helpful. Do take into account the age, and growth of the Rhododendron and map them. Any previous treatments specifically or not specifically for Rhododendron should also be taken into consideration and should be mapped.

Step 3. Depending on the details of the map, the area of priority will be determined.


Since we have prioritised are, we now need to select the best suitable and effective control and eradicating measure. In this phase we also need, to make a management plan. This management plan should include objectives of eradicating Rhododendron, monitoring its further growth, and creating suitable conditions to retrieve lost species.


This particular phase includes implementation of the selected method of control. There are various types of controlling measures that can be executed in order to eradicate Rhododendron. However, it is very important to select the eradication and control measure by considering few factors.

A Control measure should be adopted if it has three main attributes (Parrott and Mackenzie, 2013). They are:

·         It should be effective in controlling the invasive species

·         It should be cost effective (should not be very expensive)

·         It should not have; or should have minimum; negative or harmful environmental impacts

Physical Control Measures:

Physical methods to eradicate Rhododendron will involve steps like cutting the stem, or digging it out from the root (Barron, C. 2009). However, it will require further management because Rhododendron can grow back even after being cut. Burning (controlled) is another option through which one can get rid of rhododendron (Barron, C. 2009). However, burning is not the best option as it may disturb wildlife, can damage surrounding biodiversity, and can contribute to atmospheric pollution (Parrott and Mackenzie, 2013).

Digging out the soil and the roots of Rhododendron is another effective physical method of controlling and eradicating Rhododendron (Barron, C. 2009). However, digging out will create a lot of disturbance to the soil.


Chemical Measures:

There are Herbicides that can be used to control the growth of Rhododendron (Barron, C. 2009). Few of the effective herbicides are:

1.      Glyphosate:  10 litres of the Glyphosate solution is to be spread every hectare of Rhododendron. The mixture should be 2% by Volume which means every 2 litres of Glyphosate in 100 litres of water. The spraying should be such that it spreads over the leaves but does not run off.
Best time to apply: The best time to apply will be between June to September.

2.      Triclopyr: 8 litres of Triclopyr is enough to kill Rhododendron cultivation. This will need 8% solution by Volume, which means 8 litres of Triclopyr in 100 litres of water. The spraying should again be only to wet the leaves.
Best time to apply: the best time to apply Triclopyr will be between June and September. However, it will be effective if Triclopyr is applied after cutting and before any new growth appears.

3.      Ammonium Sulphamate (AMS): Ammonium Sulphamate or AMS needs to spray on the leaves and stem of Rhododendron. The herbicide will be 40% by Volume, which means 40 litres of AMS in 100 litres of Water. AMS is really a harsh herbicide and special care should be taken while spraying this on Rhododendron cultivation. Run off of this herbicide should be avoided and thus should be applied when rain is not imminent.

Best Time to Apply: AMS can be applied anytime in between April to September. However special optimization is required in between July to September, whether any new growth is taking place or not. If any new growth is visible, spray AMS again.

Stem Injections are also a new technology that is being used and is considered to be one of the most effective methods of controlling Rhododendron with regards to environmental impacts.


The last phase will include resurveying of the affected site to investigate if any further growth is visible. At the same time, restoration of the native species and maintaining suitable conditions for native species should also be processed in this phase for better results (Parrott and Mackenzie, 2013). The follow up treatment should be provided as per the management plan to prevent further re-growth (Edwards C., 2006)

Physical or chemical methods might be capable of eradicating Rhododendron, but in order to avoid its further growth, the best method will be combination of physical, chemical and managing methods (Barron, C. 2009). Example of best practice should be like application of herbicide, cutting it off, spraying again, and management that no further growth favourable environment stands up (Barron, C. 2009).


Taking this case study into account, effective and appropriate methods to eradicate and control invasive plant species can be figured. It is important to understand that eradication of the invasive species should not be done at the cost of a substantially big damage to environment. The invasive species are termed adverse or harmful because they damage the ecosystem’s balance and the native species, but with our method of controlling these invasive species, if we too harm the environment and ecosystem, then the overall achieved result would be positive by a thin margin. The above case study can also be used as a template to carry out an eradication process towards a particular invasive plant species.


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Peer-review ratings (from 1 review, where a score of 100 represents the ‘average’ level):
Originality = 175.00, importance = 150.00, overall quality = 175.00
This Article was published on 30th July, 2014 at 04:01:17 and has been viewed 3686 times.

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The full citation for this Article is:
Mishra, A. (2014). An introduction to Invasive Species and its? control with case study of ?Rhododendron- An invasive Species?. PHILICA.COM Article number 414.

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1 Peer review [reviewer #51143unconfirmed user] added 15th August, 2014 at 10:37:41

Nice and informative Work. Important case study which can be applied practically.
Please Remove ‘?’ used in the title.

Also, talk about the interaction of Rhododendron with other invasive plant species.

Originality: 7, Importance: 6, Overall quality: 7

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