The Scilly Isles are off the southwest tip of Britain, where they bathe in the warm waters of the Gulf Stream and the temperature hardly ever falls below freezing. In the twelfth century some monks from the mainland built an abbey on Tresco, an island in the center of the chain. Harassed by storms and pirates, they abandoned the island in the fifteenth century. Not much else happened there until 1834, when the island was leased by an avid amateur botanist named Augustus Smith. Smith was a friend of Sir William Hooker, director of the Royal Botanic Gardens at Kew. Hooker knew that semi-tropical plants would grow in the Scilly Isles better than anywhere else in Britain, and he persuaded Smith to plant exotic imports from Australia and South Africa. This was the beginning of the amazing garden that is today still owned by Augustus Smith's descendants.
The steep island provides two quite different habitats. On the higher slopes the air is quite dry for much of the year, the sun bright and semi-desert plants like these agaves thrive.
Closer to the water the air is often full of sea mist, and shaded valleys protect tender plants from the sun's full heat.
And there was no need to build a folly, since the actual ruins of the medieval monastery are scattered about.