Gardening: Green leaves are my delight: Brightly coloured and fashionable blooms are all very well, but the essence of a successful garden lies in its foliage, says Anna Pavord

Foliage first, flowers later. That dictum ought to be scratched into the concrete of every patio in the country, emblazoned on every garden gate, carved on the handle of every garden spade. An obsession with colour has been the distinguishing characteristic of most gardeners of my generation, brought up as we were to revere Sissinghurst, with its single-colour borders, above all other gardens, and to believe that the only true path to the gardener's nirvana lay through agonised choices about the exact nature of the flowers we put in our patches.

'Darling] Salmon] How brave]' exclaim the white-garden brigade as they sharpen their pruning knives for a horticultural mercy killing. You might as well fall on your garden fork there and then as try to explain that the point of the plant group in question is not the salmon flower but the whorls of bronze rodgersia leaves behind it.

This obsession with colour and with particular modish flowers has obscured a vital tenet of good gardening. Leaves are far more important in creating satisfying and enduring planting schemes than flowers. A garden that is all flowers is like a cake that is all icing. Especially where concrete, tarmac, noise, dust and mayhem rule, you need cool, still oases of green to set off occasional bursts of colour.

When you first start to garden, you are seduced by flowers. You open a catalogue. You go to a garden centre. You see only the colour of things. You hoover up plants indiscriminately, favouring the ones that are actually in flower at that moment.

But being impressed by a plant because of its flowers is like judging a man on the basis of his Armani jacket. Some plants, like some people, have little more to offer; they may be fine in a crowd, but you wouldn't want to spend long with them on your own.

Annual flowers, for instance, rarely have impressive leaves and consequently look much better when they can borrow the foliage of other plants, such as helichrysum or ivy. They should not be planted on their own, but you would not wish to be without them entirely. They are the curlicues, the garnishes, in a garden, but they should not constitute the main course.

So what should? To get a sustaining backbone you need plants that look their best for more than a six-week flowering period. When you swoop on a plant in flower at a garden centre, ask yourself: 'What will this thing look like without its flowers? Will it develop an interesting shape? What are its leaves like?'

If you judge a plant by these criteria, flowers then become a bonus, rather than the raison d'etre. Some of these backbone plants should be evergreen, so that in winter the garden does not entirely dissolve into a skein of skeletal branches.

The spurge Euphorbia characias is a great ally in this respect, making solid mounds of evergreen foliage whorled round stout, upright stems. At the start of the year, the stems that are going to flower turn over their tops, so that the clump begins to look like a small flock of inquisitive chickens. After that come the huge lime-green flower heads, which last for months. But even without the flowers, this is a winner and, in my garden, grows in sun or shade in soil both soggy and well drained.

This euphorbia, like the hosta, is a foliage plant with a capital F. Other plants are not so firmly labelled. You might leap on the crocosmia, for example, because of its arched sprays of brilliant red flowers ('Red? Darling] Wonderful on Chelsea pensioners of course but . . .'), but you will gradually find that the long, pleated, sword-shaped leaves are of equal consequence. You need these sharp verticals to give variety.

Use the brilliant crocosmia 'Lucifer' with the dark-leaved dahlia 'Bishop of Llandaff' which has flowers of an equally outrageous colour. You will have chosen both plants on account of their late summer display, but long before either of them comes into flower (about now) you will have had the pleasure of the contrast in foliage.

All foliage has bulk. Not all has beauty. Sometimes you might be prepared to trade off boring leaves for the sake of another attribute. I would not be without philadelphus (mock orange), despite its tedious habit and wearisome leaves, because of the smell of the flowers. You float on that smell. You feel good about the world. You love your neighbour. For that, you are prepared to let slip a leaf that is no more than leafish.

So I am not saying that you should have nothing in your garden that you could not defend in front of a foliage tribunal, only that you should have a good reason for including anything that would not stand up in such a court.

Where, as with the crocosmia, you get foliage and flowers of equal value, you are in clover (another double act). These are the plants to favour. Think of choisya, a shrub with heads of heavenly scented white flowers in May. But the leaves are also handsome, glossy, evergreen, arranged like three long fingers on the stalk. While you are waiting to see whether you will get a second crop of flowers (you sometimes do in late summer) the choisya can lend its leaves to a clematis, which is far happier wandering through some living support of this kind than plastered alone against a trellis.

Rodgersia is another winner. I am now swimming in rodgersias, having discovered that they do not need an actual bog to live in, but are perfectly happy in ordinary soil, provided it is not a dustbowl. They grow well in my heavy clay, and there is not a dud in the whole family. The flowers are coming out now, showy plush plumes of pink or white, but the plants have already paid two months' garden rent with their outstanding leaves.

Rodgersia aesculifolia has glossy bronze foliage, the leaflets radiating out like the spokes of an umbrella from a central stalk, the end of each leaflet bluntly cut like a horse chestnut leaf. Rodgersia pinnata has leaves similarly arranged, but without the gloss. The leaflets do not have the same blunt end and the plant generally is less massively built. Ferns are natural companions, either the tall shuttlecock fern, Matteuccia struthiopteris, or the more delicately built maidenhair, Adiantum pedatum.

For new ideas on gardening with tropical foliage plants, read Exotic Gardening in Cool Climates by Myles Challis (Fourth Estate, pounds 12.99). Susan Conder's Variegated Plants (Cassell, pounds 20) is a useful guide in its specific area.