While the modern concept of vertical farming – growing food in trays or pipes stacked on top of one another like a giant plant lasagne – dates from around the 1990s, it could be argued that farmers have sought ways to grow more in less space and with less soil for centuries. Step-over fruit trees – usually apple and pear trees – are grown as low as 1ft (30cm) off the ground and spread laterally to fill thin strips of space in allotments and orchards. The step-over technique is based on espalier training, which might date back to Ancient Roman grape cultivation.