Rhododendron ponticum (rhododendron)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Social Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Rhododendron ponticum L.
Preferred Common Name
Other Scientific Names
- Rhododendron lancifolium Moench
- Rhododendron speciosum Salisb.
International Common Names
- English: pontic rhododendron; purple-flowered rhododendron
- French: rhododendron pontique
- Portuguese: adelfeira
Local Common Names
- Germany: Pontische alpenrose
- Italy: rhododendro pontico
- Netherlands: pontische rododendron
- RHOPO (Rhododendron ponticum)
Summary of InvasivenessTop of page
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Ericales
- Family: Ericaceae
- Genus: Rhododendron
- Species: Rhododendron ponticum
Notes on Taxonomy and NomenclatureTop of page
DescriptionTop of page
Plant TypeTop of page
DistributionTop of page
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 30 Jun 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Planted||Reference||Notes|
|Syria||Absent, Unconfirmed presence record(s)|
|Poland||Present||Introduced||Planted||Original citation: Czekalski, 1996|
|-Southern Russia||Present, Localized||Native||Original citation: Czekalski, 1996|
|Slovakia||Present, Localized||Introduced||Planted||Original citation: Czekalski, 1996|
|United Kingdom||Present, Widespread||Introduced||1763||Invasive||Planted|
History of Introduction and SpreadTop of page
Risk of IntroductionTop of page
HabitatTop of page
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Present, no further details|
|Terrestrial||Managed||Disturbed areas||Present, no further details|
|Terrestrial||Managed||Rail / roadsides||Present, no further details|
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details|
|Terrestrial||Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural grasslands||Present, no further details|
|Littoral||Coastal areas||Present, no further details|
Hosts/Species AffectedTop of page
Host Plants and Other Plants AffectedTop of page
Growth StagesTop of page
Biology and EcologyTop of page
The chromosome number of R. ponticum is 2n=26.
Physiology and Phenology
The growth rate of seedlings is slow and can take up to two years for a seedling to become a juvenile bearing adult sized leaves. The growth of R. ponticum can occur throughout the growing season with defined periods of growth occurring between May and June, July-August and a third period of growth in September. The initial shoot extension takes place in early May, though under favorable conditions this may be bought forward to as early as February. In Mediterranean populations, the plant flowers over two months in late spring, though flowers can still be seen during the autumn-winter period (Mejias, et al., 2002). Flowers appear from late spring to late summer in the introduced range, though the influence of increased latitude and altitude will delay this process. The plant can extend both laterally and vertically, though the main growth form is the former as the stems lacks the strength to support large vertically growth unless supported by other vegetation or man-made structures. The root system consists of an extensive root mat laid down in the top 2-15 cm of the soil. This characteristic root system allows the shrub to exhaust the available soil oxygen thus exerting an indirect competition effect on surrounding plant species (Esen, 2000). The competitiveness of the root system is enhanced by mycorrhizal symbionts, which enhance mineral and nutrient acquisition.
Reproduction is mainly by seed with vegetative spread being limited to air layering, with branches rooting if buried in damp soil. R. ponticum flowers from the age of 12 years and pollination is assisted by insect species of the families Hymenoptera and Syrphidae. Rhododendron bushes 2 m high can produce over one million light small seeds each per annum (Shaw, 1984). The average weight of the seed varies from 0.02 to 0.19 mg depending upon location (Esen, 2000). Light is an essential prerequisite for the germination of seeds as is a temperature range of about 10-15°C.
R. ponticum is tolerant of a range of climatic conditions from as cold as -17°C to 25.9°C in its native range in Turkey to a much narrower climate in its introduced range. The species is intolerant of drought and grows best in areas of uniform damp climates. High humidity, mild winters and acidic soils are favoured for the growth of R. ponticum. The ability to tolerate dense shade gives R. ponticum a great advantage over its competitors. Even almost closed canopy conditions do not prevent this species from growing. A well drained humus soil type is the preferred conditions for optimum growth and R. ponticum will grow on other types of soils in its introduced range though not so vigorously.
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-20||0|
|Mean annual temperature (ºC)||-17||40|
|Mean maximum temperature of hottest month (ºC)||16||40|
|Mean minimum temperature of coldest month (ºC)||-17||6|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||0||0||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||525||1300||mm; lower/upper limits|
Rainfall RegimeTop of page
Soil TolerancesTop of page
- very acid
Special soil tolerances
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Illinoia lambersi||Herbivore||Plants|Growing point; Plants|Leaves|
|Stephanitis rhododendri||Herbivore||Plants|Growing point; Plants|Leaves|
Notes on Natural EnemiesTop of page
Means of Movement and DispersalTop of page
Natural dispersal is effective with the aid of wind and seeds can travel considerable distances, estimated at up to 1000 m, though this is considerably reduced to as little as 10 m in wooded areas (Shaw, 1984). Seeds are mature after 6 months and dispersal generally occurs 2 months after this time when the capsules open (Cross, 1981).
Vector Transmission (Biotic)
There are no known incidences of animals, other than man, acting as disseminators of seed.
There is no agricultural use for R. ponticum and therefore no agricultural practices associated with the species, but seed may be transferred by machinery involved in controlling R. ponticum infestations.
R. ponticum has only been introduced deliberately as an ornamental and then escaped accidentally from the enclosed areas. Accidental introduction may have occurred via the disposal of R. ponticum pot-plants or plant parts in landfill sites or rubbish collection sites.
R. ponticum was intentionally introduced into the UK where it was planted as an ornamental in gardens of large houses and in estates, where it provided cover for game animals. For many years, R. ponticum was more or less restricted to these artificial habitats but with the gradual erosion of large estates and the attendant workforces, the plant was allowed to spread unchecked. The warmer winters and wetter climates of the western UK appear to favor the fecundity, spread and establishment of the plant and it is in these areas where R. ponticum, within the past 40-50 years, has changed from a naturalized, prized garden plant to an invasive category 4 species (Cronk and Fuller, 1995).
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page
Environmental ImpactTop of page
Impact: BiodiversityTop of page
Social ImpactTop of page
Risk and Impact FactorsTop of page
- Invasive in its native range
- Proved invasive outside its native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts animal health
- Reduced native biodiversity
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
- Graft stock
Similarities to Other Species/ConditionsTop of page
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.Cultural Control
Grazing is not a method for control. R. ponticum is unpalatable to most herbivores due to the large amounts of toxic chemicals, particularly 'free' phenols and diterpenes in the leaves (Esen, 2000). The diterpenes, known as grayanotoxins, are not found in all species of Rhododendron but occur in the leaves, flowers and nectar of R. ponticum. Phenols are mostly concentrated in the young tissues, such as young emergent leaves and buds providing 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 also gives protection to the buds against attack by invertebrates.
Heavy machinery with long hydraulic arms is best for digging up the root material and crushing the branches. However, heavy machinery in certain habitats such as ancient woodlands compacts the forest floor causing problems such as increased erosion (Esen, 2000). Operator knowledge of the structural growth of the plant is essential to enable full eradication of the plant without leaving underground suckers. The surface rooting characteristics of R. ponticum should allow for the shrub to be lifted entirely from a site with most of the main rooting structure intact. R. ponticum removal is best suited to autumn and winter in order to make, and take advantage of, a clean uncontaminated seedbed in the spring. Due to the spread of the shrub, a systematic approach is best suited to the removal of the weed at each stage. If R. ponticum is just cut back, this will be followed by vigorous growth from the stumps during the next growing season. An approach of methodical cutting, mechanical removal and burning of root material and stumps is the preferred means to achieve a R. ponticum-free site.
Spraying cut stems with ammonium sulfamate is effective in controlling the growth of R. ponticum though surrounding vegetation may be effected. Glyphosate is commonly used to control R. ponticum in the UK after the bulk of the biomass has been removed by burning or mechanical removal, and triclopyr and imazapyr at differing concentrations have also been used with varying success (Lawrie and Clay, 1989).
Until now, biological control has not been taken into consideration as part of the management strategy for R. ponticum. However, this may offer a potentially simpler, cheaper and safer alternative approach to chemical and mechanical control, or an additional tool in the battle against this weed in the UK. Due to the horticultural importance of the genus Rhododendron, with over 500 spp. currently in cultivation in the UK alone (Mabberley, 1998) the risk associated with the classical approach using coevolved, exotic natural enemies from the native range is considered to be too high (Evans, 2000). However, a number of natural enemies have already been identified in the native range, including a rust fungus from Portugal (Farr et al., 1996). Instead, the proposed strategy is based on the evaluation of natural enemies associated with the weed in the UK, focusing on fungal pathogens for use in an inundative approach. This will involve the mass production and formulation of a selected pathogen and, in the case of R. ponticum, application of such a proposed fungal product or mycoherbicide as a site-selective stump treatment. Potential pathogens include a species of Cryptosporiopsis which was associated with dieback in Berkshire, UK (Shaw, 2003). This strategy has been implemented safely and successfully against woody invasives in the Netherlands, where BioChon, a product based on local strains of the wood-rotting basidiomycete Chondrostereum purpureum, has been marketed for the control of alien Prunus spp. as well as Populus spp. (Jong, 2000). Evaluation of this strategy against similar woody targets is also underway in both Canada and New Zealand.
ReferencesTop of page
Abrahams A, 1998. Rhododendron nectar- poisonous to both bees and man. Scottish Beekeeping, 75:19-20.
Anon., 2003. Dieback hits parks and gardens - the garden. Journal of the Royal Horticultural Society, 128:9.
Ansin R; Terzioglu S, 1994. New botanical variety of Rhododendron ponticum L. (R. ponticum L. subsp. Ponticum var. heterophyllum Ansin var. nova). Turkish Journal of Agriculture and Forestry, 18:137-140.
Bean WJ, 1976. Trees and Shrubs Hardy in the British Isles (8th Edition). London, UK: J. Murray.
Colak AH; Cross JR; Rotherham ID, 1998. Rhododendron ponticum in native and exotic environments, with particular reference to Turkey and the British Isles. Practical Ecology and Conservation, 2:34-41.
Compton SG; Key RS, 1998. Species action plan: Lundy Cabbage (Coincya wrightii) and its associated insects. Peterborough, UK: English Nature.
Cross JR, 1975. Biological flora of the British Isles, Rhododendron ponticum L. Journal of Ecology, 63:345-364.
Doyle G; Klotzli F; Walther GR, 1999. Conference on recent shifts in vegetation boundaries of deciduous forests, especially due to general global warming. Birkhauser Verlag AG, Basel, Switzerland, 237-251.
Elton CS, 1958. The Ecology of Invasions by Animal and Plants. London, UK: Methuen.
EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm
Esen D, 2000. Ecology and control of Rhododendron (Rhododendron ponticum L.) in Turkish Eastern Beech (Fagus orientalis Lipsky) forests. Thesis. Virginia Polytechnic Institute and State University.
Gritten RH, 1995. Rhododendron ponticum and some other invasive plants in the Snowdonia National Park. Plant invasions: general aspects and special problems. Workshop held at Kostelec nad Cernymi lesy, Czech Republic, 16-19 September 1993 [edited by Pysek, P.; Prach, K.; Rejmanek, M.; Wade, M.] Amsterdam, Netherlands; SPB Academic Publishing, 213-219
Humphreys DJ; Stodulski JBJ; Stocker JG, 1983. Rhododendron poisoning in goats. Veterinary records, 21:503-504.
Kelly DL, 1981. The native forest vegetation of Killarney southwest Ireland: An ecological account. Journal of Ecology, 69:437-472.
Mabberley DJ, 1998. The Plant Book. Second dition. Cambridge, UK: Cambridge University Press.
Mejias JA; Arroyo J; Ojeda F, 2002. Reproductive ecology of Rhododendron ponticum (Ericaceae) in relict Mediterranean populations. Botanical Journal of the Linnean Society, 140:297-311.
Shaw MW, 1984. Rhododendron ponticum- Ecological reasons for the success of an alien species in Britain and features that may assist in its control. Aspects of Applied Biology, 5:231-239.
Shaw RH, 2003. Biological Control of Weeds in the UK: Opportunities and Challenges. In: Child L, Brock JH, Brundu G, Prach K, Pysek P, Wade, PM, Williamson M, eds. Plant Invasions, Ecological Threats and Management Solutions. Leiden, The Netherlands: Backhuys Publishers.
Terzioglu S; Merev N; Ansin R, 2001. A study on Turkish Rhododendron L. (Ericaceae). Turkish Journal of Agriculture and Forestry, 25:311-317.
USDA-ARS, 2003. Germplasm Resources Information Network (GRIN). Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysearch.aspx
Ansin R, Terzioglu S, 1994. New botanical variety of Rhododendron ponticum L. (R. ponticum L. subsp. Ponticum var. heterophyllum Ansin var. nova). In: Turkish Journal of Agriculture and Forestry, 18 137-140.
CABI, Undated. Compendium record. Wallingford, UK: CABI
Elton CS, 1958. The Ecology of Invasions by Animal and Plants., London, UK: Methuen.
O'Hanlon R, Choiseul J, Corrigan M, Catarame T, Destefanis M, 2016. Diversity and detections of Phytophthora species from trade and non-trade environments in Ireland. Bulletin OEPP/EPPO Bulletin. 46 (3), 594-602. DOI:10.1111/epp.12331
Schlenzig A, 2005. First report of Phytophthora inflata on nursery plants of Rhododendron spp., Gaultheria shalon and Vaccinium vitis-idaea in Scotland. Plant Pathology. 54 (4), 582. DOI:10.1111/j.1365-3059.2005.01217.x
USDA-ARS, 2003. Hedychium flavescens. In: Germplasm Resources Information Network (GRIN). Online Database, Beltsville, USA: National Germplasm Resources Laboratory. http://www.ars-grin.gov/cgi-bin/npgs/html/tax_search.pl
Distribution MapsTop of page
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