The high-rise hostas of Hackney

Look up from the landscape of tarmac, brick and concrete in the east end of London and you may see a rooftop oasis
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Satellite dishes sprout thickly from the balconies and roofs of the Kingsland Estate in Hackney, east London, but you are hard pressed to find a flower. The greenest thing here is the buddleia that sprouts, irrepressible, from a railway viaduct, a grubby brick structure housing the secret burrows of French polishers, car breakers and trade finishers.

So the sight of roses and greenery bobbing four floors up at Clifford Lawton House is more than usually cheering in this landscape of tarmac, brick and concrete. Gill Moore, warden of a sheltered housing scheme here, is the person you've got to thank if you too should find yourself mooching down the Whiston Road, parched for want of plants.

Her roof terrace faces south on to Whiston road, and is a sizeable 22ft by 13ft. A high brick wall, with French windows leading into the sitting room, shelters the garden on the north side. There is a brick wall on the west and a wooden fence on the east. Along the low south road edge is a raised bed, contained by a brick wall. Apart from this one bed, all the plants grow in pots and there are more than a hundred of them.

From the road, all you can see are the plants in the raised bed. There is a fine choisya 'Sundance', not showing the slightest signs of burning in this exposed situation, and a big, exuberant bush of feathery santolina, just coming into flower.

From Ms Moore's flat, you get the full view of the garden through the French windows opening on to the terrace. Close by the door is golden- leaved summer jasmine scrambling up the fence. Beyond are lilies, hebes, stokesia, euphorbias, violets, hostas, aquilegias, osmanthus, chiastophyllum with long tassels of bright yellow flowers, camellia, cistus, saxifrages, daylilies - all growing one plant to a pot. The pots themselves are arranged in two thick bands down the east and west sides of the terrace. Ms Moore calls them her borders.

This was one of the tricks Ms Moore learnt about roof gardening when she moved from her "proper" garden in Fulham. Plants in pots look better when massed together than they do scattered about randomly. They grow better too, as the pots, being less exposed, do not dry out so quickly. All the pots are the same colour: pale terracotta, which is important if you garden in a small space. You don't want too much distraction. Most of Ms Moore's are plastic, with some good looking clay pots (presents from her husband, Fred) placed along the front.

Fred customised a small three-wheeled trolley so that Ms Moore can get her sacks of compost from the pavement up to where she needs them. There is a lift in the building, which helps. She gardens organically and the bought-in compost is supplemented by her own worm compost which cooks quietly in two small brown dustbins in a corner of the terrace.

I liked the wormery. It lent a comforting touch of the farmyard to this eyrie high above the rumble of buses and tube trains. The worms are brandlings which Ms Moore gets from a fishing tackle shop. They start off in the bin in some worn-out compost from one of the pots and get fed with vegetable scraps left over from the kitchen. Sometimes they get wads of old newspaper. She has not found that they are fussy eaters. They'll make do with The Guardian if The Independent isn't on the menu.

Every so often she tips the contents of the wormery out on to a plastic sheet on the terrace, shovels the worms back into the emptied bin with a fresh supply of goodies and uses the compost they have made to top-dress her pots.

Watering is critical - as it is for anyone gardening in pots rather than real earth. Ms Moore waters her roof garden once every two or three days. That is less than I would have expected, given the relatively small size of most of her containers and the exposed situation of the roof garden. The worm compost is full of humus, of course, and that soaks up water very efficiently. She also lines all her pots with newspaper before she fills them with compost and plants them. The newspaper blots up water and provides insulation round the inside of the pots when the plastic and terracotta begins to heat up in the summer. Most of the pots have saucers under them for the whole of the summer season.

You can also increase the water-holding capacity of pots and hanging baskets by using clay granules such as Hortag. Stand a short piece of drainpipe on end in the middle of the container, fill the pipe with Hortag, then pack compost into the container round the pipe. When the pot is full, pull out the drainpipe. Then you have a water-retaining core through the centre of the pot which will act as a reservoir for roots to tap into.

The biggest pots (16in) hold the climbers that clothe the east and west facing walls. The golden-leaved summer jasmine is planted close to the French windows so the smell of the flowers can drift into the flat. Also on the east wall is the clematis 'Prince Charles', pale greyish blue, planted with stones laid over the roots to keep them cool. Next to it is 'Wada's Primrose', a clematis that comes out in May and June with a boss of yellow stamens in the centre of creamy white flowers.

After years of growing anything she fancied, regardless of its colour, Ms Moore now restricts herself to blue, yellow and white in the roof garden. A good deal of the yellow is provided by foliage such as the jasmine, the choisya, soft mounds of the saxifrage 'Cloth of Gold' and the variegated osmanthus 'Goshiki'. This has the handsome holly like foliage of O. heterophyllus but the dark green leaves are mottled all over with pale yellow. I have grown the ordinary variegated version, with leaves margined in cream, in a pot for six years. It does well in sun or shade.

One of the advantages of having the pots massed along east and west sides of the roof garden is that none of the plants has to take blazing sun all day. Half get morning sun, the other half basks in the afternoon. If they get too big, they are shipped down to Ms Moore's son's garden in Kent. A massive bronze fennel recently went up the road to the herb garden at the Geffrye Museum.

If you change your mind about the position of a plant in a real herbaceous border, you have a certain amount of puffing and blowing to dig the thing up and move it to a different place. If Ms Moore suddenly thinks "Too much yellow this end" she simply picks up the pot in question and shuffles it along to new quarters. She does the same with plants that come in and out of season, such as aquilegias and the viola 'Freckles', both of which are now taking back seats in the rows of pots.

Because the roof garden so immediately fills the view from the French windows of the sitting room, Ms Moore likes plants that will look handsome throughout the year, such as the osmanthus. She has evergreens such as Helleborus corsicus and the small-leaved hebe called 'Amy' which has dark, coppery foliage and short spikes of intense purplish-blue. She has a bay tree and clumps of sword-leaved sisyrinchium. She even has a vegetable patch of tomatoes, peppers and sugar-snap peas. I hope Ms Moore runs out of space soon. Then she might overflow on to some other roofs in Whiston Road. It could do with more of her touch.