R. G. Stapledon was one of the most profound of twentieth-century thinkers on rural and landscape issues. Scientist, philosopher and teacher, he was an unconventional figure who, with a dedicated team of loyal colleagues, established the basic biology and breeding systems of the Graminae at the Welsh Plant Breeding Station in the 1920s and 1930s. They subsequently bred a range of grass and clover varieties which were to transform grazing lands throughout the British Empire. To Stapledon the grasslands of an island nation were of vital strategic importance and if Britain were to prepare for a future war, her grasslands would require dramatic improvement if they were to provide a reserve of fertility. In proposing and implementing a vast scheme of renovation of the uplands of Wales, against the fervent opposition of the economists, as a means of both improving agricultural output and stemming the haemorrhage of depopulation, he demonstrated the practical feasibility of land improvement. In so doing he provided a template for the massively subsidised post-war programme of land improvement which was so dramatically to change the face of Britain. This article describes the evolution and conduct of the Cahn Hill Improvement Scheme which was not only one of the most ambitious outdoor ‘experiments’ of the last century, but in a sense, a metaphor for nation-building itself.