Rhododendron is a highly invasive exotic evergreen, threatening landscape character and biodiversity, and also an important host for Phytophthera ramorum, the fungus which causes "sudden oak death" disease. It is native to the Mediterranean and was introduced from Gibraltar in 1763. (It was present in Britain in previous interglacials but did not re-colonise after the last glacial period.)
Rhododendron spreads by layering and copious seed production (over 1m seeds per bush). It produces toxins, and suppresses other plants by poisoning the soil as well as year-round shading. Using the same mycorrhizal fungal associates as heather to maximise nutrient uptake from poor acidic soils, it is particularly invasive on heathland because suitable fungi are already present. In woodland, rhododendron prevents seedlings establishing, so when old trees die they are not replaced and the natural and highly biodiverse structure of ground layer, field layer, understorey and canopy disappears.
Few insects or other herbivores eat rhododendron, due to its toxicity. Most of the 31 British insects found on it are widespread generalists - only six introduced species are rhododendron specialists.
On the (comparatively minor) plus side, rhododendron provides dense cover (many historical introductions were for game cover on estates). Various warbler and thrush species feed and nest in rhododendron stands, and finches and thrushes favour it for winter roosting sites. It provides cover for deer, badger setts and fox earths. The abundant flowers are highly attractive to insects (but rhododendron honey can be poisonous!).
Rhododendron is the biggest threat on some heathland sites, but on Greenham Common invasions from adjoining properties are localized and have mostly been dealt with. However, it is best to tackle rhododendron before it becomes entrenched, because the toxic leaf litter can prevent re-establishment of native plants for years after a stand has been removed. So we check for re-growth in the cleared areas, and look out for isolated specimens. We don't burn the cuttings because even bonfire smoke from rhododendron is said to be toxic!