Your garden is on top of a roof. How do you turn it into a jungle?

WORKSHOP Anna Pavord's garden problem solver

"I am the proud owner of a roof terrace measuring roughly 17 feet x 13 feet," began Pamela Danvers, "it is open to the north, overlooking the District and Piccadilly lines which are out in the open here, running from West Kensington station. The south and half of the east side are sheltered by an eight-feet-high, zinc-covered wall which runs up to the roof; the west side is brick, about 10 feet up to the eaves, and the remaining half of the east abuts the next-door (apparently unused) terrace which is separated from mine by some netting, rising to a height of about six- feet.

I have inherited from the previous owner various bits and pieces in the way of window boxes and small tubs with (mostly) unidentified things growing in them, and brought with me an avocado, three bonsai citrus trees grown from pips - which are at least five years old and have always done well outside (though that was in Buckinghamshire) - and a cyclamen.

I fancy living in a jungle (although it may have to be Himalayan rather than equatorial) and intend to put up some baskets on the brick wall, and grow ivy over the railings, but what would you do?"

Some problems are common to all roof gardens. Turbulence is one of the worst, but Ms Danvers's roof garden was cosier than most I have seen, as it is walled round on three sides. It faces north, but there are advantages in that. Shade is usually difficult to arrange on a roof-garden but this one, with shadows cast by the relatively high walls around it, had its own built in shade. That would make growing good foliage plants easier, for Ms Danvers's dream of a jungle could not be achieved without introducing a lot of leaf into the place.

This roof garden, though, had some particular problems of its own. The sheets of metal that wall in the terrace on the north- and west-facing sides get very hot in summer and are inhospitably chilly in winter. There seemed to be no way of fixing any plant supports to their smooth surfaces. Similarly, the guttering did not look stable enough to support a hanging net of chicken wire. Then there was the surface of the flat roof, which looked like a layer of tar, painted with a silvery, light reflecting paint. The colour was sufficiently faded not to draw too much attention to itself, but Ms Danvers had found that pots of any weight scored and marked the surface (it may have softened in the exceptional summer heat). She was concerned that water might find its way through these grooves and into her flat below. On the other hand, she did not want to do what her surveyor had recommended: resurface the entire roof garden.

She had found that by standing pots on spare cork tiles, she could get round the problem of marking the roof's surface. She had also managed to get an old table, four-feet square, on to the roof, and her photographs show that this had been a very successful focus of the roof garden display last summer. She filled the entire surface of the table with pots of petunias, bought cheap from the Columbia Road market, breaking them up with pots of striped spider plants, which, together with other houseplants, such as waxy crassulas, spend their summers outside.

Ms Danvers had also had great success with seed saved from a bag of red peppers she had bought in Berwick Street market. She had sown them in clutches of three to five seeds in five-inch pots, let one seedling grow on, and been rewarded with a remarkable harvest. I was rather jealous of those, having failed miserably with my own peppers last year.

My general impression, standing on the roof was that it had great potential, but that there were too many small containers and not enough big ones. This is easy to say when you are not the person humping the pots (and the compost to fill them) up three flights of vertiginous stairs. It also seemed that Ms Danvers could build on the success of her massed table- top display by creating massed effects in other areas of the roof terrace.

To do this, she needs to import more greenery, which could act as a foil and a background for the flowers which she has already learnt can grow surprisingly well in her eyrie among the chimneys. Photographs show tobacco plants, foxgloves and pansies flowering, but looking a little lost and naked against a backdrop of sky, brick, sheet-metal and chimneys.

Three evergreens in matching pots, placed along the north-facing wall, would soften the effect of the sheet-metal and give an impression of a well-furnished space. They would also create a backdrop more evanescent annuals or bulbs in smaller pots, which could be shunted in and out in season and placed in front of the evergreens. Because the roof garden is shady and not too exposed, I would be inclined to experiment with camellias or bay trees, bottom heavy pyramids rather than top heavy mops. Both plants take very happily to life in pots, and camellias in this location would not be exposed to early-morning sunshine in spring.

The best wall for displaying plants was undoubtedly the brick one, formed for the mostpart by the chimney-breast of the adjoining house. The bricks, yellowish London stock, had a pleasing texture. The table takes up the whole central section of this wall, but to the right was a clear stretch on which you could train an evergreen wall-shrub, to provide extra leafiness. Only foliage could give the feeling that this roof is a luxurious, properly furnished space and provide a sympathetic backdrop for flowers.

The brick wall, however, faced east, opposite the only open section of the east side of the roof-garden, straight into the teeth of potentially harsh winds. Whatever was to grow there would need to be tough, and planted in a container big enough to remain stable. Loam compost helps, as it is much heavier than soilless types. It dries out less quickly too. Paradoxically, evergreens in this kind of situation are often more at risk in winter than in summer, because few people think of watering in winter, although winds can be as drying as the summer sun.

Pyracantha - easy to train, hardy, evergreen and equally good in flower or fruit - is my first choice and would display itself well against the brickwork. Vine eyes would be the neatest way of tying it securely to the wall. 'Orange Glow' does well on an east wall in our garden and is still covered with brilliant orange-red berries in March. Pyracantha rogersiana has sweet smelling flowers and the 'Flava' variety bears lovely milky yellow berries, but it is not always as reliably evergreen as 'Orange Glow'.

The four horizontal bars of the iron railings and four long and narrow concrete troughs, which Ms Danvers had inherited from the previous owners, stretch along the open north-facing side of the roof-terrace. She had fixed chicken-wire along the bottom of the railings, to provide support for plants and to stop pots blowing overboard through the gaps. This was where she was thinking of growing ivy.

That is certainly a possibility, but before long the ivy would fill the concrete boxes with matted roots, and Ms Danvers would have to use other pots for her flowers. Given that this was the only open aspect, I felt that ivy might create a claustrophobic atmosphere and suggested growing a mass of sweetpeas in the troughs. Their tendrils might not be strong enough to keep them upright, but with some judicious tying in, you could get round that. The seeds could be sown directly into the concrete boxes, but the plants would need extra feeding through the summer.

Ms Danvers hadn't been sorry to leave behind her garden in Buckinghamshire. "Too much mowing," she said, and being rather more interested in houseplants than gardens, felt the roof terrace an appropriate compromise. She evidently likes growing things from seeds and pips, and may build on her triumph with the peppers and try some tomatoes this year. A bush variety such as 'Tumbler' would get round the need for staking, and they, at least, might benefit from the reflected heat of the metal walls. At the cost of only a slight crick in the neck, travellers on the District and Piccadilly lines can keep on eye on their progress.