The genus Rhododendron is the largest in the Ericaceae family with approximately 1200 species. There are two recognised subspecies of R.ponticum, subsp. baeticum and subsp. ponticum, with a number of varieties recorded. R. ponticum var. album has white rather than the usual purple flowers, R. ponticum var. cheiranthifolium is compact with narrow and wavy leaves, R. ponticum var. lancifolium is compact with narrow leaves, R. ponticum var. variegatum has variegated foliage, and R. ponticum var. heterophyllum (Ansin and Terzioglu, 1994).
R. ponticum is a large evergreen shrub, which can grow 2-8 m high, and can consist of several major axes forming from an irregular base (Cross, 1975). It forms a compact shrub in open areas, whereas in the shade it adopts a larger lateral spread. Stems lack the strength to support large vertical growth unless supported by other vegetation or artificial structures. The bark is rough, brown to dark brown, and the wood is hard and light brown. The root system consists of an extensive mat in the upper 15 cm of the soil. Leaves are glabrous, up to 22 cm long, glossy dark green above, paler beneath, oblong to elliptic in shape. Flowers can vary in colour from lilac pink to various light and dark shades of purple, spotted with brown and orange, in compact racemes borne from the glabrous pedicel, with 10 stamens with curved filaments. The fruit is a woody capsule containing several seeds that can persist on the plant for up to three years.
SIMILARITIES TO OTHER SPECIES
There are a number of Rhododendron species found where R. ponticum has been introduced which may be common and with which it may be confused. These include R. annae, R. arboreum, R. campanulatum, R. niveum, R. oreodoxa and R. wallichii, though differentiation is not generally difficult with available keys.
R. ponticum is a perennial, woody, geophytic shrub, which propagates vegetatively or by seed.
BIOLOGY AND ECOLOGY
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. In native 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. Rhododendron bushes 2 m high can produce over one million small light seeds each per annum (Shaw, 1984), with an average seed weight 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. Growth rate of seedlings is slow, and it can take two years for a seedling to bear mature leaves. R. ponticum can grow throughout the growing season, though with defined peaks in May-June, July-August, and September. The chromosome number of R. ponticum is 2n=26.
R. ponticum is tolerant of a range of temperatures, as low as -20°C and up to 40°C where native, but with a much narrower climatic range where introduced. The species is intolerant of drought and grows best in areas of uniform damp climates, with high humidity and mild winters also favoured for optimum growth. The ability to tolerate dense shade or even almost closed canopy conditions gives R. ponticum a competitive advantage over other plants in such situations. A well-drained, acidic humus soil is preferred, though R. ponticum will grow on other types of soil in its introduced range, but less vigorously. It can grow from sea level to 2000 m altitude.
R. ponticum is associated with areas with a warm to hot wet summer and a cool to cold wet winter. It is hardy to zone 6 (-23 to –18°C). It is associated with the vegetation zones: temperate deciduous forests.
In its native range, R. ponticum occurs in deciduous and evergreen forests in the Pontic Mountain ranges of Turkey, pine woods in Lebanon and on granite slopes in Spain and Portugal. Where introduced, R. ponticum has colonized a range of habitats including managed and ancient woodland, scrub woodland, moorland, heathland, peatland and urban areas in the UK (Foley, 1990; Doyle et al., 1999), and has spread from gardens to invade woodland and national parks (Thomson et al., 1993). R. ponticum is principally a weed in natural forests, but may also be a weed in agricultural land, and is also present in managed grasslands; natural grasslands; urban areas; coastal areas; rail and roadsides, and wastelands.
CROPS / OTHER PLANTS AFFECTED
R. ponticum does not pose a significant threat to plants in agricultural systems, but has become a serious pest in commercial land management (Rotherham, 1990) and has proved to have dramatic effects on habitat structure and native biodiversity in the areas where it has been introduced. Where R. ponticum is introduced in an area all plant species are threatened. Wild hosts include Calluna vulgaris (heather), Fagus sylvatica (common beech), Ilex aquifolium (Christmas holly) and Pteridium aquilinum (bracken).
PATHWAYS FOR MOVEMENT AND DISPERSAL
The wind can transport seeds 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).
There are no known incidences of animals other than man acting as disseminators of seed or vegetative propagules. Accidental introduction may have occurred via the disposal of R. ponticum plants or plant parts in landfill or rubbish collection sites.
Seed may be transferred by machinery involved in controlling R. ponticum infestations.
Movement in trade
R. ponticum has been intentionally introduced as an ornamental for planting in parks and gardens, and numerous varieties and cultivars are readily available as potted plants in shops, markets and garden centres, and as seed and live plants from mail order catalogues and via the internet.
USES AND BENEFITS
R. ponticum continues to be a very popular ornamental species with numerous varieties and cultivars selected and sold for their colourful, abundant and showy flowers. It used to be also planted as cover for game animals in the UK, and today may still offer nesting sites for birds and animal species. Some medicinal applications have been suggested.
EPPO region: Belgium, Bulgaria, France, Georgia, Ireland, Norway, Poland, Portugal, Russia (Southern Russia), Slovakia, Spain, Turkey, United Kingdom.
Asia: Georgia, Lebanon, Turkey.
HISTORY OF INTRODUCTION / SPREAD
R. ponticum subsp. ponticum has a wide native distribution covering an area south of the Black Sea, including Bulgaria and Turkey (Bean, 1976; Terzioglu et al., 2000), Georgia and the Krasnodar region of southern Russia (Czekalski, 1998) and Lebanon (Cross, 1975), and may be or may have been also native to Syria. Small pockets of native habitat are found in south western Spain and central and southern Portugal where R. ponticum subsp. baeticum is found (Milne and Abbott, 2000).
R. ponticum has spread to other parts of Europe but not to other continents, where it was introduced deliberately as an ornamental and has then escaped accidentally from the enclosed areas. R. ponticum was first introduced to the UK from Gibraltar in 1763 by the nurseryman Conrad Lodiges (Cronk and Fuller, 1995), and recent molecular studies support Iberia as the origin of UK material (Milne and Abbott, 2000). By 1803, R. ponticum was being sold on London markets as a flowering pot-plant and became established throughout England as an ornamental in stately homes. R. ponticum was also introduced to Ireland around 1800 (Doyle et al., 1999), though fossil records of Irish peat deposits show R. ponticum was a native to Ireland during the Gortian Interglacial period.
For well over a century, R. ponticum was more or less restricted to such confined and artificial habitats as parks and gardens, 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 favour the fecundity, spread and establishment, and it is in these areas where R. ponticum, since the mid-1900s, has changed from a naturalized, prized garden plant to an invasive species (Cronk and Fuller, 1995). The invasiveness of R. ponticum in Wales, UK, began to be documented in 1943 (Thomson et al., 1993; Gritten, 1995), and it is invasive in many parts of the UK and is found throughout Ireland (Kelly, 1981). R. ponticum is now also naturalized in Belgium and France (Tutin et al., 1972; Cross, 1975), and known to be present but localized in Norway, Poland and Slovakia. Further spread is possible due to its invasive characteristics, though is unlikely if adequate control and management is applied.
The economic costs of eradicating R. ponticum in an area can be many millions of Euros. The cost of clearing in Snowdonia National Park, Wales, UK alone has been estimated at over Euro 50 million (Mabberley, 1998). On the island of Colonsay, Scotland, UK, R. ponticum is spreading at a rate of 20 ha per year, and the cost of prevention and eradication of the weed is estimated to be approximately Euro 200,000 per year for a planned 16 year programme. On the small island of Lundy, Devon, UK, 105 days work and approximately Euro 50,000 is required every year to clear R. ponticum from cliffs (Compton and Key, 1998). Ingestion of R. ponticum leaves has also been known to poison sheep, cattle (Black, 1991), goats (Humphreys et al., 1983) and dogs (Frape and Ward, 1993), due to the large amounts of toxic chemicals, particularly ‘free’ phenols and diterpenes present (Esen, 2000), and the nectar of R. ponticum is poisonous to man, bees, mice and cats (Abrahams, 1998). An additional impact of R. ponticum is as a host to other pests, such as the vine weevil Otiorhynchus sulcatus, the serious forest fungal pathogen Heterobasidion annosum, and more recently to sudden oak death, Phytophthora ramorum (Anon., 2003).
Footpaths can become blocked by R. ponticum restricting public access especially in natural and national parks, and invasion in such areas also reduces their natural composition and value as tourist destinations. The nectar of R. ponticum is poisonous to man, and leaves and nectar are poisonous to domesticated animals.
Impact on biodiversity
R. ponticum is widely recognized as being very environmentally and ecologically damaging to semi-natural habitats, woodland, heathland and managed plantations. R. ponticum threatens forests throughout the UK and in its native range in Turkey (Colak et al., 1998) and where conditions are suitable for R. ponticum to grow, it will out-compete most native plant species eventually producing a monocultural stand. Ground vegetation disappears first, such as wild forest flowers, ferns and mosses, and once such native plants have disappeared then associated fauna is also threatened. Specifically, R. ponticum alone is known to pose a significant threat to the rare endemic Lundy cabbage, Conincya wrightii (Brassicaceae), and a flea beetle (Psylliodes luridipennis) associated with this plant on the island of Lundy, UK (Compton and Key, 1998).
RISK AND IMPACT FACTORS
R. ponticum is noted to have negative impacts on biodiversity, environment, forestry production, rare or protected species, native fauna, native flora, transport and travel, and tourism.
SUMMARY OF INVASIVENESS
R. ponticum has become established in the UK and other parts of Europe where it is threatening natural and semi-native habitats and the associated flora and fauna. In its native range, R. ponticum is also spreading and causing problems in the forests of Turkey where it is displacing other native vegetation. Present control methods are expensive and labour-intensive and classical biological control is not an option due to the many economically important species in the genus.
Is the species invasive in its native range?
Has it proved invasive outside its native range? (i.e. is it an invasive alien species)?
Is it highly adaptable to different environments?
Does it have high reproductive potential? (e.g. for weeds; prolific seed production, high germination rate, reproduction by rhizomes, tubers, stolons or root/stem fragments).
Is it highly mobile locally? (i.e. for weeds, propagules capable of moving long distances by wind, water, attachment to machinery, animals or humans).
Can its propagules remain viable for more than one year?
Does it tolerate, or benefit from, cultivation, browsing pressure, mutilation, fire etc?
Is it competitive to agricultural and plantation crops or pasture plants?
Does it cause impacts on ecosystem processes? (e.g. hydrology, sedimentation, fire risk, nutrient cycling etc.).
Does it adversely affect natural communities? (biodiversity, native populations, endangered or threatened species) by competition or hybridization (underline one or both).
Does it adversely affect community structure? (e.g. effects on the food chain, elimination or creation of a canopy).
Does it adversely affect human health? (e.g. allergies, effects on water or air quality).
Does it have sociological impacts on recreational patterns, aesthetics, property values?
Is it harmful to animals? (e.g. poisonous plant parts or vector of animal diseases).
Does it produce spines, thorns or burrs (or other discomfort)?
Is it a host or vector to recognised pests and pathogens of agriculture or forestry etc?
Is it highly likely to be transported internationally (a) accidentally? (e.g. as a contaminant).
Is it highly likely to be transported internationally (b) deliberately? (e.g. as an ornamental)
Is it difficult to identify / detect as a commodity contaminant? (e.g. due to small seed size)
Is it difficult to identify / detect in the field? (e.g. similarities to other species, inconspicuousness)
Is it difficult / costly to control? (e.g. resistance to pesticides)
If R. ponticum is cut back, this is followed by vigorous growth from the stumps during the next growing season and as such roots must be removed. Heavy machinery with long hydraulic arms are used effectively to dig up roots and crush branches, but adequate knowledge of the structural growth of the plant by the machine operator is essential to enable full eradication of the plant without leaving underground suckers. The surface rooting characteristics of R. ponticum should allow the lifting of a whole shrub with most of the main rooting structure intact.Autumn and winter removal is preferred, to take advantage of a clean uncontaminated seedbed in the spring. However, heavy machinery is unsuitable in certain habitats such as ancient woodlands as it compacts the forest floor causing increased erosion (Esen, 2000). Hand clearing is very expensive and time consuming, but a systematic approach appears most suitable, such as methodical cutting, mechanical removal and burning of root material and stumps.
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 in the UK after most of the biomass has been removed mechanically or by burning, and triclopyr and imazapyr at differing concentrations have also been used with varying success (Lawrie and Clay, 1989).
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 classical biocontrol is considered to be too high (Evans, 2000). Instead, a proposed strategy is based on an inundative approach, such as a mycoherbicide as a site-selective stump treatment. There are few natural enemies associated with R. ponticum even in its native range, though Judd and Rotherham (1992) found a total of 31 phytophagous insects associated with the shrub throughout its introduced range, the most closely associated being Stephanitis rhododendri, Illinoia lambersi (Hemiptera) and Melolonthamelolontha (Coleoptera). Farr et al. (1996) identified a number of fungi associated with the shrub including a rust fungus from its native range in Portugal, and others include Chrysomyxa ledi var. rhododendri (Rhododendron-spruce needle rust), Colletotrichum spp., Phytophthora cactorum, Armillaria mellea and Chondrostereum purpureum.
R. ponticum is described as an invasive category 4 species in the UK by Cronk and Fuller (1995).
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