No garden should be without a tree and now is the time for planting one.

Taller trees set off lower-growing shrubs and flowers, adding an extra dimension to your garden design. They may have blossom and autumn colours to put on a show and birds and other wildlife appreciate the shelter and food available. During a warm and sunny spell, trees offer welcome shade and in the autumn and spring, other plants get protection from howling gales.

Choosing a new tree is above all fun. The principal considerations are its final height and whether your garden is sufficiently big. In a tiny garden a 5m-high tree might be too tall, its branches spreading too widely and casting too much shade. The roots spread just as widely, sucking up all available moisture and nutrients.

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Mistakes are all too easy to make and might take years to become apparent. Other than horrors like Leyland cypress, most trees grow very slowly and you'd scarcely notice the shady pall that blocks out the sun. Some species, such as Acer palmatum or Japanese maple, give you whatever height and colour and shape of leaf you fancy, ranging from 1m to 6m in height. You need to be careful with betulas (birches). Although a tree like Betula pubescens grows slowly to a respectable 3-5m, it eventually reaches 10m. Read the plant label carefully in the garden centre or be prepared to get out the chainsaw if your tree outgrows your space after 20 years.

There are also your neighbours to consider. A large tree could make the next-door garden cool and shady, and spreading roots might steal nutrients from beyond your boundary, eventually causing structural damage.

Planting a tree in October lets it establish some roots and settle in before winter, readying it for a good start in spring. Although a tree is largely dormant after leaf fall, its roots keep growing until the middle of December. In 2007, scientists at the University of Basel in Switzerland found tree roots grew while the soil temperature was higher than 6C. Last year, soil temperature here gradually fell during the autumn, averaging 12C in October, 8C in November and below 6C in mid December. February was the coldest month, but by the middle of March the soil became warm enough for tree roots to start growing.

By planting over the next few weeks, you give your tree the best chance of thriving. The Basel researchers also found unexpectedly high spring temperatures encouraged shoots and buds to burst into life. When buds began to open, they lost moisture through transpiration, so needed to draw liquid up from the roots. While this wasn't a problem for trees with established or establishing roots because they could supply the necessary moisture, trees planted in late winter had much smaller root systems which couldn't supply enough moisture and the tree became quite badly stressed.

This advice applies both to bare root and container-grown trees and is essential for the former, which are the best type to buy because they are much cheaper and nurseries offer a broader choice of varieties. When the tree arrives from a nursery, its roots are wrapped in a hessian or plastic bag to conserve moisture. Be sure to keep the roots moist until planting.

Prepare a square hole three times the diameter of the rootball and the depth of a spade. Loosen the soil to ensure good drainage, otherwise the water that enters the hole over winter could drown the young tree. Tease the roots apart and place in the centre of the hole.

The point where the roots join the trunk is known as the root collar – the slight swelling round the trunk. When packing soil round the tree, keep the collar above ground level. The whole trunk, including the collar, needs to breathe and the roots, which have to exchange oxygen as well, will die if they're too far below the surface. Deep planting is much worse than shallow planting. Gradually pack the excavated earth round the tree, gently firming the soil round the roots with your boots. Completely fill the hole, tamp round the roots and check you've planted the tree to the correct depth. Then water in.

Young whips – single-stemmed trees with few if any side branches – are cheapest and easiest to plant. If you're planting a larger tree, you'll require a stake to prevent the newly planted specimen from rocking in the wind, thereby damaging new root growth. Drive a 1.5m post diagonally into the soil, close to the tree on the windward side, with the centre of the post ending up opposite the trunk, then join the tree to the post with a special tree tie. n

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