Trees are the most long-lived growing features in any garden. Once they are well established, it is very difficult to move them; pruning them if they become too big is difficult, needing skilled workmanship, and is never a permanent solution to the problems of excessive roots and over-extensive shading that arise.
Since Victorian times the gardener’s problems in tree-planting have been made much easier by the introduction from western China in particular, as well as Japan, and also by hybridization and selection, of a wide new range of trees which are of moderate size. These include excellent maples, whiteheams, rowans, cherries and ornamental apples (crabs), as well as birches. Many of these also provide what is wanted in a small area, a tree that has more than one season of interest, such as decorative bark in mid-winter, attractive unfolding foliage in spring followed by a period of flowering, then brightly coloured fruit and finally gay colouring of the leaves before they fall. Trees often have at least two if not three seasons of interest.
Evergreen broad-leaved trees are of particular interest in winter, and many have variegated or coloured-leaved forms, and the number available is now greatly increased. All are least satisfactory in towns where air pollution takes away the shine of their foliage.
The same applies to conifers a number of which are of too great a size and too slow growing for gardens, and are seen at their best in forests and pineta.
For road planting and use in smaller gardens narrow (fastigiate) forms of many trees have been selected and are propagated as cultivars. They are also useful in planting on a large scale on account of their beautiful shape. This applies, also to the numerous weeping trees available.
Soil Hardy trees are surprisingly tolerant of soil conditions provided drainage is good. Many come from mountains where soil is not deep, except in the river valleys.
Where the soil is well drained, the limiting factor for a number of species is the amount of lime present. Many trees growing naturally on acid or neutral soils will grow equally well on soils with a moderate lime content, particularly if the soil is deep and fertile. But a certain number are, like rhododendrons among shrubs, strongly calcifuge (lime-hating) plants, particularly on rather shallow, chalky soils as are found in many fine gardens in such areas as the Chilterns, Cotswolds and South Downs.
Apart from the degree of soil alkalinity (see pH) depth and soil structure affect the kind of trees that can be grown. In general, with certain notable exceptions, conifers prefer acid or neutral soils. The Rosaceae, however has many genera that are often associated in nature with alkaline soils such as Malus (apples), Prunus (cherries, plums, peaches, almonds, etc), Pyrus (pears), Crataegus (thorns) and Sorbus (rowans and service trees).
It is curious that though many calcifuge plants will not live in calcareous soils (containing lime), most of those that are calcipholous (lime-loving) will grow well in neutral and acid soils.
The following list indicates the preferences of some commonly cultivated genera, particularly those that will, or will not, grow on soils with a moderate lime content, and those that on no account thrive on shallow, chalk soils.