Gardening: Planting for architecture

We have just moved into a new loft apartment with a very sunny south-west-facing terrace of 50 square metres. The building in which the loft is located is not the usual industrial model, but is based on the High Museum, Atlanta, designed by Richard Meier. This makes many gardening ideas inappropriate - and any plan will need to be rather formal, which isn't my personal inclination.

Additionally, the paving on the terrace is a pink mixed with grey which does not blend well with terracotta pots. I have bought a grey metal table and chairs. We also have two metal sculptures. Otherwise I would envision one or more large containers to hide the pots. In theory, nothing is supposed to obtrude over the height of the railings, but plants do grow. There are also glass partitions at each end of the terrace which need hiding with climbers.

Given that the terrace will be visible all the year round, I believe I need some evergreen and a good shape to the whole enterprise. I would like to have some camellias, and would love some small citrus trees if they could survive. Otherwise I have no strong preferences and quite conventional aversions. Can you help?

The loft apartment about which Bobbi Clark writes is in trendy Clerkenwell, once an Italian quarter of London, near the City, and intricate with narrow streets, old printing works and textile factories. She and her husband, Richard, occupy an extraordinarily bold space added to make fourth and fifth floors on top of a Thirties factory building. The apartment is roughly triangular, with the terrace wrapped along the outside of the longest, south-west-facing side of the triangle.

The entrance, the dining space and the living space are all angled towards this outside wall, which is made of glass, dropping the height of the two levels of the apartment. Over the main part of it, there are no blinds. Here, more than is usual in gardening terms, the outside is the inside. There is no visual interruption in the way the eye flows over the intensely restrained interior, with its few carefully placed pieces of furniture, to the outside terrace, paved, as Ms Clark explains, with squares of pinkish- grey Italian granite.

"I'm a very visual person" says Ms Clark, forcefully. "I like things to look good." But plants, unlike pieces of furniture, have their own agendas. A large and sculptural indoor plant, a Ficus benjamina, placed so that shadows of its leaves, illuminated by its own spotlight, would be reflected on the ceiling, was beginning to moult in an alarming way. Similar behaviour in any plants that might take up residence on the roof terrace could not be tolerated. Being on show all the time, they would be playing too important a part in the appearance of the apartment.

On the terrace, Ms Clark was looking for "theme and shape. Perhaps one or two subtly flowering things. Mostly foliage". What she didn't want were lots of little, unconnected events. That seemed to call for some large, simple planting boxes, that could take more than one large specimen. They would probably have to be free-standing structures, pretending to be built in, for there is nothing to build them against. On one long side is the glass wall of the apartment; on the parallel outside edge are horizontal rails, six rows of them, cantilevered inwards for safety. The two narrow ends of the terrace are closed off by frosted glass walls.

Homely terracotta certainly wouldn't be the thing here, but I can see containers faced in zinc or stainless steel working quite well in this context. They would have to be insulated, so that the compost inside would not heat up in full sun. And given the fact that Ms Clark perhaps wants a garden more than she wants to do gardening, I recommend that she should buy good-sized plants at the outset.

That is the opposite of what I would usually say. But the terrace, facing into the prevailing wind and very hot in summer, is not the ideal place to nurse plants up. With big plants, too, you get a better idea of the effect you have in mind. A trip to Architectural Plants, near Horsham, is called for. Camellia sasanqua, phormium, trachelospermum, arbutus, trochodendron, eucalyptus and Pinus patula would all be worth seeking out. So would acacia, agave, myrtle, sempervivum, cordyline, Euphorbia mellifera, fatsia, bay, lomatia and yucca. Richard Meier-esque architecture would not faze these plants in the least. They all have plenty of architecture of their own.

The terrace is at least four times longer than it is wide: not easy proportions. I wondered whether Ms Clark had thought about using plants to divide up the space, creating, for instance, a separate area for the outside dining table and chairs. No, she says, she hadn't, and all things considered, she thinks she'd prefer the space left open. Given the nature of the apartment, that is not surprising.

Then there is the need for shade. Ms Clark has a parasol, but she could also use climbing plants outside to create a shady summer sitting place. The supports would have to be carefully designed to fit in with the severely modern architecture of the building itself. Once they were in place, either Actinidia chinensis or Cobaea scandens would do a good job covering them.

How much of a load, I wondered, could this roof bear? Would it be a good idea to use a light, inert growing medium, such as Grodan, rather than compost? Should there be an automatic irrigation system? The answer is probably yes. But there are more technical aspects to roof gardening than you find in any other branch of horticulture. It is not the right place for experiments in DIY. Ms Clark needs a garden designer who knows the problems and can recognise the opportunities of this extraordinary setting.

I suggest Dan Pearson, whose garden at Chelsea last year was modelled precisely on this kind of environment: the rooftop space of a London warehouse recently converted into apartments. His design was dramatic and modern, and used plants specifically chosen to cope with the demanding conditions of high-rise living. No petunias, no climbing roses and definitely no hanging baskets.

Help with roof gardens: Dan Pearson, 80C Battersea Rise, London SW11 1EH. Composts: Grodan and other hi-tech garden goods, from Growth Technology Ltd, Fremantle House, 21-25 Priory Avenue, Taunton, Somerset TA1 1XX (01823 325291). Plants: Architectural Plants, Cooks Farm, Nuthurst, Horsham, West Sussex RH13 6LH (01403 891772). Structures: Stuart Garden Architecture, Burrow Hill Farm, Wiveliscombe, Somerset TA4 2RN (01984 667458). Irrigation: Sales & Co, 8 Fauchonberg Rd, Chiswick, London W4 3JY (0181-742 8855) which specialises in and advises on environmentally-aware ways of watering plants. Water-retaining gels: Greenacres Horticultural Supplies, PO Box 1228, Iver, Bucks SL0 0EH (01895 8352356). Lighting: Gardena UK, Dunhams Lane, Letchworth, Hertfordshire SG6 1BD (01462 686688), John Cullen Lighting, 585 King's Road, London SW6 2EH (0171-371 5400).