High society: Roof gardens can be a great option for the green-fingered city-dweller. Just make sure that your building has a lift, says Anna Pavord

A roof garden has always seemed to me the epitome of urban chic and glamour: nobody tells you about the stairs. I once attempted to green-up a large balcony, about 17ft long and 12ft wide attached to a third-floor shoe box in London and I know now that roof gardening is about brawn, not glamour.

It is about carrying things, there being no storage space on balconies of this sort. It is about hauling and heaving and forgetting things that you have left in the boot of your car, parked three streets away. It is about staggering along landings like a pot-bellied Buddha, tubs clasped to your chest.

On the third flight of stairs in this particular building, a sack of compost caught on a banister rail and I watched with fascinated horror as the contents poured down the stairs like lava from Vesuvius. A door opened. A staggeringly chic man emerged and picked his way in shining loafers through the debris, disgust written in every carefully chosen step.

Then, there is the wind. It is far more ferocious and damaging at third-floor level than it ever seems on the ground. The plan for this balcony was modest, depending more on foliage than flowers. Flowers do not seem as important as leaves in this entirely hard landscape where trees wave like drowning swimmers distantly over a sea of rooftops.

Though it probably got me marked down as a voyeur of the worst sort, I found a session on the balcony with field glasses extremely useful in sorting out which plants could cope with roof-garden life and which could not. Rhododendrons were the most obvious casualties. You could imagine how irresistible they must have seemed to flat dwellers seeing them in full spring flower in the garden centre. But by nature they are woodland plants. Everything about a roof terrace is wrong for them. They hate full exposure to sun and wind. They hate drying out, which is more likely to happen on a rooftop than not. In contrast, box, clipped into pyramids and balls, held its own remarkably well, marooned three floors up.

My plant list included a claret-coloured vine to train against the west-facing wall, trailing ivy-leaved geraniums with chunky fresh green foliage, and two standard bay trees, lollipops on top of stout trunks four feet high. Evergreens are obviously a good idea in this kind of setting and the formality of the standard bays seemed to suit the unreal surroundings of this small space.

The trees started life in large black plastic pots and within the first week, had blown over five times. I substituted even larger terracotta pots, packed with a heavier type of compost. Though the pots at least stayed upright, the trees, with their neat root balls, blew out of the pots. I tried guy ropes lashed to house wall and railings, but though that stopped the trees blowing out entirely, they leaned in a pathetic way and the roots, with all the jiggling about, never got a chance to anchor themselves in their containers. Eventually, the trees were reprieved from this miserable existence and went to the country. Later, I learnt that heavy flat stones placed on top of the compost either side of the main stem are the best counterbalance to sudden gusts of wind.

I recently went to Bermondsey Wall in London's Docklands to see how a professional garden designer copes with the problems of making a garden on a roof. First lesson: work only in buildings with lifts. Jo Thompson was commissioned to make what her client called an "urban meadow" in a south-facing space of about 50sqm. Double glass doors lead from the flat on to the roof, which is decked with bleached ipe timber. There had to be room for a hot tub, which effectively divides the space into two separate areas, one on the east side of the hot tub, the other on the west. Tall containers of flat-topped box provide a notional screen.

The planting is confined to the edges of the roof garden, where the weight of compost in containers can be spread most safely. Ms Thompson designed planters of light marine plywood, painted white and covered with the same ipe hardwood as had been used for the floor. These are lined inside with fibreglass containers; holes in the bottom of the containers link up with pipes that drain the rest of the roof garden.

The planters run all the way round the edge of the roof garden, so that you look out at tower blocks and city spires through waving plumes of molinia, panicum and Miscanthus nepalensis. The containers are relatively narrow, which does not allow much opportunity to group plants into foreground and background, but Ms Thompson has been pleased to see how well most plants have coped in this unnatural environment. Only the red hot pokers she included in her original scheme have refused to cooperate. Achillea and echinacea, prostrate rosemary, Hebe pinguifolia ' Pagei', Geranium ' Johnson's Blue' are all doing their best to provide the meadow of their owner's dreams.

Bamboo was vetoed, but big steely grey agaves stand like metal sculptures at the corners of the roof garden. Tall bronze phormiums and leathery-leaved pittosporum provide extra evergreen structure to the planting. Generally, the colour scheme is as cool as the pared-down surroundings, although alliums and the stalwart Verbena bonariensis add seasonal splashes of purple. As in many schemes of this kind, hard landscaping is the dominant factor. Plants occupy less than 10 per cent of the space.

For Ms Thompson, this was "a dream job". The client didn't meddle. There was an architect on hand to worry about supporting and installing the hot tub and Merriments Landscaping built the garden and project-managed the whole job. She had all the fun garden design can provide and few of the headaches. The only products that caught my eye were some terrific lime-green cubes - outdoor seating - made by the Italian designer Paola Lenti. The cost, which included craning in the hot tub and all the timber floor, was an eye-watering 70,000.

On the balcony I was trying to green up, there was no money to spend at all. Everything grew in pots. Though each pot had its own big saucer underneath, watering remained the chief difficulty, since it all had to be carted in cans from a very small galley-kitchen. I thought at the time of covering the white ceramic tiles on the balcony (they were hideous) with the old coir matting that had been ripped out of the flat. The pots would have been easier to manage as the coir would act like capillary matting and create a damper environment around the plants. They would have liked that. But would the tenant underneath? It might have been the loafer man. I could not have faced another dose of distilled distaste.

Jo Thompson's website is at www.english-garden-design.co.uk or you can contact her on 07834 196193