Gardening: Nothing the secateurs can't cure: High above the traffic, all the rooftop retreat required was a little pruning, and a tree, says Anna Pavord

We are a young professional couple with no previous gardening experience. We started a garden on our roof in October 1991, and have been adding to it ever since. First, we put up trellis front and back with the idea of growing climbers to stop the wind; only partly successful. Then we filled as many pots as possible with any plant that took our fancy, ignoring all advice about what can survive in a pot.

The advantages of our garden are: most things seem to grow; we never get frost; lots of sunshine; it's very private; we can match soil to plant, hence the beautiful camellia.

The challenges and problems are: it's very exposed to wind and sun; it has no earth other than what we carry up three flights of stairs; there is a limit (not yet exceeded) on the weight we can place on the roof; aphids; cats; lack of time (watering alone takes up an hour each day in the summer) and we only have time for an occasional repotting binge. We have considered automatic irrigation.

What we want are: a tree (oh for a tree); birds other than pigeons (bird box as yet unused); shade; thick windbreak climbers; low maintenance]]]

THE exclamation marks after that last phrase are a giveaway. Katherine William-Powlett, who gardens with her husband, Joshua Danziger, high among the chimney pots of Goodge Street in central London, knows that low maintenance is a hollow objective with a roof garden. Plants on roofs are like astronauts in a space capsule - they need everything brought to them.

These two have obviously been bringing in the right stuff, because the plants on their roof are thumpingly healthy. Sturdy square trellis protects the north and south sides, and cuts off a small slice to make a working area: a place to store tools and hide plants that are resting; it also filters the sniffy blasts from the Greek and Japanese restaurants down below.

The trellis is vital, but is twice as effective, they say, now that they have put fine mesh netting around the outside. The climbing plants grow more vigorously and can hang on to the supports with less trouble. The west-facing boundary of the roof is a 6ft brick wall, lined with tall clay chimney pots. You feel as you climb out on to the roof that you have arrived on the set of the musical Cats.

Better not mention cats. Even here, three storeys up, said Ms William-Powlett, they are a nightmare. Her husband had just installed a deterrent that beeps or barks when disturbed. I only heard it in beeping mode, but thought it scarcely scary enough to keep away the tough boulevadiers of Goodge Street.

In view of the amount of greenery on the roof, I supposed that Mr Danziger had built up some beds with retaining walls. But no, everything was growing in pots, without saucers under them. Summer-flowering jasmine and 'Alberic Barbier' rose made an arbour over a seat. Clematis montana had ramped away over wires to make a shady corner facing south-east. Ivies, climbing hydrangea, fig, bay, vines, ceanothus, fuchsia, a huge camellia, the rhododendron 'Grumpy' were all flourishing in pots. The only disaster had been a choisya, still looking sickly in the recuperation ward.

Repotting all the plants this spring was a huge exercise that they are not keen to undertake again - and with the plants already in 12in or 14in pots, they will not have to. They can simply refresh the compost by replacing the top 2in. But they have made this job more complicated than necessary by using clay granules to top-dress all the pots. The granules, though, do a good job in that they retain moisture; drying-out remains the chief problem for plants on rooftops.

Ms William-Powlett and Mr Danziger have done a brilliant job of getting things to grow ('We only buy things that are voracious,' Ms William-Powlett said), but they are unsure how to manage the growth and at the same time unwilling to sacrifice any of the greenery they have so laboriously coaxed into being.

Some things, however, need tackling. The fig on the north-facing trellis has a fringe of leaves like a halo at the extremities of its branches, but nothing in the centre where the foliage should be providing a leafy screen.

If Mr Danziger steeled himself to cut off one of the main branches next March, he would find the fig would obligingly produce a young replacement, feathered all along with leaves. This, tied in, would provide better cover than the grey, bare stems of the mature growths.

Then there is the jasmine in the bower. 'It has stopped in a hedgehoggy bush,' said Ms William-Powlett. But it would not have done so if they had pruned some of the old growths each summer after it had flowered. Having, as novice gardeners, succeeded triumphantly at the first fence, they must now work out how to get over the second, by persuading their plants to grow as they want them to. This usually involves some pruning (though not of camellias or rhododendrons).

The jasmine was planted to cover a bower, but it does not know that. It just knows how to tangle its way up to the sky. However, if its new long shoots are tied to the bamboo struts Mr Danziger has carefully put in place, it will soon do the job.

But what about the tree that Ms William-Powlitt has set her heart on? One grows on a neighbouring roof and she envies it daily. Looking at it through field glasses, I thought it might be Pittosporum tenuifolium, which grows to about 15ft. The weight of a full-grown tree might pose problems: the maximum load on the roof is supposed to be 150lb a square metre. Like many New Zealanders, this tree copes well in difficult conditions, but I would start it off in a half-barrel rather than a small pot.

Mr Danziger already has a eucalyptus in a pot too small for it. The foliage is superb. Why didn't he whack that into a half-barrel, stake it so that the lead shoot knew it was boss, and grow that on as The Tree? It could replace a slightly unsatisfactory cupressus tucked into a corner where the entrance to the roof met the east-facing party wall.

Though potentially a chilly gap, the eucalyptus leaves would look splendid silhouetted against the sky, and it would grow fast. Perhaps too fast. Think of the traffic news in a few years' time: 'Chaos in West End as crane lowers giant tree from Goodge Street roof.'

The eucalyptus foliage would be too sparse to provide the shade on the 'want' list, but there is the beginning of a shady patch under the net of Clematis montana in the south-east corner. Without too much difficulty, you could make a miniature pergola with two bamboo uprights and a few cross pieces to extend this green roof parallel with the whole length of the south- facing trellis. The winter jasmine, honeysuckle and ivy already growing on the trellis would help the clematis with the roofing.

The only obvious gap in the windbreak of climbers is on the south-facing side where the clematis, rather than looking at the view into Goodge Street, has shot up on to wires strung over the top of the corner. I suggested the big-leaved vine, Vitis coignetiae, to fill the space. Its foliage would contrast well with the fussier leaves around it, and the autumn colour might be good. A pot would restrain its growth, which would otherwise be over-exuberant in this position.

An automatic irrigation system would make life easier for the couple, but I hope they do not get one. It would not see the aphids, or tweak a tendril of clematis in the right direction. While they are watering, these two novices are looking and learning about their plants, turning themselves into proper gardeners. That is worth an hour a day.

(Photograph omitted)