In cities, window boxes are vital reminders of nature. So don't let them wither in winter: there are plenty of things to plant. By Anna Pavord

Town houses need window boxes in a way that country houses do not. In a terrace of town houses, fronting straight on to a pavement over a sub-basement, a window box may provide the only possible way of greening up the view. This was clearly understood at the time that many city streets were first being filled with new houses. "What are the casements lined with creeping herbs/ The prouder sashes fronted with a range of orange, myrtle?" asked the poet William Cowper. He answered himself a few lines later. "Are they not all proofs, that man immured in cities, still retains/ His inborn, inextinguishable thirst/ Of rural scenes, compensates his loss/ By supplemental shifts, the best he may?"

At the height of the window box boom in the 1870s, Shirley Hibberd, gardening guru of the day, was recommending at least four changes of plants in a year. We, in an era which has seen an unprecedented boom in garden spending, should at least be able to manage two. The petunias are rotting, the geraniums are wondering where the sun has gone, the nasturtiums have thrown in the trowel. It is time for one of Cowper's "supplemental shifts". But what to?

That depends where you live. In London, where central heating escaping from inside keeps window boxes in an almost Mediterranean micro-climate, you can use plants generally grown indoors to great effect in a window box. Big florists' cyclamen, for instance, enjoy the outside cool of a window box rather more than they do the over-heated dry atmosphere of a window sill inside. But they will melt at the first touch of frost.

Displays made from plants that are all the same colour have more impact than three or four plants all of different shades. Use a pale winter flowering heather to fill in the gaps. The cyclamens' own juicy, marbled leaves contrast well with the wispy foliage of heather. If you want white cyclamen, choose a dark-flowered heather. Prices of cyclamen fluctuate wildly, the best bargains of course being in street markets such as the Sunday scrum at Columbia Road in London's east end.

The bushy little winter cherry, Solanum capsicastrum, a half-hardy sub- shrub with brilliant little round fruits of orange, red and green, is also happy to be left out - but only in places where it is not likely to get struck down immediately by frost. It only grows about a foot high, so is ideal in a window box. One such plant had self-seeded itself into the paving of Janis Leggott's garden, featured in this column last week, and she said it had grown outside very happily for the last few years. You could combine it with pale variegated ivy, which tips over the edge of window boxes to make soft curtains of greenery.

For a box outside a kitchen window, you might think during the winter of having some evergreen herbs that you can reach out and pick when needed. Thyme, rosemary, sage and marjoram will all grow successfully in window boxes. You could use mounds of purple or variegated sage interplanted with the decorative kales that have become so popular. Or plant the narrow- leaved grey sage 'Hidcote', with bright green moss, curled parsley and a scattering of pansies for colour.

For bravery in the face of the enemy, scarcely anything can beat a pansy. I was watching some in a window box in Elgin in the north of Scotland recently, when terrible westerly gales were ripping through the streets. Though blown horizontal by the wind, the pansies stuck grimly to their posts and continued to fly their standards. You would not think that such a large flower could stand that kind of battering, but it does.

Winter flowering pansies available in garden centres now, are grown from seed sown in May or June, and there are certain seed strains - such as Universal Plus, Floral Dance and Ice Queen - which produce pansies in a wide range of colours. If you were planting pansies with coloured kales, you might choose 'Universal Plus Ivory Rose Blotch', which has flowers with deep pink moustaches standing very clearly out on a cream ground. Or you might want to make a more sumptuous statement by planting deep purple pansies amongst arching clumps of the black grass Ophiopogon planiscapis, 'Nigrescens'. It is evergreen and not more than nine inches high.

Height is an important consideration when planting out flower boxes. Flowers naturally turn towards the light, so from inside you are backstage, as it were, looking at the supports rather than the painted backdrop of the window box productiom. You can sometimes get over this difficulty by dropping the level of the window box, but this will depend on the type of window and sill that you have.

Flowers are generally happier in window boxes that are in a sunny position, though cyclamen will thrive in shade. If you have a very dark area, perhaps a sub-basement where you would like to try a window box, think of ferns. You could grow the shiny strap-leaved harts tongue fern together with the frilly, lacy fronds of a Polypodium such as 'Cornubiense', which is evergreen. Ivy is a natural companion for ferns. Try the pale greyish- leaved 'Adam' for a cool effect, or the golden variety 'Buttercup' for a warmer display. Scatter snowdrop bulbs in the gaps for early spring.

Although plants such as florists' cyclamen and winter cherry will give instant colour to see you through the next couple of months, before you plant them you need to think about what happens after winter. Set a quiet time bomb ticking under the winter cherries by planting a thick layer of bulbs at the base of the container. You need to plant much more thickly in a window box than you would in a border. When the display is over, lift the bulbs (which will be exhausted by the cramped conditions) and plant them out in the garden. If you don't have one, give them away to somebody who has! They will eventually fatten up and get back into flowering fettle.

Concentrate on bulbs that perform early - by May you will be getting itchy fingers and eyeing up the summer bedding. Think also about the relative heights of the plants. Flowers that are too tall will look ridiculous outside as well as in and will be far more likely to snap off in the wind. Hyacinths are gorgeous in a window box because they are themselves so waxily unreal, and you can cheat the seasons by buying bulbs which are already in bud. On warm still days, you will be able to open your windows to let the rich, swoony smell of the flowers drift indoors.

Tulips of the Kaufmanniana family are ideal as they have such interesting leaves, striped and mottled like snakeskin. They are rarely more than eight inches high and mostly flower in March. Greigii tulips such as the famous scarlet 'Red Riding Hood' are equally suitable, as well as some of the showier species, such as T. praestans, 'Fusilier'. Most other tulips will be too tall for window boxes. Crocus work well, as do dwarf iris such as Iris reticulata. Start winter plants off in fresh compost. Old compost is as dispiriting as someone else's sheets.

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