WORKSHOP The Dysons have a problem with the bottom of their garden. Anna Pavord advises
"We have been working on our garden for 16 years, gradually turning it from an overgrown jungle into an informal place where the family can relax. There is one part we still have to tackle and that is the bottom of the garden round the old shed - used as a munitions factory by the local ladies of Barnes during the First World War. They made shells there. It is covered with an old and wonderful wisteria that rampages nearly to the top of a large poplar tree beside it. The garden boundary is an 18th-century wall of mellow brick. Between the end of the shed and the wall is a dark corner shaded by a high holly tree which is protected by a preservation order.

"Our problem is to work out how to plant this area. It has for ages been used as the compost and bonfire corner. As the wall is only a few feet away from our next-door neighbour, we have moved the bonfire and compost heap, thinking them a bit unneighbourly where they were. This opens up quite a large space, where there are all sorts of possibilities: ferns, a collection of shade-tolerant plants, something a bit mysterious. What would suit it best?"

Kate Dyson and her husband, John, live in a tall house looking over the river at Barnes, in southwest London. The garden is all at the back of the house, about 25ft wide and six times as long. The boundary on the south side turns in a dog leg so that the bottom of the garden is a good 15ft wider than the top. The bottom boundary, as Mrs Dyson mentioned in her letter is formed by a magnificent old brick wall of 1720, at the moment rather overpowered by ivy.

The previous owner left the house in her will to the Society of Elderly Vegetarians. I thought this must be a Beachcomber invention, but the Dysons assure me it is not. Miss Doubleday, the elderly vegetarian in question, had lived in the Barnes house all her life and did not believe in killing things. The Dysons inherited a jungle with the bones of a late Victorian sunken garden buried deep within it. Crawling on their hands and knees through the undergrowth, they found the munitions hut, complete with patriotic flags, rise-and-fall lights, maps and boxes of earphones. A family of foxes lives under it now.

Despite the 16 years of clearing, the garden still has a dark, secretive air. It is richly planted with old roses and delphiniums, hollyhocks and geraniums, with big trees rising around the boundaries. Close to the house is a birch and in the far right-hand corner, an ash. The corner that the Dysons want to tackle is guarded by a superb tall holly. If you creep under its branches, you come to the empty corner where the bottom wall meets the left-hand boundary. It is hidden from the rest of the garden and I felt something should be happening there - a grotto perhaps or a strange obelisk - that would be a surprise and a pleasure to find after the long, meandering walk from the back of the house. Mrs Dyson has an antique shop, so might be able to put her hand on the odd grotto or crumbling stone statue.

The holly, a fabulous tree, needed some expert trimming, to make it more of a lure and less of a barrier. The Dysons had had the top taken out of it, leaving some stringy branches sprouting awkwardly from the truncated crown. If these were cut out, it would enhance the profile of the tree. The canopy of leaves could be lifted a little, too, so that, without losing any of the mystery, it would be easier to get under the tree into the secret corner.

The way to lift the canopy of a tree is not to whizz round the bottom with a chain saw, like a chef trimming pastry from a tart, but to run your hand up each individual branch and cut it where it joins on to a larger branch. In this way, you retain the natural, drooping, fringed outline that is characteristic of holly. A good arboriculturist goes with the flow of a tree. A bad one reduces everything - ash, elm, oak, holly, to interchangeable butchered blobs.

To the right of the holly tree lay the mounds left by the bonfires and compost heaps that used to occupy the area. They looked like what they were. They should be levelled out and the bonfire ash used round the borders in the rest of the garden. But looking at this specific area that the Dysons were wanting to tackle, you could see that in fact it was no good thinking about that corner in isolation. Whatever went on in the corner had to work with whatever was to happen along the rest of the bottom of the garden.

The Dysons were planning an island bed, with summer flowers on the right, where the garden was quite open and sunny. The holly was the only dominant feature on the left, together with the old brick wall. I suggested they stripped the ivy off the wall (there is plenty of undergrowth elsewhere in the garden for wildlife) and used it for plants which would be bold enough to sing out, even from the distance of the kitchen, on the first floor of the house.

Taking as the key the holly, at its best from autumn onwards with its shining leaves and berries, the Dysons could orchestrate an autumn and winter crescendo here at the bottom of the garden, throwing a giant-leaved Vitis coignetiae at the wall, and lacing it through with late-flowering Viticella clematises and the white-flowered Solanum jasminoides 'Album'. In autumn the leaves of the vine turn a fiery scarlet before falling. They would look superb with the holly.

Because of the burning and the composting going on down at the bottom of the garden, there hadn't previously been much room for shrubs, though there was a weary-looking hypericum there. To me, hypericum smells of nothing but bus stations and roundabouts. Out with it, I urged. In with a jagged-leaved mahonia to bloom now with cowslip-scented flowers and perhaps a bold hydrangea, such as H sargentiae which has leaves as rough as sharkskin, as big as dinner plates. The hydrangea would extend the season backwards a little as it flowers in July and August.

With these two landmark shrubs in place and the vision of the wall behind, clothed in bold climbers, you could begin to see how this space would work. You would wander down through the garden, drawn on initially by the imposing bulk of the ancient wisteria. This ate its original supports long ago and it is now jacked up from underneath with some very nifty carpentry carried out by Mr Dyson.

Coming round the wisteria, you would then be led on towards the holly with the mahonia and the hydrangea grouped to the right of it. There, an informal narrow path would lead past the shrubs and bend round to the left, under the holly, to bring you to the corner shrine. The main path would lift round to the right, run roughly parallel with the bottom boundary and curl itself round the summer island bed.

Since the holly will be taking a fair amount of moisture out of the ground and the soil itself is light and free-draining, the Dysons should not perhaps be too ambitious with their planting between the shrubs and the holly. They could introduce a few ferns, such as polypodys, which would not mind the dry situation. They would give a little height and a great deal of finesse to the scheme. Then they could carpet the ground underneath with masses of bulbs such as snowdrop, scilla and the corms of spring- and autumn-flowering cyclamen. The grotto/obelisk would moulder quietly away in its corner, attracting mosses and lichens, perhaps with ivy licking around its feet. Although you would not be able to see it from the house, the Dysons would know it was there - a powerful, hidden secret.

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