A tall story: Frenchman Patrick Blanc is the undisputed master of the vertically-planted garden

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For once, I was glad of the traffic jam that stretched all the way along Piccadilly. Sitting at the top of a No 9 bus, it gave me plenty of time to admire the living wall that Patrick Blanc, the French architectural botanist, installed last year on one side of London's Athenaeum hotel. I went to the grand opening, but was so mesmerised by Blanc's vivid emerald hair, I took too little notice of the wall itself. With any type of garden though, you need to judge it when it has settled (or not), rather than at its primped and preened christening.

The wall stretches for 10 stories from pavement to roof and is the tallest living wall in the country. It's planted according to the system that Blanc has made his own: he doesn't use compost, but sets his plants in slits cut in a thick, felted kind of fabric stretched on batons over the wall. The roots anchor themselves in the felt and, by capillary action, suck up what they need from a solution of nutrients dissolved in water which drips its way from the top down to the bottom. There's a protective barrier behind, of course, to stop damp seeping into the expensive bedrooms, and a very firm armature, to stop bits dropping on pedestrians below.

That's a massively simplified description, but perhaps explains the difference between Blanc's methods and the system favoured by a firm such as Scotscape. They use modules (500mm x 500mm) packed with compost, each with a cover punched with holes in which the starter plants are set. The modules can be planted up while horizontal, then fixed into position vertically on a supporting frame. The deepest modules (150mm) give most room for plant roots to spread, but saturated with water can weigh at least 65kg each. Whatever support system is in place, living walls need irrigating, which means finding room for a header tank, a pump, and a gutter along the bottom of the wall to collect the drips.

Writing in the Garden Design Journal, Paul Hensey reckoned the starting price for a living wall was £500/sq m with irrigation extra. So the Greater London Authority's policy requiring "major developments to incorporate living roofs and walls where feasible" could add substantially to a developer's bill. I'm not weeping for them. They will just pass on the cost to whatever poor devil finally pays the rent. But are living walls feasible for ordinary gardeners?

Given the rash of living walls that has appeared in show gardens at the Chelsea Flower Show, designers would like us to think so. Most of the ones there, though, were crass, in-your-face things. But one, a soft dark mass of moss and fern, half hidden to the left of a showy Malaysian pavilion, was superb. It was in the garden designed by James Wong and David Cubero and formed part of their boundary with Thomas Hoblyn's adjoining show garden. It had taken forever to plant, said Wong, but the texture was gorgeous, and it dripped gently in the way that moss-covered rock faces do.

But if you are having to pay for that water, then maintaining a living wall, let alone installing it, becomes expensive. If you can harvest rainwater to use in the header tank, then you gain on two counts. The water is free and you can more confidently boast of the green credentials of your wall. Living walls are always seen as a Good Thing in environmental terms and certainly I'd prefer any day to look at a tapestry of plants than a raw stretch of concrete. But they are thirsty things and cannot, even in our dampish climate, survive without their daily pipe-fed drips.

Most gardeners, even in the middle of cities, have an easier way of greening up their space than by using a living wall. We use pots, where we haven't got earth, and grow plants in those. If we can dig into real soil, we plant climbers and train them up walls rather than installing expensive systems in order to let them hang down. Even ivy, which gardeners have been taught to distrust, has now emerged as a positive benefit on house walls. We always knew it was good for wildlife, but now it's being praised as an insulator, too.

Patrick Blanc started making his living walls inside buildings such as Les Passages shopping centre at Boulogne-Billancourt and Club Med's offices in Paris. In these controlled environments, he used mostly tropical plants, drawing on his travels in Taiwan and India, Guadeloupe and Malaysia to put together magnificent combinations of ferns and aroids. Schefflera, medinilla, billbergia, anthurium and asplenium all flourished. He's successful because he thinks of the needs of the plants before anything else. He remembers how he has seen particular things growing in the wild, and positions them on his walls accordingly.

Outside, conditions can't be controlled, but Blanc has found at least 300 different plants that he uses to create the futuristic swirls and arcs of his extraordinary tapestries. His current top star is Iris japonica, which he would never have thought of including had he not seen it hanging off a rock in Japan, looking far happier than it usually does in a garden. It contrasts well with the softer foliage of other favourites such as Fuchsia hatschbachii and F. regia. They are all on the Athenaeum wall. So is the maidenhair fern (Adiantum capillus-veneris) and more surprisingly, the Chilean bromeliad, Fascicularia bicolor. Look up next time you are passing. By Blanc's count, there are more than 250 different plants growing here. That's quite a garden.

The Athenaeum is at 116 Piccadilly, London W1. For more about living walls go to verticalgardenpatrickblanc.com or to scotscape.com, or see Patrick Blanc's book, 'The Vertical Garden' (Norton, £34)