Rhododendron ponticum is the most expensive alien plant conservation problem in Britain and Ireland. It was introduced in the eighteenth century, probably in 1763 from Spain, and was then described as a not fully hardy plant. It was expensive to buy. It was made hardier by artificial and natural selection and by hybridisation with Appalachian and other Rhododendron species. It is easy to propagate and became cheap and popular in the mid and late nineteenth century as an ornamental, for game cover and as a root stock for other ornamental rhododendrons. The lowest price was in about 1880 by which time it had escaped widely. The escapes were ignored by botanical recorders for over 50 years. It was scarcely recognised as a problem until between the two world wars. Major control projects date from the second half of the twentieth century.

Wildlife impacts This species can make dense stands within which light levels will not be sufficient for other plants to flourish; due to the presence of 'free' phenols and diterpenes (grayonoterpenes) the plant is unpalatable or possibly even toxic to mammals and probably invertebrates; these chemicals are present in the leaves, flowers and nectar. Phenols are most concentrated in the young emergent leaves and buds.

All this being said, however, R. ponticum is said also to provide suitable habitat for songbirds.

 Would you ever suspect such a pretty plant to be an invasive allien??? 

sources:

http://www.habitas.org.uk/invasive/species.asp?Item=3888

http://www.erica.demon.co.uk/EH/EH1212.html