Gardening: Wonder wall

In the third part of our series on tackling garden problems, Sarah Raven explains how to soften the appearance of walls and paving

YOU KNOW what it's like when you meet a man who is too clean-cut? Who looks as if he's never broken sweat? It's all wrong, isn't it? What you want is a bit of rough, some proof that life is being lived there. The same goes for a garden. Paths and walls look too clean and too perfect when they're new. The corners haven't been softened by frost, and there is no sign of a plant. In summer, when there's lots going on in the flower beds, the expanse of stone, brick or concrete looks like a motorway carved through the middle.

The fast route to avoiding the immaculate look is to build them imperfectly. Use more expensive reclaimed brick if you can, and don't be a slave to the spirit level. We took this to such an extreme with a new wall we were building here that visiting council planners said it looked like a medieval monastery.

For a terrace, path or steps, the same applies. Don't get them perfectly set in hard concrete, with the pointing laying flush with the brick, pavers or stone. Use a soft mix with a ratio of five parts sand to cement (instead of the usual three) and lay the slabs on top, leaving some 1-2in cracks in between. The softer base allows the pavers or bricks to sink or rise a little, creating a bit of irregularity.

If you have inherited a path, terrace or wall when moving to a new house, you can use a hammer and chisel to knock some corners off and create gaps so that the plants can set seed and start colonising. Chisel out small sections of mortar an inch long and an inch deep in the wall or steps: such small holes won't affect their structural integrity.

There are a whole series of self-seeding plants which will transform virgin walls and terraces within a couple of years. You want fluffy, cascading things, which billow out and flow down vertical steps or walls and soften the edges of terraces and paths. Eryngium, or sea holly, is a great self- seeder, but it is too upright and spiky for this job. The fluffy acid- green flowered plant Alchemilla mollis (Lady's Mantle) will scatter itself into paths and Geranium dalmaticum also grows well in cramped conditions, but these take time. A few more rampant plants are required to get started.

For a completely fail-safe path and wall-cladding plant, go for valerian. This flowers right the way through late spring and summer in dark pink (Centranthus ruber), red (C.r. `Coccineus') or white (C.r. `Albus'), producing a confetti of seed from one plant. Bees and butterflies love them, and they will flower for longer if you deadhead. Cut them hard back to the base straight after their first flowering and before they have had a chance to seed, and you will get a spectacular second crop of flowers in September.

Valerian finds its way into the tiniest nook or cranny and will germinate in the least hospitable-looking place. It is so promiscuous that it will clad a wall in five years, but its fleshy roots will spread in the mortar and widen cracks that are already there. English Heritage won't let it near an ancient monument.

The red valerian looks superb on a wall with the small blue bellflower, Campanula portenschlagiana. This used to be called C. muralis and is a robust, evergreen perennial, perfect for spreading into cracks and colonising walls. For a deeper purple-blue, with larger flowers, look out for the cultivar `Birch Hybrid'. Both this and valerian thrive in bright shade, so you can plant them facing north or east, and in the sun.

If these sound too invasive, try the perennial wallflower Erysimum `Bowles' Mauve'. This has lovely purple flowers through the spring and summer and evergreen, grey-green leaves. You could mix this with Erinus alpinus, the fairy foxglove, which looks like a glorified toad flax in pink, purple or white. It likes dry rock crevices and freely self-seeds.

Particularly good in steps and paths is the white-daisy-like flower Erigeron karvinskianus (syn E. mucronatus). This carpeting, bushy plant flowers in only three months from seed, and continues to do so until the autumn frosts. The flowers open white and deepen to rich pink as they age. It will self-seed in less than a year: E.k. `Profusion' is a free-flowering form.

You can grow the valerian, campanula, erinus and the erigeron from seed (available from Chiltern seeds Bortree Stile, Ulverston, Cumbria LA12 7PB; tel 01229 581137). Sow them in the spring or early autumn in a cold frame without any heat, or place the seed trays against a shady wall. They will germinate in less than a month and form a couple of pairs of true leaves two to three weeks after that, when they should be pricked out. Do this into individual 1-2in cells, choosing the size according to the holes where you will plant them. Push the seed plugs into the wall in September or October. If any die during the winter, sow again in early spring, putting the seedlings out in late March/early April to fill vacant holes.

The erysimum is a sterile hybrid, so you can't grow it from seed. Propagate it from semi-ripe cuttings taken now. Remove 2-3in non-flowering side branches from a plant bought at the garden centre, strip any bottom leaves and push the stems into a well-drained gritty mix of compost. They should root in a couple of weeks. Pot them on and slot them into the wall in October with your seed plugs. Keep a few plants back in case some die over the winter, potting them on regularly. When you put them out in the spring, make sure you water them in properly.

Jam soil into any other cracks between paving stones or in walls. Once established, plants will shower the expanse of bricks or stone with their seeds and their offspring will colonise any hole with enough room and food for them to grow.

This week

1 July is the perfect time for garden visits. The wall beside the entrance of Harold Peto's Italianate garden at Iford Manor, Bradford-on-Avon is covered in a brilliant mix of valerian and campanula. They have delicious cream teas too. Tel 01225 863146

1 The flower-covered steps and walls of Christopher Lloyd's garden at Great Dixter in East Sussex are also looking their best right now. It is well worth a visit to the huge collection of clematis for sale at the nursery. Tel 01797 252878

1 The moat walk of Sissinghurst Castle Garden, Sissinghurst, Kent has a wonderful scattering of Erysimum `Bowles' Mauve' all the way down its length and now is the time to see the white garden. Tel 01580 715330