Rhododendron ponticum and hybrids

What action is going on for this species?

The following will give you information on what work has been taking place through the Species Action Framework:

• Updates on recent action 
• Action up to start of SAF, April 2007 

Species background

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.

Why is this on the Species Action List?

This species meets criterion 2 of the Species Action Framework as an invasive non-native species which presents a great risk to biodiversity.

It is widespread and abundant in woodlands throughout Scotland, where it has led to a serious decline in the abundance and diversity of the associated native plant communities.  It spreads rapidly, and regenerates from small fragments of material.  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.  Although it is difficult to control, there are management options available.  Removal of the species can have a beneficial effect on a range of ground flora, mosses, liverworts and other species.

Habitat, distribution and abundance

It is found in wooded and open areas on acid soils in areas of high humidity.  Within Scotland the west coast is worst affected, but it is found throughout the country.

R. ponticum is native to parts of Iberia and the Pontic region of eastern Europe and western Asia.  It is present in 1,787 10km squares in Great Britain, between 0-600m altitude.

History of invasion and expansion

R. ponticum was first introduced from south-west Spain in 1763.  Subsequent introductions have also occurred, 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 British populations of rhododendron often contain the genes of R. catawbiense, a North American species.  The level of such mixing is much greater in eastern Scotland and it is suggested that this may confer greater cold tolerance, which may have allowed it to colonize 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

R. ponticum colonises new habitat - especially woodland and heathland - quite rapidly, and can regenerate via seeds, suckers or rootlets.  It forms extensive dense thickets which cast a very deep shade, leading in woodland to loss of ground flora, epiphytic bryophytes and lichens, modifying the fauna and preventing regeneration of trees.  In addition to the effect of shade, it may produce biochemicals which can affect other plants, inhibiting the germination or seedling establishment of other species.  There is also evidence for the prevention of mycorrhizal development in the roots of seedlings of competing plant species. R. ponticum may act as an intermediate host for the Phytophthera sudden oak death fungus; although this has not yet been found in Scotland, it is causing considerable concern in England.

R. ponticum is identified as a serious threat to upland oakwood in the UKBAP. It is also identified as a threat for several lower plants and fungi in the UKBAP including Acrobolbus wilsonii, Arthothelium macounii, Lejeunea mandonii and in the grouped plan for tooth fungi.

The 'Species Lead' at Scottish Natural Heritage

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