The world tends to divide into two when it comes to vertical gardening. On one side are those to whom the sight of a wall covered with greenery is something of beauty. On the other side are those who consider any climbing plant a nuisance, something that will damage the masonry or pull down the fencing or, at the very least, make them reach for the secateurs.
Yet, in an age when gardens are getting smaller, especially in towns and on new estates, surely it makes sense to use vertical surfaces as well as horizontal? And what if we discovered that using plants to clothe walls and roofs could actually protect the environment, and save us money, by reducing heat demand?
These are themes being explored in several gardens at this year's Chelsea Flower Show, which opens on Tuesday. Designers are using vertical planting techniques as a way of increasing the space available for greenery, to provide a habitat for wildlife, and to screen urban gardens from intrusion and noise.
Mark Gregory, designer of The Children's Society Garden, is looking at solutions for the modern, urban home. His garden includes two living-wall panels that use a range of foliage colour from green to purple, accented by the coral-pink flowers of heuchera. Plants include sedums, ferns and euphorbia.
A similar approach has been adopted by Philippa Probert, whose urban garden design, Green Living, has roof and walls planted with a mixture of flowers and foliage not only to create a spectacular vista but also to help muffle noise from traffic and neighbours.
Gregory and Probert say they were inspired in part by the work of Patrick Blanc, the French botanist who has pioneered the use of green walls worldwide, both for interiors and exteriors. Examples of Blanc's murs végétals include the Siam Paragon shopping centre in Bangkok, the Caixa Forum in Madrid, the Quai Branly Museum in Paris and a vast new green wall in the Rue d'Alsace, also in Paris.
Thanks to the work of people such as Professor Nigel Dunnett at Sheffield University, we in the UK have become accustomed to the idea of green roofs that use drought-tolerant, low-growing succulents including sedum, or wildflowers that don't require a high level of nutrients.
Blanc, on the other hand, has taken green walls to a whole new level. He uses a hydroponic technique and grows the plants not in soil, but felt, mounted on 1cm PVC panels held in place by a metal framework. The plants are fed and watered via an irrigation system, using run-off from the roof. He got the idea from observing plants in the wild, particularly in the tropics, where many plants grow on rock, tree trunks or soil-less slopes, so long as they have access to water, light and carbon dioxide.
The benefits are not only thermic and phonic insulation, but also aesthetic. Different colours of foliage is used to form patterns of stripes or swirls, while on other walls, large-leaved plants contrast with small-leaved species to give a textural contrast. The overall effect is incredibly rich, like a tapestry or a carpet.
Blanc's designs are more easily installed if they are incorporated as part of a new development, and far more affordable if you are a large corporation or institution. What about those of us who live in 1960s estates, or 1930s semis, or Victorian terraces? Well, Mark Gregory's living walls are basically wooden shelf units, wall-mounted bookcases. The plants are in soil-filled compartments, with drought-tolerant species at the top, and plants that like moist conditions, such as ferns, at the bottom.
Philippa Probert feels it's important for Chelsea show gardens to inspire ordinary, domestic gardeners. "Most people these days have smaller gardens, and it's nice for them to go to a show and be able to take pieces of ideas home with them."
Gregory agrees. "Patrick Blanc and his 'living walls' have been an influence on this garden, but I'm trying to create vertical planting in a way that can be copied and replicated by people coming to the flower show."
The Japanese designer Ishihara Kazuyuki, a regular exhibitor at Chelsea, uses a different technique for the living walls on his design this year, entitled Midori No Tobira (The Green Door). He has planted moss, ferns and acer (maple) seedlings into a framework of chicken wire filled with spaghnum moss.
The idea that an urban lifestyle causes us to lose touch with the natural world is a very strong theme in Ishihara's designs, and by surrounding the garden with greenery on both the vertical and the horizontal planes, he accentuates the feeling of a calm, private, green oasis.
If you still feel creating a living wall might be a bit beyond you, you can still take advantage of "facade greening", a new name for the old-fashioned technique of growing climbing-plants up the side of a building.
Livingroofs.org, an independent UK website that promotes the use of green roofs and walls, says climbing-plants can dramatically reduce the maximum temperatures of a building by shading walls from the sun. Evergreen climbers such as ivy also provide winter insulation, not only by maintaining a pillow of air between the plant and the wall, but by reducing wind chill on the wall surface. Even the interwoven bare stems of deciduous climbers, such as wisteria or climbing hydrangea, can reduce windchill by trapping air.
Although stem-borne roots of ivy can damage masonry and brickwork in bad repair it is usually all right on a rendered or concrete surface. Non-clinging climbers, such as vines or wisteria, can be trained on wires or trellis.
The garden designer and writer Noël Kingsbury has been studying at Sheffield with Professor Dunnett and is co-author with him of Planting Green Roofs and Living Walls (£25, Timber Press). He feels green walls are still a new field in Britain, compared to Japan, where vertical planting has become almost mainstream, and Germany and Switzerland, where facade greening is more common on new buildings than here.
"There has been a lot of work in Japan on vertical planting, but in the UK it's very early days and there has not been so much evidence-based research on which plants will work. In Japan, it's hot and humid, so things grow," he said.
Facade greeninghas been undergoing a huge change thanks to high-tensile support systems. "There are Swiss companies who are using climbers to cover six-storey buildings," says Kingsbury.
On a large scale, he says, climbers that have a substantial woody structure, such as wisteria, kiwi (Actinidia kolomikta), and vines, work best. He also likes climbing hydrangea, and for domestic use, he recommends the dreaded ivy. "It's self-clinging, evergreen and it does provide a lot of shade. But you need to maintain it: it needs clipping to keep it looking neat."
From roses round the door to creepers round the office block: horticulture has been on a long journey during the past 20 years. And it seems that, in the future, the only way is up.