Rhododendron (Rhododendron ponticum)

Present distribution

Scientific name:	Rhododendron ponticum L.
Common name(s):	Rhododendron

This weed is not known to be naturalised in Victoria

Habitat:

Native to areas of Turkey and Spain, Rhododendron has become widespread in Britain where it has been planted for ornamental purposes and game cover (Milne & Abbott 2000; Shaw 1984). Reported to invade forest, woodland, riparian vegetation, heath, grasslands, coastal dunes and forestry on acidic soils but not in well established pasture (Cross 1975; Dehnen-Schmutz et al 2004; Weber 2003).
Invasive potential has a strong correlation with climate as the plants slow growing seedlings are sensitive to desiccation over a hot dry summer period (Shaw 1984).

Potential distribution

Potential distribution produced from CLIMATE modelling refined by applying suitable landuse and vegetation type overlays with CMA boundaries

Map Overlays Used

Land Use:
Forest private plantation; forest public plantation

Broad vegetation types
Coastal scrubs and grassland; coastal grassy woodland; heathy woodland; lowland forest; heath; box ironbark forest; inland slopes woodland; sedge rich woodland; dry foothills forest; moist foothills forest; montane dry woodland; montane moist forest; sub-alpine woodland; grassland; plains grassy woodland; valley grassy forest; herb-rich woodland; sub-alpine grassy woodland; montane grassy woodland; riverine grassy woodland; riparian forest; rainshadow woodland; wimmera / mallee woodland; on soils with a pH of less than 7

Colours indicate possibility of Rhododendron ponticum infesting these areas.

In the non-coloured areas the plant is unlikely to establish as the climate, soil or landuse is not presently suitable.

Map showing the potential distribution of rhododendron

Red= Very high	Orange = Medium
Yellow = High	Green = Likely

Impact

QUESTION	COMMENTS	RATING	CONFIDENCE
Social
1. Restrict human access?	Can form dense impenetrable thickets which are difficult and costly to remove and control (Tyler, Pullin & Stewart 2006). This would pose a serious impediment to even machinery.	h	h
2. Reduce tourism?	Has been noted to alter aesthetics, which may be viewed as a positive because of its display of flowers (Shaw 1984) Its ability to form thickets may impact on some recreational activities however there is no reported evidence of this.	m	m
3. Injurious to people?	The tissues of rhododendron contain chemicals which are toxic and have caused death in livestock that have consumed it. (Connor 1977; Forsyth 1968). The nectar is also toxic which can mean honey made from it will be poisonous (Forsyth 1968). While it is more common for it to cause skin and eye irritation more severe cases of rhododendron poisoning have been reported (Connor 1977; Shepherd 2004).	h	mh
4. Damage to cultural sites?	Ornamental species could alter aesthetics (Shaw 1984).	ml	mh
Abiotic
5. Impact flow?	While it has been reported to be a riparian species in areas of its native range, it does not tolerate long-term waterlogging and therefore can not persist instream and therefore would have little to no impact upon water flow (Cross 1975; Mejias, Arroyo & Maranon 2007).	l	mh
6. Impact water quality?	Does contain toxic compounds in leaves which may then be released into the water and effect the aquatic life; however the impact of this occurring has not been quantified (Rotherham & Read 1988). As a dense evergreen shrub it would provide heavy shade and as it is not deciduous it would not clog the system with a glut of leaves during autumn like some other introduced species (Shaw 1984). Therefore it is unlikely to impact significantly upon water temperature or dissolved oxygen.	m	m
7. Increase soil erosion?	Shrub with extensive root system therefore likely to have a stabilizing effect (Cross 1975). It can reduce ground cover, and therefore potentially expose the soil surface to some erosion but not of a large scale (Cross 1981).	l	mh
8. Reduce biomass?	Forms dense thickets (Tyler, Pullin & Stewart 2006). When invading an open habitat this would be an increase in biomass, It is also however able to reduce regeneration of canopy species because of its dense shading (Eşen et al 2004). Which would likely mean that over all invasion by Rhododendron would cause an overall net replacement of biomass.	ml	mh
9. Change fire regime?	As rhododendron forms dense thickets and shades out ground cover species (Cross 1981; Tyler, Pullin & Stewart 2006). This would alter fuel dynamics creating dense elevated fuels and reducing much of the ground fuel except for litter. Rhododendron also has the ability to quickly recover from fire by resprouting from root stock and an ash bed can be taken advantage of for germination (Shaw 1984). It is unknown to what extent Rhododendron could alter the fire regime in an Australian ecosystem but it is believed to have the potential to have some impact.	m	m
Community Habitat
10. Impact on composition (a) high value EVC	EVC= Damp Sands Herb-rich Woodland (V); CMA= East Gippsland; Bioreg= East Gippsland Lowlands; M CLIMATE potential. Rhododendron can form monocultures; forming dense thickets of pure stands up to 65ha in Britain where it is also an invasive species (Cross 1975). IOt is also reported to be capable of shading out the lower strata and reducing the regeneration of the upper strata (Cross 1981; Eşen et al 2004). As there is only small areas within Victoria with a high or very high climate match rhododendron may not have as severe an impact.	h	mh
(b) medium value EVC	EVC= Valley Grassy Forest (D); CMA= East Gippsland; Bioreg= East Gippsland Lowlands; M CLIMATE potential. Rhododendron can form monocultures; forming dense thickets of pure stands up to 65ha in Britain where it is also an invasive species (Cross 1975). IOt is also reported to be capable of shading out the lower strata and reducing the regeneration of the upper strata (Cross 1981; Eşen et al 2004). As there is only small areas within Victoria with a high or very high climate match rhododendron may not have as severe an impact.	h	mh
(c) low value EVC	EVC= Damp Forest (LC); CMA= East Gippsland; Bioreg= East Gippsland Lowlands; M CLIMATE potential. Rhododendron can form monocultures; forming dense thickets of pure stands up to 65ha in Britain where it is also an invasive species (Cross 1975). IOt is also reported to be capable of shading out the lower strata and reducing the regeneration of the upper strata (Cross 1981; Eşen et al 2004). As there is only small areas within Victoria with a high or very high climate match rhododendron may not have as severe an impact.	h	mh
11. Impact on structure?	Rhododendron can form monocultures; forming dense thickets of pure stands up to 65ha in Britain where it is also an invasive species (Cross 1975). IOt is also reported to be capable of shading out the lower strata and reducing the regeneration of the upper strata and therefore having a major impact upon more than 60% of the flora strata (Cross 1981; Eşen et al 2004) As there is only small areas within Victoria with a high or very high climate match rhododendron may not have as severe an impact.	mh	mh
12. Effect on threatened flora?	Rhododendron is impacting upon threatened heathland communities of southern England and threatening the survival of rare Atlantic bryophytes in west Ireland woodlands (Cronk & Fuller 1995; Peterken 2001). There is no evidence of it impacting upon a threatened Australian species.	mh	m
Fauna
13. Effect on threatened fauna?	There is no evidence of Rhododendron specifically impacting upon a threatened Australian species, there is evidence to suggest that the species can alter the soil chemistry to the detriment of earthworms which could include the Giant Gippsland Earthworm (Cross 1975). However in Britian where it has also been introduced, very few fauna species are associated with the plant and therefore the significant habitat change is impacting upon fauna in general and would impact threatened species (Peterken 2001; Yela & Lawton 1997).	mh	m
14. Effect on non-threatened fauna?	In Britian where it has been introduced, very few fauna species are associated with the plant and therefore the significant habitat change can impacting upon fauna species (Peterken 2001; Yela & Lawton 1997). There is evidence to suggest that the species can alter the soil chemistry to the detriment of earthworms (Cross 1975).	mh	m
15. Benefits fauna?	May be visited by some insects and may provide some shelter, in Britain however very few species have been observed in association with Rhododendron (Peterken 2001; Yela & Lawton 1997).	h	mh
16. Injurious to fauna?	Has caused stock illness and death (Forsyth 1968). Nectar can be toxic to bees (McBarron 1977). There is evidence to suggest that the species can alter the soil chemistry to the detriment of earthworms (Cross 1975).	h	mh
Pest Animal
17. Food source to pests?	Due to its toxicity it provides little in the form of food for pest species.	l	mh
18. Provides harbor?	Forms dense thickets (Cross 1971). Was valued in Britain as cover for game, which was largely focused on pheasants but could provide cover for species such as foxes and rabbits (Shaw 1984).	mh	mh
Agriculture
19. Impact yield?	Reported to reduce regeneration and growth rates in eastern beech forests in Turkey and effect forestry in Britain including of conifer species (Eşen et al 2004; Shaw 1984). It is not reported to what extent competition with Rhododendron impacts upon growth rates. Has caused stock death in New Zealand and Britain which have been reported to be very unpleasant including projectile vomit, however there have only been a few reported cases and only a small proportion was affected (Connor 1977; Forsyth 1968).	m	mh
20. Impact quality?	Honey produced from its nectar is toxic (McBarron 1977). It is not known if this is a contaminate issue for Australian honey.	m	l
21. Affect land value?	It is unknown to what extent large infestations of rhododendron could impact upon property values.	m	l
22. Change land use?	May cause some silvicultural enterprises to become unviable (Shaw 1984).	mh	mh
23. Increase harvest costs?	It can increase the cost of a timber crops establishment to the point where it is almost not economically viable and it can interfere with other silvicultural operations (Shaw 1984).	h	h
24. Disease host/vector?	Rhododendron has been reported to be affected by Phytophthora inflate and Phytophthora kernoviae, however these are not of major concern in Australia (Beales et al 2006; Schlenzig 2005). Rhododendron is affected by a powdery mildew, however it is not known if this would impact upon any commercial crop (Inman, Cook & Beales 2000).	ml	mh

Invasive

QUESTION	COMMENTS	RATING	CONFIDENCE
Establishment
1. Germination requirements?	Seeds are released in winter and spring and considering there inability to persist in the environment germination is thought to occur largely on a seasonal basis (Shaw 1984).	mh	mh
2. Establishment requirements?	Has a preference towards specific seed beds such as thin moss beds and after fire. It can establish on bare ground but the seed and seedlings are vulnerable to desiccation and need to remain moist or not be exposed to large variations in temperature for up to 3-5 years. Establishment can occur under moderate shade, but seeds do not tolerate being covered with litter or buried (Shaw 1984).	m	mh
3. How much disturbance is required?	Reported to have invaded natural to near natural heaths in Britain (Dehnen-Schmutz et al 2004). Natural heath vegetation is taken to mean before invasion these were healthy functional ecosystems.	h	mh
Growth/Competitive
4. Life form?	Other; evergreen shrub-tree (Weber 2003).	l	mh
5. Allelopathic properties?	Rhododendron has been reported to contain chemicals with in its tissues with potential of having an allelopathic effect, it is unknown however to what extent this aids the plants competitive ability as a mature plant also has the ability to shade out the majority of other flora beneath it (Rotherham & Read 1988; Cross 1981)	m	mh
6. Tolerates herb pressure?	Leaves contain toxic substances which makes them unpalatable and generally left alone. Overgrazing by sheep and deer in England is considered a contributing factor in the spread of Rhododendron (Shaw 1984). Its role as an ornamental species also make biological control an unlikely prospect (Shaw 1984).	h	h
7. Normal growth rate?	During the establishment phase seedlings have little competitive ability and can be smothered by low growing vegetation or leaf litter (Cross 1975). A 3-5 year old plant it quite insignificant and it takes 10-12 years to reach maturity (Shaw 1984). This growth rate would be exceeded by many other shrub species	ml	h
8. Stress tolerance to frost, drought, w/logg, sal. etc?	Fire will kill seedlings, however damaged mature plants will regrow or coppice (Shaw 1984). Tolerant of frost (Cross 1975). Drought tolerance is dependent upon adaptation to the habitat, a plant in a more exposed position it is more tolerant than one is a moist protected position (Cross 1975). Seedlings are very sensitive to drought (Mejias, Arroyo & Maranon 2007).. Growth is poor in waterlogged soils; however it can tolerate temporary flooding (Cross 1975). The plants ability to invade dunes 200m from the sea suggests a tolerance to salt (Fuller & Boorman 1977).	mh	h
Reproduction
9. Reproductive system	Flowers and produces seed (Shaw 1984). Can reproduce vegetatively through layering, as branches in contact with damp soil can set root (Cross 1975).	h	h
10. Number of propagules produced?	A single flower head can contain between 3000 and 7000 seeds and a large bush can produce several hundred flowers in a season, resulting in a potential seed production close to a million (Shaw 1984).	h	h
11. Propagule longevity?	Rhododendron has very small seeds which do not survive well as a seed bank from one year to the next, even under artificial conditions (Shaw 1984).	l	h
12. Reproductive period?	Can flower as between 10 and 12 years and live to 50 years and possibly more (Shaw 1984).	h	h
13. Time to reproductive maturity?	From seed a plant takes 10-12 years to flower (Shaw 1984).	l	h
Dispersal
14. Number of mechanisms?	Very light seeds, approximately 0.066mg, dispersed by wind and also externally on animals by attaching with a frill of hairs at each end of the seed (Cross 1975; Shaw 1984).	h	h
15. How far do they disperse?	Seeds have been observed to occasionally disperse more than 1km, however under general conditions dispersal is most effective over distances of 100m (Shaw 1984).	mh	h

References

Beales P.A., Lane C.R., Barton V.C. & Giltrap P.M., 2006, Phytophthora kernoviae on ornamentals in the UK. EPPO Bulletin. 36: 377-379.

Connor H.E., 1977, The Poisonous Plants in New Zealand. E.C. Keating, Government Printer. Wellington.

Cronk Q.C.B. & Fuller J.L., 1995, Plant Invaders, the threat to natural ecosystems. Chapman & Hall. London

Cross J.R., 1975, Biological flora of the British Isles. Rhododendron ponticum L.. Journal of Ecology. 63: 345-364

Cross J.R., 1981, The Establishment of Rhododendron ponticum in the Killarney Oakwoods, S.W. Ireland. Journal of Ecology. 69: 807-824.

Dehnen-Schmutz K., Perrings C. & Williamson M., 2004, Controlling Rhododendron ponticum in the British Isles: an economic analysis. Journal of Environmental Management. 70: 323-332.

Eşen D., Zedaker S.M., Kirwan J.L. & Mou P., 2004, Soil and site factors influencing purple-flowered rhododendron (Rhododendron ponticum L.) and eastern beech forests (Fagus orientalis Lipsky) in Turkey. Forest Ecology and Management. 203: 229-240.

Forsyth A.A., 1968, British Poisonous Plants. Her Majesty’s Stationary Office. London.

Fuller R.M. & Boorman L.A., 1977, The spread and development of Rhododendron ponticum L. on dunes at Winterton, Norfolk, in comparison with invasion by Hippophae rhamnoides L. at Saltfleetby, Lincolnshire. Biological Conservation. 12: 83-94.

Inman A.J., Cook R.T.A. & Beales P.A., 2000, A Contribution to the identity of Rhododendron Powdery Mildew in Europe. Journal of Phyopathology. 148: 17-27.

McBarron E.J., 1977, Medical and Veterinary aspects of plant poisons in New South Wales. Department of Agriculture New South Wales.

Mejias J.A, Arroyo J. & Maranon T., 2007, Ecologcy and biogeography of plant communities associated with the post Plio-Pleistocene relict Rhododendron ponticum subsp. baeticum in Southern Spain. Journal of Biogeography. 34: 456-472.

Milne R.I. & Abbott R.J., 2000, Origin and evolution of invasive naturalized material of Rhododendron ponticum L. in the British Isles. Molecular Ecology. 9: 541-556.

Peterken G.F., 2001, Ecological effects of introduced tree species in Britain. Forest Ecology and Management. 141: 31-42.

Rotherham I.D. & Read D.J., 1988, Aspects of the ecology of Rhododendron ponticum with reference to its competitive and invasive properties. Aspectso of Applied Biology. 16: 327-335.

Schlenzig A., 2005, First report of Phytophthora inflate on nursery plants of Rhododendron spp., Gaultheria shalon and Vaccinium vitis-idaea in Scotland. Plant Pathology. 54: 582.

Shaw M.W., 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: 231-242.

Shepherd R.C.H., 2004, Pretty but poisonous. Plants poisonous to people, an Illustrated Guide for Australia. R.G. and F.J. Richardson. Meredith

Tyler C., Pullin A.S. & Stewart G.B., 2006, Effectiveness of management interventions to control invasions by Rhododendron ponticum. Environmental Management. 37: 513-522.

Weber E. 2003, Invasive plant species of the world: a reference guide to environmental weeds, CABI Publishing, Wallingford.

Yela J.L. & Lawton J.H., 1997, Insect herbivore loads on native and introduced plants: a preliminary study. Entomologia Experimentalis et Applicata. 85: 275-279.

Global present distribution data references

Australian National Herbarium (ANH) 2007, Australia’s Virtual Herbarium, Australian National Herbarium, Centre for Plant Diversity and Research, viewed 22 Mar 2007 , http://www.anbg.gov.au/avh/

Eremeier A. & Bruelheide H., 2004, Comparison of native and invasive Rhododendron ponticum populations: Growth, reproduction and morphology under field conditions. Flora. 199: 120-133.

Global Biodiversity Information Facility (GBIF) 2007, Global biodiversity information facility: Prototype data portal, viewed 22 Mar 2007, http://www.gbif.org/

Mejias J.A, Arroyo J. & Maranon T., 2007, Ecologcy and biogeography of plant communities associated with the post Plio-Pleistocene relict ponticum subsp. baeticum in Southern Spain. Journal of Biogeography. 34: 456-472.

Milne R.I. & Abbott R.J., 2000, Origin and evolution of invasive naturalized material of Rhododendron ponticum L. in the British Isles. Molecular Ecology. 9: 541-556.

Missouri Botanical Gardens (MBG) 2007, w³TROPICOS, Missouri Botanical Gardens Database, viewed 22 Mar 2007, http://mobot.mobot.org/W3T/Search/vast.html

Nadezhdina N. Tatarinov F. & Ceulemans R., 2004, Leaf area and biomass of Rhododenrdon understory in a stand of Scots pine. Forest Ecology and Management. 187: 235-246.

Stout J.C., Parnell. J.A.N., Arroyo. J. & Crowe T.P., 2006, Pollination ecology and seed production of Rhododendron ponticum in native and exotic habitats. Biodiversity and Conservation. 15: 755-777.

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