Scaffolding poles are ideal in urban spaces - paint them black and they vanish
TRAIN JOURNEYS into town are interesting if you're a garden designer - you get to have a nose into other people's backyards. It's good therapy if you're worried about the number of people taking up the profession, because there are still plenty of gardens in need of urgent attention. Some people make an effort with their space, but there's always the odd pergola. The reality of a rickety structure collapsing under the weight of a malevolent Russian vine is a million miles from the ideal of a walkway dripping with bloom. The fact is, building a pergola is a fiddly task and - unless you take time to do a good job yourself - the installation isn't necessarily cheap.

It's worth pointing out that I'm talking about pergolas and not arbours. As I see it, a pergola is a planted structure you can walk under, connecting two or more spaces. Built around the whole garden, it becomes a cloister in which strategic seating can create a framed and enlivened view. An arbour sits in isolation as either a focal point or a resting station. Each can bring an element of height to a flat garden but can equally add a touch of ... how should I put it? Well, a touch of tackiness.

Getting the proportions right is the most difficult exercise. It needs to be at least two metres high, so you can walk under it comfortably. This sounds obvious but I have experienced pergolas that make you bow as if entering a Wendy house.

The crossbeams need to be thick enough to span the path (which should be no less than 1.5m wide) and support the weight of the plants. Typical dimensions using timber are 75mm (3in) by 75mm (3in) for the uprights, with crossbeams measuring 150mm (6in) by 150mm (6in) by 50mm (2in) thick.

Thicker uprights make the structure more stable but the overall appearance can become lumpy, especially if you only plan to grow something like clematis up it. Thicker uprights generally require an increase in the width of the overhead boards, and with all that bulk, a thick-stemmed climber such as wisteria or a voluminous rose would be more suitable. Wider crossbeams make the structure look more comfortable and allow a little creativity with the overhangs.

A modern design with uprights and overhead beams all of the same dimensions will work in the right setting but attention to detail on the joinery is crucial. Wires can be used instead of crossbeams but can look pathetic and unless they're given suitable tension they will sag under the weight of any vine. The thought of a pergola without plants might seem odd but, well-built and well-placed, such a construction can make a powerful focal point - bands of canvas can be used to provide shade.

Overall scale should relate to the size of the garden and nearby buildings. Rustic poles have a charm of their own against a cottage garden; brick or stone piers with stout oak beams are common in large estates. Well- made metal arches can work beautifully but you get what you pay for and plastic-coated replicas look cheap and are usually more difficult to erect.

Durability will depend on the materials you use. Timber in the ground will eventually rot and, unless using sustainable hardwood, must either be pressure treated with preservative or fixed to a metal shoe set in concrete. Metal scaffolding poles are ideal uprights, especially in a contemporary setting. Painted black they almost vanish, making them most useful for urban spaces. In fact, the single most effective thing you can do to improve an existing pergola is to give it a paint job. Unless the colour ties in with the house or the boundaries, a darker colour will recede and let the plants do the talking. E