If I buy just one tree, which should it be?

Westonbirt Arboretum, near Tetbury in Gloucestershire, is a wonderful place to look at dogs. On the Autumn Colour Trail this week was the most elegant Gordon setter I have seen for a long time, his black and tan coat gleaming against the brilliant yellow foliage of a shagbark hickory. From the understorey of box bushes burst wild braces of springer spaniels, flushing out the pheasant that existed only in their overheated minds. Staffordshire bulls with chests as impressive as Sly Stallone's rolled their way down the wide rides, between tall stands of Bishop's pine.

Like a threnody woven through the birch and the beech, the oaks and the acers, was the wailing voice of a woman in a Barbour jacket and boots. "Candy, Candy, here, here, HERE." Candy, a staggeringly capricious cream- coloured labrador, dashed across Willesley Drive in the south part of the Silk Wood with the Barbour jacket in close pursuit. She was there again at The Link, and, still screeching, in the Broad Drive. As the labrador lollopped past us for the seventh time, it turned its head and gave the closest thing to a wink I've ever seen on a dog. The two of them may be there still. Certainly the dog looked as though he could keep up the game for days.

What I was supposed to be doing was looking for an answer to a perennial question, one that crops up often in readers' letters. If there is room for only one tree in a garden, which tree should it be? For me, it would be a pear. Not the poncy, silver-leaved kind, but a proper pear, with snow white blossom in spring and melting russet fruit in autumn.

I like the shape that a pear tree makes, rather narrow in proportion to its height. In a small garden, that is a useful attribute. So is the fact that a pear tree has more than one season of interest. That is easy to forget in the great lemming rush to the garden centre at Easter. But ask yourself always, "What comes after the blossom?" In the case of many flowering cherries, the answer is a deafening silence.

Be clear in your mind what you want the tree to do. If it has to screen some hideous eyesore, then height will be an important consideration. Do you want to sit under it and eat supper in summer? If so, you will not want a tree that drops its branches too low to the ground.

Above all, you need to be realistic about the tree's size and shape when mature. Weeping willows may look romantic in a nursery bed, but are totally unsuitable for all but the largest gardens. They grow fast, not only up but out. If you must have one, choose not the common weeping willow, Salix chrysocoma, but the manageable S. purpurea 'Pendula', the purple osier which, trained as a standard, makes a charming small weeping tree that stops growing at 16ft.

The blue Atlas cedar, Cedrus atlantica 'Glauca', is another species that looks very fetching when it is small. Do not be taken in by this winsomeness. It will be a cuckoo in most suburban nests, growing to at least 80ft. If you want an evergreen, plant box or holly, both natives, but very slow growing. This is what puts people off them. But if we plant only instant, short-lived trees, what will be left for future generations to enjoy?

There were some superb hollies at Westonbirt such as 'J C van Tol' with dark, shining, almost spineless leaves and huge crops of berries. I've also got a weakness for the silver hedgehog holly called 'Ferox Argentea'. It is well-named, ferociously difficult to deal with as it has prickles not only on the edges but also on the surface of its leaves. The spines in this variety are creamy white and there is a band of the same colour round each leaf. But it is certainly not a tree for impatient gardeners. Even for a holly, this is slow.

Purple-leaved trees can be oppressive in small gardens. They are spectacular when the leaves first emerge in spring but as summer progresses, the colour becomes ever more heavy and dismal. If your garden is exposed, then avoid exotic species such as catalpa, the Indian bean tree, which has large leaves that are rather thin in texture. When the wind blows, they will tear rather than flutter. Robinia has small feathery leaves, offering no wind resistance at all, but the wood is brittle and liable to snap in a gale.

Nurseries sell trees as whips, feathers, half-standards and standards. Some also offer extra heavy standards, or what they call advanced stock, which can mean a tree up to 20ft high. This will be difficult to establish and keep on its feet without expert care. Whips have a single stem, feathers have small side branches. A half-standard will have a stem clear of branches four to five feet from the ground. A standard has a clear stem of five to six feet. Both kinds of standard should have been pruned to give a well balanced canopy of branches. Whips and feathers are much cheaper, but you have to take on the responsibility of training them.

When choosing a tree at a garden centre, bear in mind that biggest is not always best. In its natural state, a tree has a root system as big as its top canopy of branches. Where trees are for sale, this is rarely feasible, but the bigger the disparity in the proportion of roots to shoots, the bigger the difficulty in getting the tree established.

Where trees are container-grown, a good nursery will re-pot them as they grow. As a rough rule of thumb, the tree should not be more than five times higher than the width of the container. Avoid top heavy plants.

Trees kept in pots have a distinct tendency to make roots which whirl around in a restrictive spiral. It is difficult to straighten these out as you plant, so the tree never has a chance to anchor itself securely in the ground. Bare-rooted trees, carefully lifted any time after leaf fall, will often have better root systems than large container-grown trees. Some trees however, such as davidia, liriodendron, eucalyptus and nothofagus (southern beech) resent disturbance at the roots and are better bought in containers.

Nurseries: Burncoose and South Down Nurseries, Gwennap, Redruth, Cornwall TR16 6BJ (01209 861112). Dulford Nurseries, Cullompton, Devon EX15 2DG (01884 266361). Thornhayes Nursery, St Andrews Wood, Dulford, Cullompton, Devon EX15 2DF (01884 266746). Barnsdale Plants, Exton Avenue, Exton, Oakham, Rutland, LE15 8AH (01572 813200). All do mail order.

Westonbirt Arboretum, Tetbury, Gloucs, (01666 880220) is owned and maintained by the Forestry Commission. Open every day 10am-dusk. Admission pounds 2.60 (pounds 3 in October).

1Snowy mespilus (Amelanchier lamarckii) Twiggy low-domed tree with masses of small starry flowers in April. The leaves turn rust red and yellow in autumn. Height 15ft, spread 10ft. Thrives in any good garden soil.

2Strawberry tree (Arbutus unedo) Small evergreen tree which eventually grows into a gnarled Arthur Rackham shape. Bell-shaped white flowers in autumn and red strawberry-shaped fruits. Bark shreds and peels. Not reliably hardy. Height 20ft, spread 10ft. Plant in a sheltered position. Protect young plants with mesh or bracken.

3Thorn (Crataegus prunifolia) A small, compact tree eventually developing a broad head. Showy round red fruit which stay on the tree well into winter. Leaves colour richly in autumn. Does well in light shade. Height 17ft, spread 14ft.Crataeagus x lavallei is equally good.

4Crab apple (Malus hupehensis) A neat small tree, with stiff, upright branches. Scented flowers in abundance are followed by yellow fruit flushed with red. Height 16ft, spread 12ft. Does best in full sun.

5Kashmir mountain ash (Sorbus cashmiriana) Hanging clusters of white flowers in late spring and feathery, pinnate foliage, similar to our own native mountain ash. Clusters of fruit, pearl-white on red stems, persist well into winter. Height 13ft, spread 12ft. Sorbus hupehensis is as good, but more vigorous.

6Judas tree (Cercis siliquastrum) Clusters of purplish flowers without stems break directly from the branches in spring. Very pretty rounded leaves, which turn yellow in autumn. Light grey green pods of seeds persist through the winter. An elegant, slow-growing tree, best in light shade. Height 20ft, spread 20ft (but not for 50 years or so).