Think small, but think long

A tree is the single best thing you can add to a small garden. It gives height, shade, shelter for birds, a place to hang a swing, and a focal point no amount of bedding plants can match. The catch is that most trees are sold by the size of the pot rather than the size they will reach in twenty years. A modest-looking sapling can double in height in a decade and quietly fill a courtyard.

Before you fall for anything, find three figures: the tree's ultimate height, its canopy spread, and how far its roots tend to travel. Then measure your plot. In a garden roughly 7 metres by 6 metres, a tree that reaches 4 metres tall with a 3-metre spread sits comfortably and still leaves room for a border, a path and a patch of lawn. As a working rule, keep at least half the mature canopy spread between the trunk and any boundary.

Read your soil and light first

Most disappointment with trees comes down to a mismatch between the plant and the ground it went into. Spend a few minutes on this before you spend money.

• Heavy clay: wet in winter, baked hard in summer. Choose trees that tolerate both extremes, and improve the planting hole with grit and garden compost rather than digging a neat bucket-shaped pit.
• Light, sandy or chalky: free-draining and often alkaline. Woodland-edge and Mediterranean trees do well, but they will need watering through their first two or three summers.
• Sun and shelter: six hours or more of direct sun suits flowering and fruiting trees. A warm, south-facing wall is a bonus, effectively shifting you half a climate zone south.
• Shade and damp: anything under deciduous cover, or on the north side of a building, needs woodland species that evolved beneath a canopy. Only a handful of trees will sit happily with wet feet.

Good choices for sun and free-draining soil

• Amelanchier lamarckii — frothy white blossom in spring, berries in June, fiery autumn colour. Happy in most soils, including clay, and only about 5 metres at maturity.
• Acer griseum — the paperbark maple. Slow-growing, with peeling cinnamon bark that earns its place in winter.
• Malus 'Evereste' — a crab apple with white blossom, orange-red fruit and good disease resistance.
• Pyrus salicifolia 'Pendula' — the weeping silver pear. Compact and elegant, but it insists on full sun and sharp drainage.
• Cercis siliquastrum — the Judas tree, flowering on bare stems in May. Wants a warm, sheltered corner and lime-rich soil.

Trees for shade and heavier ground

• Taxus baccata — yew is the most forgiving evergreen for shade. Clip it as a cone or a loose cloud-pruned shape and it will sit happily for decades.
• Ilex aquifolium — native holly. Slow, dense and valuable for wildlife; choose a named female form if you want berries.
• Acer campestre — field maple, our only native maple. Tolerates clay, shade and pruning, and turns buttery yellow in autumn.
• Sorbus aucuparia — rowan. Slender, tough and generous with berries, and content in a damp or exposed spot.
• Betula utilis var. jacquemontii — the white Himalayan birch. Best planted in a group of three, and it copes with damp clay where drainage is reasonable.

Roots, boundaries and neighbours

Roots are rarely as dramatic as people fear, but they deserve thought. Trees draw moisture from a wide area, and on shrinkable clay this can, in extreme cases, affect foundations. Surveyors' guidance — often called the 45-degree rule — is worth checking if you are planting within a few metres of a house or a neighbour's wall. As a general principle, plant no closer to a building than half the tree's mature height, and further for anything vigorous.

Some trees are more trouble underground than above. Poplar, willow, eucalyptus and cherry plum send out questing roots, and the last two are notorious for suckering across a lawn. If you have your heart set on one, plant it behind a root barrier or, better still, in a large container. If a branch or root crosses a boundary, your neighbour may cut it back to the line, but not enter your garden without permission. Talking through your plans first prevents most disputes.

Planting well and keeping it in scale

Bare-root trees, available from late November to March, establish faster and cost less than container-grown ones. Plant on a mild, frost-free day into a square hole rather than a round one — the corners encourage roots to spread outwards — and set the tree so the old soil mark sits level with the ground. Firm it in, water thoroughly, and mulch with well-rotted organic matter, keeping it clear of the trunk.

Water generously for the first two summers; this is the single biggest reason young trees fail. Stake only on windy sites, and remove the tie after two years.

Finally, remember that you are allowed to shape a tree. A light annual prune, timed to suit the species, will keep a naturally larger tree in proportion for years. Fruit trees on a dwarfing rootstock, trained as a cordon or fan against a wall, will crop in barely a square metre of ground. Even a small garden can hold a tree — the trick is choosing one that suits the space you have, rather than the one that suits the pot it came in.