The thin end of the hedge: Anna Pavord comes up with a plan for her long, narrow border, which will save guests from the arms of the spiky berberis

Every textbook on garden design warns against the too-narrow border. You lose the balance between a border's width and its length. You lose the opportunity to build up contrasting groups of plants. Flowers look as if they have been lined up for the school photograph.

Sometimes however, you have no option; it is narrow or nothing. I have this problem with a border that runs for 50ft along an east-facing wall of the house. It is barely 3ft wide because it gives on to the drive that swings round to make a parking place by the back door. There is just room, in theory, for two cars to park alongside each other.

But I have been trying to widen the effect of the border by letting things spill over on to the drive. Consequently, whoever arrives in the second parking slot is pushed into the border on the other side of the drive and climbs out into the prickly embrace of a Berberis aggregata. On a January night screaming with gales and rain this does not make for a warm welcome.

The narrow bed has been predominantly yellow and orange: potentillas, crocosmia, loosestrife, day lilies, early 'February Gold' daffodils, with brilliant blue tradescantia, Michaelmas daisies, alchemilla and a range of mad tulips for contrast. It was the sort of bed you accepted, like a very old coat, because it was familiar. You could say little in its defence.

It was also riddled with ground elder which came in on two gifts of plants. A poisoned chalice that] No amount of weeding or dabbing with glyphosate would get rid of it. Since the bed was anyway unsatisfactory and a dead japonica on the wall needed to be extracted, the best thing seemed to be to dig the whole thing up and start again.

The difficulty has been in deciding how to replant. To make the most of herbaceous perennials, you need more room for manoeuvre than a narrow border gives you. But having turned the front borders into high maintenance, bedding-out areas, I did not need another screaming for that sort of attention. And for the same reason that I tore the mixed planting out of the front beds, I wanted something that looked different from the rest of the garden and would make a virtue out of the necessity of the long, narrow shape.

Half a clue came from the hart's- tongue ferns that seed themselves into the back of the border by the house wall. Their long, shiny strap leaves are always handsome, particularly now when the foliage of most perennials has the allure of decaying cabbage.

Another lead came from the pyracantha planted underneath the kitchen window. One tier of branches is trained as a ruff along the windowsill. The rest is trained in a chequerboard on the wall to mirror the window next to it. The training is easy. You just wait for a likely shoot to present itself and then tie it in in the right direction. Any new growth that you don't want, you cut off.

The ferns and the pyracantha were all I wanted to keep from the original planting: they are bold, unfussy and evergreen. The strong geometry of the chequerboard is pleasing on the plain stone wall of the house, but it calls for something equally bold under it. I think box is going to be the answer, cut into alternating cones and balls. They can march single file the whole length of the border, which will get round the problem of trying to shoehorn tiers of different plants into insufficent space.

The flowing shapes of the hart's- tongues behind will offset the regular geometry of the box and there will be plenty of opportunity to provide more colour in plantings between the box shapes. But if I think of perennials for that, I am back where I started. If there was no room to juggle them before the box, there will certainly not be after. And then they would start creeping forward again and I would be back in the arms of the berberis.

Eventually, I foresee the pyramids and the balls being rather fat and splendid, taking up a good deal of the ground on which they are sitting. But in the short term there will be several feet between them. Perhaps bulbs to provide three different washes of colour at three different times of the year would be the most effective. I could start with aconites, as nothing is more cheering at this time of the year. And the clear, bright yellow would contrast well with the sombre bulwarks of the box. They are flowering now in a different part of the garden, pushing through turf round a yew tree. They deserve a medal for bravery.

I would need a lot of them to fill the entire border around the box shapes. And preferably a supplier who will send them in the green for, like snowdrops, they move best while they are growing rather than as dried tubers. They will enjoy the rather damp situation in the narrow border and put up with the semi-shade.

So would snowdrops, and the white would be very cool and elegant with the dark green foliage round it. But I have masses of snowdrops elsewhere and not many aconites. White can wait until early autumn, when I would want white Cyclamen hederifolium, as much for the marbled leaves as the shuttlecock flowers. The leaves persist until late spring.

They too would enjoy the growing conditions here, for the border is shut off from the south by a projecting arm of the house, which casts a shadow over it for at least half its length. Cyclamen relish shade and the leaves are much bigger and lusher in shade than they ever are in sun. So between them, the sheet of aconites and the sheet of white cyclamen take care of the beginning and the end of the season. The problem lies in the middle.

Lilies would be showy, but prohibitively expensive as they would need to be replaced regularly. They never last in our heavy, damp soil and they are massacred by small black underground slugs. I grow a few kinds in pots where they can have the kind of soil that suits them and sigh over the rest in catalogues.

Brodiaea would be nice, but would probably sulk away from full sun. They have strong wiry stems that never flop, grow to about 18ins and have heads of blue flowers, a bit like miniature agapanthus. But they come from California where they are used to bulb-baking summers. I have them in another part of the garden, where they hang on grimly, but do not increase. They flower in June or July, depending on the species.

English iris might be another possibility. They are like the Dutch iris you see in florists' shops, but have bigger flowers that come out in July. Alone among the bulbous irises, they positively enjoy cool, damp soil, but they also like sun. Even so, I am rather tempted by the thought of them spearing up between the box shapes.

But they are tall and would do nothing to cover the soil. By the time they are out, the cyclamen leaves will have disappeared. Perhaps the iris should emerge from a low sea of sweet woodruff, tucking itself around the box.

Woodruff (Asperula odorata) is a British native which favours damp soils and woodland. It grows no more than 6ins tall, with whorls of fine leaves topped with a froth of tiny white flowers in May and June. Yes, I think I will go for the iris/woodruff combo and see how they cope. Unfortunately the English iris are not as cheap to buy as the Dutch.

For Christmas I was given a flat stone mask, a streetwise cherub of the 17th century with flowers in his hair. He has a jaundiced, seen-it-all-before look on his face. He will be just the thing for this border. Once we have worked out how to fix him on the wall, he can look out of one of the panes of the pyracantha window.

Orchard Nurseries, Flint House Road, Three Holes, Wisbech, Cambs PE14 9JN (0354 8613) sells aconites in the green, pounds 5 for 25, pounds 9 for 50. Postage and packing pounds 1.50, delivery February and March.