What is the Rhododendron Ponticum plant?
Rhododendron is a large evergreen shrub (up to 8m tall) introduced to Ireland as an ornamental plant in the 18th Century from Asia and north-west China. However, not all are invasive species. Although there are more than 900 species of Rhododendron, only one type, Rhododendron ponticum, is invasive in Ireland. This species has dark green waxy, oblong leaves and large pinkish purple or lilac flowers on 2-4 cm stalks. Flowering occurs in spring and summer, and individual plants can produce large quantities of viable seed, which can persist in creating a seed bank in the soil.
How does this invasive species propagate?
Rhododendrons can also propagate by vegetative means, by suckering from roots and layering wherever branches touch the ground. Rhododendron thrives on peaty, sandy and acidic soils and is extremely hardy. Large areas of woodland and bogs in Ireland have been infested by Rhododendron ponticum. The leaves of Rhododendron contain toxins which makes it unpalatable to grazers. It can form dense impenetrable thickets, creating large shaded areas. This toxicity and creation of shade enable Rhododendron to out-compete native plant species (Maguire et al., 2008).
How can the spread be treated?
Treatment programmes can be divided into three main stages: initial removal, control of stems and roots, and follow-up. Our ecologist recently carried out Rhododendron control on a blanket bog site. The plant was multi-stemmed, and flower buds were present, indicating that it was likely older than ten years. Each of the stems was cut and removed by hand, cutting as close to the ground as possible to remove above-ground growth. The cut material was removed from the area to allow for effective follow-up work and prevent regrowth.
As rhododendron regrows vigorously when cut, stump treatment with a herbicide was necessary to kill the plant. When possible, the direct stump treatment method is favoured over general broadcast spraying, which is not as effective and has the potential to impact surrounding non-target species. Rhododendron leaves are thick and waxy; therefore, for foliar herbicide treatment to be effective, each leaf must be thoroughly wetted with herbicide to kill the plant. Additionally, the dead Rhododendron may persist in situ for 10-15 years, which can hinder the recovery of the site.
How much of a treat is Rhododendron Ponticum?
Rhododendron ponticum is listed under the 3rd Schedule: Part 1 – Plants; Non-native species subject to restrictions under Regulations 49 & 50. Regulations 49 and 50 of the European Communities (Birds and Natural Habitats) Regulations 2011 make it an offence to plant, disperse, allow dispersal or cause the spread of certain species. Additionally, rhododendron is listed on the “Most Unwanted: Established Threat” and the “High Risk: Recorded Species” list compiled by Invasive Species Ireland, a joint initiative by the Northern Ireland Environment Agency and NPWS.
For more information
Using an experienced, registered Professional Pesticide User to carry out invasive species treatment is important, as control efforts can sometimes lead to the spread of certain invasive species. For further information on invasive species, please see Biodiversity Ireland and Invasive Species Ireland.