Green walls: how to grow a garden vertically

The urban green revolution is all around us. From floriferous balconies to lush roof gardens, green walls and innovative urban food-growing schemes, the plant-love in our cities continues to grow.

And rightly so. Every tiny patch of planting brings a benefit to those who live and work there. A challenging world of flash floods and unforgiving, heat-baked concrete is transformed by foliage-dappled sunlight and fresh scents. Through a host of small utopias, cities can become an ever-changing palimpsest of greenery.

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Benefits of green walls

The evidence is overwhelming. Vegetation in cities helps regulate air temperature and combats air pollution by trapping particles on leaves. It also reduces local flooding by absorbing rainwater, both at root level and by holding it in the canopy of foliage, while planted areas are known to increase local biodiversity.

^{Not just pretty: green walls like Westminster City Hall’s can improve the health of a city (ANDREW CROWLEY)}

On a domestic level, few need convincing but, if the warm glow of environmental rectitude is not enough, there are also good business reasons to seek out an urban greening specialist. Foliage on or around a building acts as an insulating jacket which keeps the building warmer in winter and cooler in summer. This not only reduces carbon emissions but saves on heating and air-conditioning bills.

There are also human benefits, such as improved mental health and reduced stress. “You will get things from a green building that you won’t get from a conventional green space,” says Gary Grant of the Green Roof Consultancy, who designed London’s largest green wall, at the Rubens Hotel in Victoria, London (below left). “It is better for the people using it and the building is more attractive so, for example, if it is next to a restaurant, then you get a busier restaurant.

“One of our clients discovered that it reduced staff turnover. Companies think about saving pennies by changing to low-energy light bulbs, but the money spent making their workers more comfortable or putting a spring in their step ends up having a major financial benefit in terms of reduced recruiting costs.”

Towering six storeys over the busy street below and covering an area of 350 square metres, the living wall at the Rubens contains more than 10,000 plants from 23 species, including evergreen ferns, ivy and flowering plants such as geraniums, crocuses, buttercups and strawberries. Installed and maintained by expert urban greening company Treebox, the wall is checked at least four times a year. In the meantime, a rainwater-collection system on the roof, text-alarmed in case of failure, stores up to 1,200 litres of water to irrigate the planting modules.

Designed to create drifts of colour throughout the year and provide forage for pollinators, the vertical garden imbues a once featureless façade with charm and personality.

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Upwardly mobile

There are three main ways to make a green wall.

1. Conventionally, plants are grown up a trellis or wire, as at the Oval cricket ground in London. This is an inexpensive option but it takes time for plants to establish.
2. Then there are two modern alternatives, both of which involve fixing irrigated planting modules to a frame that is separated from the building with an air gap and geotextile membrane.

^{The living wall at the Rubens Hotel in London}

Schemes like the one at the Rubens Hotel work a bit like a vertical bedding scheme using thousands of flowerpots filled with growing medium. The other method involves cages of crushed bricks which are seeded or planted up with tiny plugs. This creates a more naturalistic effect similar to the wild flora of mosses and ferns that appear around a broken downpipe.

Green wall controversy

Green walls are, however, not always welcomed and there are frequently objections from people who have reservations about the amount of money that will be spent – typically around £400 per square metre – or who misunderstand the process.

“Changing perceptions is a challenge and there is also a prejudice that plants should not be anywhere near a building – that they will destroy it. But it is not a case of shoving soil and vegetation onto the fabric of a building willy-nilly,” says Gary.

“In the context of construction, the cost is minimal and the benefits are considerable,” he adds. “A green wall will protect the underlying structure from frost, sun and rain and will extend the life of the building. Ultra-violet light is very destructive. Stone does very well in the face of UV exposure, but plastics decay, waterproofing fails.

“Green installations could double, or triple, the life of the surface to which they are attached.”

As with anything, green installations have a design lifespan. The hardware does not last forever and the plants must be periodically managed or replaced, yet Gary is adamant that the numbers stack up.

In addition to the hardware and engineering, large urban greening schemes require a fair bit of gardening know-how, not least because they are far more exposed than conventional gardens. “It is amazing how many things do grow,” says Gary. “But a lot of people in construction don’t really get it; they want to buy a product that you can just roll out and it is finished. It is not like buying wallpaper; there is a need for horticultural and architectural skills.”

As vertical greening becomes more popular and more ambitious, the horticultural industry, too, must adapt to its demands. The palette of proven plants is likely to increase, but sourcing can be an issue.

“I might be looking for plant diversity, functionality or pollinator benefits,” explains Gary. “Some nurseries are responsive and say, ‘We can do that if you can wait a little while,’ but many large growers just want to shift stock. This is a problem when you are implementing a project. You have to install what you can get hold of.”

Also there is the matter of manpower. Horticulture has an image problem: gardening is often seen as something done by old men and the breadth of career options is poorly known. As a result, the workforce is ageing and nurseries struggle to find enough young British employees with sufficiently good skills. And it seems unlikely that the ability to abseil off the top of a building to plant up a green wall was on their selection criteria.

“There is work to do,” admits Gary. “Horticultural professionals are not as well respected as engineers or architects in Britain. Countries like Germany are way ahead. A potential lack of skilled horticulturists is a problem, but I am very optimistic. I think we should stimulate a market and then stimulate investment in training.”

The future’s green

Britain has already come a long way and, as the technology becomes accepted and companies start to invest, the future looks bright. Now both fashionable and possible, urban greening might just open the door on a new world of gardening opportunity. But ultimately it may be less the grand objectives and big bucks that make the difference, as much as the gradual accumulation of smaller projects.

“It is a very, very exciting time,” says Gary. “Ten years ago, people thought it was far-fetched. They didn’t believe it was possible. Now we have different arguments – and we will overcome those too. There will be a remarkable change over the next 10 or 15 years, not just in the major projects, but as lots of small projects coalesce into a greener city.”

How to go vertical

Green walls are an increasingly popular feature in domestic gardens and many benefits of larger systems still apply: they can be fed by stored rainwater, reduce airborne particulate matter and, to a certain degree, can help insulate the building to which they are attached. Vertical planting is an excellent use of space and can substantially increase the planting area in a small garden or roof terrace. And for the grow-your-own enthusiast, herbs, veg and smaller fruit such as strawberries thrive.

Sarah Shuter and David McIver have had their 3m x 2.5m green wall for two years. It is mounted on a formerly blank wall outside a garden office and they are enthusiastic. “Everyone thinks it is fantastic,” says David. “It contains things like heuchera, ferns, grasses and ivy which filled the space and gave instant cover. There was one little problem when someone accidentally turned off the irrigation system, but Treebox [who installed it] has been great. I love it!”

But there are a few considerations. As in other areas of the garden, the plants still need to be fed, watered and trimmed or replaced. The weight of the fully planted and watered system must be assessed. South-facing installations risk overheating and plants may suffer if watering is intermittent.

Large-scale green walls are built and maintained by professionals but for ordinary gardeners, cheaper planting pockets and modular systems are now available. These can be hung from house walls or attached to a fence.

Vertical gardening technology has advanced to take in both the design needs of gardeners and the drainage and root-ventilation needs of plants; Wally Planters from woollypocket.co.uk are modules with self-watering tanks, and Burgon and Ball sells hand-watered Verti-plant pockets in various colours. Treebox supplies an Easiwall system and it also has a nice line in vertical allotments with optional irrigation.

For further information, visit greenroofconsultancy.com