"Solar peach walls" were developed by nineteenth-century fruit growers of Montreuil (France):
Their secret lay in the construction of a honeycomb of solar walls. As Suzanne Freidberg writes in Fresh, the Montreuillois enclosed rectangular plots “in walls of plaster — a material that absorbs heat much more effectively than brick — and oriented them all north-south, so as to capture the most sunlight.”More details at Edible Geography.
This gridiron of sun traps were surprisingly effective, according to Freidberg:
Indeed, both day and night the gardens were warmer than their surroundings by several degrees Celsius. In this microclimate Mediterranean fruits thrived. Peaches ripened a month before others on the market, when prices were still sky-high. In addition, the villagers trained their espaliers to stretch out across the east-facing walls like giant fans cradling each peach in a perpetual sheltered sunbath.