The gardens of ancient Babylon should probably take the credit for being the first vertical gardens, but thanks to recent innovations - and growing eco-awareness - the idea is catching on today. Celebrated architects, including Renzo Piano, have made them part of their design vocabularies, and they are springing up all over the world.
It's possible to have a more modest domestic system, too. A wall of greenery can bring to life a bleak courtyard, grey street or cold modern building, as shown in a new book, Vertical Gardens (Thames & Hudson). An array of textures and patterns can be created that would be impossible to appreciate in a conventional garden viewed from the side. The benefits aren't just visual: the planting can improve air quality, deaden sound, and provide insulation.
They can also reduce temperature in city centres. Here, a framing system allows plants to colonise the walls of buildings, inside or out, via relatively lightweight metal frames that support layers of 1cm thick PVC and felt, which make for even water distribution. This soil-less system is essentially hydroponics: the plants' roots grow into and on to the felt, and absorb the nutrients being fed into the water supply trickling over them.
It's such an ingeniously simple idea you have to wonder why it took so long to catch on.
A domestic system suitable for even the smallest wall is available from ELT Living Walls, 0049 721 485 3423 (eltlivingwalls.com). It uses a series of panels divided into cells containing soil
Water is fed from the top of a living wall, emulating the natural world. Typically, plants such as these ferns grow on slopes and rock faces, so make ideal candidates
A living wall can be a place to experiment, just like in a garden. Here are bergenias, an old-fashioned herbaceous border favourite. Or try tough-as-old-boots sedums, a popular choice for green roofs
Unlike climbers, which tend to be rampant and unruly, this system uses plants of a more predictable size, allowing you to create patterns and control the end result more accurately
Due to access problems, it helps to rely on greenery rather than flowering plants to keep down maintenance. Evergreen ferns and shrubs, such as this fatsia, keep the wall fresh year round