Rhododendron ponticum is the most damaging and most widespread non-native terrestrial plant in Britain. Left untreated, control costs are estimated to double every 20 years. Although control projects have been undertaken throughout Britain and Ireland, the cost of these has been very high and rhododendron continues to be a widespread problem which compromises biodiversity, forestry and other land management objectives.
Many rhododendron populations are still actively spreading, and it appears likely that a changing climate will favour further expansion. The threat of Phytophthora, to which rhododendron plays host, has made the need to control rhododendron at the national level all the more urgent.
Rhododendron has a severe impact on the biodiversity of native woodlands and open ground habitats. Once plants have become established, native tree seedlings, understorey plants and moorland and grassland vegetation are suppressed and further regeneration will fail to become established under the rhododendron canopy. Only holly, yew and ivy are able to survive under a dense canopy of rhododendron. However they cannot out-compete rhododendron which has extensive advantages, such as the ability to increase leaf area under shady conditions.
A number of methods have been used over the years to kill rhododendron. Some of these have used chemicals which are now banned, such as imazapyr and ammonium sulphamate. Where chemical treatments are used, the herbicide usually specified is a formulation of glyphosate approved for forestry use (such as Roundup Pro Biactive 450). Glyphosate is readily translocated vertically (e.g. from leaves to roots), but not transversely. Triclopyr, although no longer in production, but still an approved herbicide, is more effective than glyphosate on cut stumps, and as a foliar application during wet weather (Edwards, pers. comm.) although a review by Tyler & Pullin (2004) found it to be less effective when used as a foliar spray.
There has been a lot of recent research on the use of bio-control against other invasive non-native plants, but this is not considered to be an option for rhododendron, because of the impact it would have on horticulture.