Invasive Alien Species in Northern Ireland

Rhododendron ponticum, Rhododendron

Rhododendron ponticum
© Paul Hackney
Rhododendron ponticum
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Rhododendron ponticum L.
Rhododendron ponticum is a large evergreen shrub or small tree introduced to Britain and Ireland in the 18th century. It is a handsome dark green leaved shrub with showy trusses of lilac flowers, growing in its native habitat as an understorey plant in mixed forest, with other shrubby species such as cherry laurel, or as a dwarfed form above the snowline, at over 2000m. The plant is now found as a native in two distinct zones: one extremely extensive - eastern Europe (SE Bulgaria and NW Turkey) eastwards to beyond the Black Sea to Transcaucasia, and south to North Anatolia (hence its specific epithet ponticum, from Pontos); and a second, smaller area - the Iberian peninsula. These two zones are the surviving parts of a much larger range in western Eurasia in the Tertiary period; the fossil record shows it to have existed in Central Europe and as far north as Ireland even more recently, in one of the interglacial periods. Some authorities see the Iberian population as a distinct subspecies, subsp. baeticum, distinguished from the species by invariably having a densely short-hairy stem to the inflorescence.
Several possible dates can be put forward as the date of introduction to England, between 1763 and 1780. Although often viewed now as an escaped garden ornamental, it was also in earlier periods extensively planted as game covert, and was also used as the rootstock onto which many later Rhododendron hybrids were grafted. All of these means of introduction into the wild mean that it has taken hold in many areas of Britain and Ireland, and particularly forms expansive thickets in the milder areas. It has also hybridised with several other species, for example R. maximum and R. catawbiense.
Evergreen shrub or small tree to c. 8m tall:
leaves - long-oval, 10-20cm long, 2-6cm wide (or larger in the wild); dark green above, paler and hairless beneath, leaf stem 1-3cm long;
inflorescence - at the end of branchlets, with up to 15 flowers on a stalk c. 2.5cm long, flowers opening in late spring;
flowers - calyx very small, with 5 blunt teeth c.25 mm long; corolla bell-shaped, c. 5cm wide, 3.5-5cm long, lilac, often with pinkish or purplish tinge, and with green-yellow spots in throat; individual flower stalks c. 2-4cm long, hairless or slightly glandular-hairy; 10 stamens, filaments hairy at base, ovary and style hairless.
Country of origin
According to the German botanist Pallas, material in English gardens in 1784 originated from Gibraltar, and it does appear that both the Asian and Iberian subspecies are present in British and Irish populations.
Location in Ireland
Locally abundant, especially in demesne woodlands, and naturalising elsewhere.
Life cycle
This plant produces large quantities of viable seed - one source estimates that each inflorescence can be responsible for the production of from 3-7000 seeds, which can persist to make an enormous seed bank in the soil. It can also propagate itself by vegetative means, both by suckering from rootstocks and, more importantly, by layering, wherever branches touch the ground.
Wildlife and habitat impacts
This species can make dense stands within which light levels will not be sufficient for other plants to flourish; due to the presence of ‘free’ phenols and diterpenes (grayonoterpenes) the plant is unpalatable or possibly even toxic to mammals and probably invertebrates; these chemicals are present in the leaves, flowers and nectar. Phenols are most concentrated in the young emergent leaves and buds.
All this being said, however, R. ponticum is said also to provide suitable habitat for songbirds.
Human impacts
Cases are said to have been reported of poisoning by ‘toxic’ honey from rhododendrons, which are usually short-lived, the severity of the reaction probably being proportionate to the amount of the affected honey digested and the health and susceptibility of the individual concerned.
Key vectors
Originally a deliberate introduction to moorland and woodland for both game covert and ornamentation, whence it has spread by self-seeding. Where it was employed as a rootstock for less vigorous hybrids, it will, if untended, send up sucker growths which swiftly outcompete the scions for space and light.
Advice to specific groups relevant to this species
Local action plans could perhaps be put in place to eradicate small populations before they become a bigger problem. Recolonising from nearby properties which are not being managed is, however, likely.
Management measures
Programmes to eradicate populations in affected areas can be initiated, but the physical clearance and removal of shrubby material is insufficient if subsequent basal regrowths and generation of seedlings from seed in the ground are not also treated with appropriate herbicides. In County Kerry, the native oak woods and Arbutus unedo populations are affected adversely by the vigorous colonisation of Rhododendron ponticum; management policies are in place to try to eradicate the plant. A case has been made for the removal of soil ‘contaminated’ with toxins from R. ponticum after clearing infestations, but in areas of thin soil this is hardly practical.
Further information
All the following institutions/sites list projects investigating the spread or control of R. ponticum: Forestry research on Rhododendron control; Centre for Evidence Based Conservation; Centre for Conservation Science.
The Offwell Woodland and Wildlife Trust
See the Invasive Species Ireland web site for further information -
Text written by:
Catherine Tyrie, Curator, Botany, Ulster Museum