Throughout the 1700s the great houses of England had their huge estates fashioned as gardens by Lancelot Brown He was known as Capability Brown for advising his clients that their land had “great capability” of “improvement” ( these days his work is not without some controversy – as here). A feature of his style was sweeping “natural” vistas and dense trees, with colours restricted to shades of green. It contrasted with the formal “Renaissance” style then preferred in Europe, as here.
But explorers of the New World were bringing back colourful Azaleas and Rhododendrons, which were found attractive as a great contrast to the green of the now classic “English Garden”. Gardens so planted were called “American Gardens”. A feature of these plants is that they grow in acid soil. Botanists crossbred the plants with remarkable effect. In flower in May they can be spectacular. Though the expression “American Garden” is now rarely used, several of such gardens remain highly prized in London: Langley Park near Slough, “Isabella Plantation” in Richmond Park, Kenwood House in Hampstead, “Rhododendron Dell” in Kew Gardens and, still using the classic name, “The American Garden” (also link here) in Dulwich Park.
This article from the Independent Newspaper explains things well.
And for more than you ever wanted to know about where rhododendrons come from, see here.