Roots manoeuvres: Choosing the right tree for your garden is only the beginning

The Westonbirt Arboretum, in Gloucestershire, is a wonderful place to look at dogs. On a trail there last week was the most elegant Gordon setter I have seen for a long time, his black and tan coat gleaming against the last brilliant yellow leaves 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 big 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 hot pursuit. She was there again at The Link, and, still screeching, in the Broad Drive. As the labrador lolloped 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 several days.

What I was supposed to be doing was looking for an answer to a question 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 russeted fruit in autumn. I like the shape that a pear tree makes, 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?")

Be clear in your mind what you want the tree to do. If it has to screen some hideous eyesore, then height will be important. 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 mature size and shape. Weeping willows (Salix babylonica) may look dreamily romantic in a nursery bed, but are unsuitable for all but the largest gardens.

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 25m. 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?

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 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 – trees up to 6m high. They will be difficult to establish and keep on their feet without expert care. Whips have a single stem, feathers have small side branches. A half-standard will have a stem clear of branches 1.2m-1.5m from the ground. A standard has a clear stem of 1.5m-1.8m. 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.

The National Arboretum at Westonbirt, Tetbury, Glos GL8 8QS, 01666 880220, is owned and maintained by the Forestry Commission. It is open every day (9am-5pm), admission £8. See forestry.gov.uk/westonbirt

Six of the best: Trees to plant

Shadbush (Amelanchier ‘Ballerina’): twiggy, low-domed tree with masses of small starry flowers in April. The leaves turn red and purple in autumn, with fruits ripening to the same colours. Height 6m, spread 8m. Thrives in any good garden soil.

Flowering cherry (Prunus ‘Shogetsu’): a relatively small cherry, with a wide, flattish head, producing in late spring hanging clusters of double pink and white flowers. It is one of the last of the cherries to bloom. Leaves turn orange and red in autumn. Height 5m, spread 8m.

Thorn (Crataegus persimilis ‘Prunifolia’): a small, compact tree eventually developing a rounded head. Showy round red fruit which stay on the tree well into winter. The polished oval leaves colour richly in autumn. Does well in light shade. Height 8m, spread 10m. Crataeagus x lavalleei is equally good.

Crab Apple (Malus transitoria ‘Thornhayes Tansy’): a neat small tree, with masses of white blossom, amber coloured fruit and foliage that colours well in autumn. Height 8m, spread 10m. Best in full sun.

Vilmorin’s Mountain Ash (Sorbus vilmorinii): a dainty tree with wide heads of white flowers in late spring and feathery, pinnate foliage, similar to our native mountain ash. In autumn the leaves turn burgundy red. The clusters of fruit are at first red, then pink, finally white. Height and spread 5m. Sorbus commixta ‘Embley’ is as good, but more vigorous.

Judas 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 winter. An elegant, slowgrowing tree, best in light shade. Height and spread 10m (but not for c.50 years).

All the trees above are available from Kevin Croucher at Thornhayes Nursery, St Andrews Wood, Dulford, Cullompton, Devon; 01884 266746 or thornhayes-nursery.co.uk Most stock is field grown and lifted from November onwards. Delivery can be arranged on orders over £100. The catalogue (which includes a useful list of “Trees for the Terrified”) is free.