Photo: Joe Caffrey
Rhododendron ponticum, like so many invasive species, was first intentionally introduced to Ireland as an ornamental plant. Sourced from Asia and the Iberian Peninsula, it was first planted in the landscaped parks and gardens of Ireland and Britain in the 18th century. This evergreen plant creates dense thickets, blocking light and reducing biodiversity. Leaves, flowers and nectar contain chemicals which are unpalatable to mammals and insects. The presence of R. ponticum changes soil chemistry and so discourages flora and fauna from colonising the nearby soil. It is also a carrier of a fungus-like pathogen Phytophthora ramorum that causes ‘Sudden Oak Death’ and attacks a wide range of plants and trees causing significant damage to native flora.
Rhododendron is very shade tolerant, making it a pest species in both woodland and open areas. It will thrive even in nutrient poor soil as it has fungal associations with plants such as heathers. After approximately twelve years this plant produces beautifully colourful flowers, which is one of the main reasons that it was introduced in the first place.. Eradication should be attempted before it reaches this stage as the flowers are highly fertile. As many as 7000 seeds can be produced per branch and these are readily spread by wind. However, R. ponticum can also spread vegetatively, thus increasing its invasive potential. Methods of eradication include removal from the root base, chemical spraying of stumps and injection of shoots with herbicide.
Rhododendron ponticum is widespread in Ireland at present and is becoming more common along the banks of rivers, particular in the west, north-west and south-west of the country. In these situations the dense growth adversely impacts on productivity in the river and impedes access to anglers or other water users.
Distribution of Rhododendron in Ireland