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Rhododendron ponticum and hybrids

Rhododendron ponticum (and its hybrids) is a non-native small tree or large shrub which is common in parks and gardens but has spread into woodland where it has become a pest species.

Distribution and abundance

The true R. ponticum is native to parts of Iberia and the Pontic region of eastern Europe and western Asia, but there is evidence that at least some Scottish plants are complex hybrids produced through interbreeding with other species, including some from North America.

Rhododendron is widespread and abundant in woodlands and adjacent open habitats throughout Scotland, especially on the west coast, where it has led to a serious decline in the abundance and diversity of the associated native plant communities. It colonises new habitat quite rapidly, and can spread by seed or by layering.

History of invasion and expansion

R. ponticum was first introduced from south-west Spain in 1763. Subsequent introductions have also occurred, including some from the Pontic region - although genetic evidence suggests that British plants are entirely Iberian in origin.

R. ponticum was known in the wild by at least 1894 and spread widely in the 20th century. The rate of spread has accelerated in the last 50 years, possibly as a result of increased disturbance to natural communities from forestry, the impact of myxomatosis on rabbits and over-grazing. Its distribution is now considered stable by some, in that it has not been detected in new 10km squares, but the species is still expanding aggressively within individual patches of habitat.

Genetic analysis suggests that some British populations of rhododendron contain the genes of R. catawbiense, a North American species. The level of such mixing is much greater in eastern Scotland and it has been suggested that this may confer greater cold tolerance, which may have allowed it to colonise colder parts of Britain. As hybridisation is common in Rhododendron species, it is also possible that interbreeding from some of the 500 other Rhododendron species cultivated in Britain has occurred through accidental or deliberate crossing in cultivation.

Impacts on other biodiversity and conservation interests

Rhododendron forms extensive dense thickets which cast a very deep shade, leading to loss of woodland ground flora, including epiphytic bryophytes and lichens, modifying the fauna and preventing tree regeneration.

R. ponticum is a cause of unfavourable condition 68 features on designated sites in Scotland. About half of these are woodlands.

Control

There are a range of management options available for Rhododendron. Whatever technique is used, it is essential that control is coordinated across ownership and administrative boundaries, and that follow-up action is sustained to prevent recolonisation of cleared ground. Removal of the species can have a beneficial effect on a range of ground flora, mosses, liverworts and other species.

Recent and current work

Rhododendron was one of the species on the Species Action Framework from 2007 to 2012. SNH worked in partnership with Forestry Commission Scotland and others, focussing on two projects, the Argyll, Loch Lomond and Trossachs Rhododendron Partnership (2008-2011) and the Highland Rhododendron Project (2010-2013), which developed a strategy  PDF document for controlling Rhododendron in Highland. Over five years they concentrated on stimulating rhododendron control on designated sites in unfavourable condition, where additional support was most likely to have a positive impact. Funding for such work was available through the Scotland Rural Development Programme (SRDP). Where necessary, these projects were able to fund survey of priority sites to inform the development of control programmes, as SRDP did not fund such work.

In parallel, Forest Enterprise Scotland (FES) launched an ambitious strategy to control rhododendron on the National Forest Estate across the whole of Scotland.

Forestry Commission Scotland (FCS), in partnership with SNH, commissioned an independent critical review of Rhododendron control in Scotland external site , which reflects on the experiences and outcomes of rhododendron control projects, draws conclusions about current practice and published research, and offers recommendations for a national strategy and the potential for future funded support.

FCS is now leading the development of a national strategy for Rhododendron control, in partnership with SNH and many other stakeholders. Progress on the strategy will be reported here.

The 'Species Lead' at Scottish Natural Heritage

Jeanette.Hall@snh.gov.uk   Tel 01463 725000