In 1644, John Evelyn, a young Englishman travelling in Europe, let himself into the garden of a French nobleman’s chateau. He was surprised to be collared by a gardener: in Britain, he explained, everyone opened their private gardens to the public.

On his return to Britain, Evelyn set about creating an encyclopedia of gardening. His Elysium Brittanicum said a garden was the terrestrial enjoyment “most resembling Heaven, and the best representation of our lost felicitie” – that is, of course, the lost Eden. In Eden, it was believed, every flower in the world grew. And what Evelyn understood was that the flora scattered at The Fall could be reunited in Britain. Why? Because we have a uniquely temperate...