Gardening: Flower power on high

What plants do best in the window box? Cathy Packe provides the answers

Traditionally, window boxes have been regarded as the playground of the urban gardener: a tiny patch where those unfortunate city-dwellers who have no land to call their own can seek temporary refuge in a rural dream.

But this is no cheap alternative for those who are deprived of the real thing: in fact, many would say that the smaller the area involved, the greater the gardening skill required.

Geraniums, petunias, fuchsias, nasturtiums and trailing lobelia are currently filling the window boxes around my part of central London. Many of the displays are beautiful, but in a few more weeks most of the plants will be past their best, if not dead, leaving the gardeners responsible with a choice: to do without a window display until next summer, or to spend money on buying more seasonal plants.

The gardens round here are small, so we have room for either a greenhouse or flowerbeds, but not both. As a result, most of us have nowhere to raise seedlings and nowhere to overwinter any window-box plants that may be worth saving. On the face of it, paying pounds 3 for half-a-dozen bedding plants does not seem a huge expense; but if you have a home that lends itself to window boxes - I have seven boxes that I like to keep planted all year round - buying new stock every season can be a pricey business, which makes gardening an expensive hobby.

If you work for a big city company - a large bank, for example - you will be used to seeing the plantscape around you changing regularly. Carefully maintained window boxes are an important part of a corporate image, but at a price that is not realistic for most domestic gardeners.

Windowflowers, the company that adorns many of the buildings in the City of London, as well as several hotels and major stores, is responsible for planting three-and-a-half miles of window boxes. They change their clients' displays five times a year (making an annual total of 17-and- a-half miles) to give a non-stop supply of seasonal colour and interest.

With a little advance planning, most of us could achieve a similar effect, but at a fraction of the cost. The secret is to create a framework that can remain in the box throughout the year and can then be embellished with a few bulbs or bedding plants according to the season.

A good window display is meant to be appreciated mainly from the outside, so you should consider the shape of the window and the style of the house when you are planning the design. Symmetrical Victorian- or Georgian-style houses usually look best with a formal style of planting, whereas a modern house can cope better with a more haphazard approach.

Height in a display is important, but remember that if your sitting-room is on the other side of the window, that two-foot conifer or box tree that looked so attractive in the garden centre could deprive you of a certain amount of light. It is also important to have a number of trailing plants to bring the eye down below the box itself. This makes it pointless to spend a lot of money on a container which is a work of art in its own right, since, in a good display, most of it will be covered up anyway.

The obvious choice for a trailing plant is ivy; this can be far more interesting than it sounds if you branch out from the common Hedera helix in its plainest form and choose the variegated types such as 'Glacier' (green and white) and 'Gold Heart' (green and yellow), or the more interesting Hedera helix 'Sagittifolia', whose leaves are shaped like an arrowhead. When these get overgrown, you can chop a bit off and pot them up to keep in the house, or pass on to a neighbour in exchange for another plant.

A more fragrant alternative to ivy is trailing rosemary, which looks and smells good all year round, and has blue flowers in summer; catmint can also be persuaded to trail without much difficulty; and there are various kinds of thyme that have attractive leaves and which will soften the edges of the container - a good one to try is the yellow-tinged x citriodorus 'Aureus'. Any of these can be interspersed next summer with whatever bedding - trailing geraniums, lobelia, fuchsias - you care to add in.

Structure and height can be achieved by planting almost any small, evergreen plant, according to whether you want it to be simply a shape, like a small conifer, or a subject of interest in itself, such as a small variegated hebe or euonymus. These can be used to provide a framework around which you can add various seasonal extras.

Once you have created a permanent structure, it is pointless to keep pulling it apart just because the parts of it have reached the end of their natural life. A way around this is to add in a few empty pots when you plant up the box for the first time. You then plant up pots of the same size with whatever you choose for the particular season - pompon chrysanthemums, perhaps, as autumn approaches, to be followed later in the year by hardy cyclamen, then Iris reticulata, Primula malacoides and finally petunias. As the contents of each pot begin to fade you just replace it with a new selection without having to replant the whole box. An advantage of this method is that you can always pop in something special for a celebration: white flowers for a family wedding, or red for Christmas.

There are different approaches to the question of colour schemes in window boxes. You can go for as many colours as possible; this is a technique that for some reason is used frequently outside pubs, and often to great effect; but it can look overwhelming in the average house. Experienced window-box planters reckon that a range of colour is important, but you shouldn't try to represent the whole spectrum in a small space.

The ingredient most vital to any window box is water. Even in a wet summer like this one, container plants can quickly dry out. The smaller the box, the more often it is going to need watering: Jill Brown, who designed the winning window-box display at the Chelsea Flower Show this year on behalf of the Coventry Fuchsia and Geranium Society, reckons to water hers at least once a day. The professionals at Windowflowers water once a week, except in a heat wave, but their secret is to use the largest box they can fit into the window. They also advise blocking up any drainage holes in the base of the container, and drilling some at the sides instead, about half-way up. This creates a reservoir at the bottom, which you can keep topped up to prevent the whole box drying out.

As this year's bedding plants start to fade over the next few weeks, it is worth considering the size of the financial investment which is, in effect, heading straight for the compost heap. Most of us would be appalled at the prospect of replanting a flower bed every summer; a bit of planning and a careful browse round the garden centre this autumn could mean that we don't have to keep replanting our window boxes, either.

Windowflowers can be contacted on 01628 667227