The Illustrated Encyclopedia of Trees

North American readers come to this European book from a privileged position and a different viewpoint. In the first place, the native tree flora of the North American continent is immensely rich and varied where that of Europe is small and reduced. Natural woodlands across temperate America regularly exhibit a fine array of different trees all growing together. In Europe they tend to be dominated by a few or sometimes monotonously only a single species.

The reason for this difference lies in the last Ice Ages and in the different conformation of our mountains and valleys. In North America, the principal mountain ranges run north-south. As the ice descended from the north, plants were able to retreat before its advance, and then repopulate their old territories once the ice retreated. In Europe the major physical barriers lie east–west: the Alps, Pyrenees, most other ranges and the northern shore of the Mediterranean all barred the way to plants' retreat before the ice, condemning the majority of warmth-needing species to extinction. When the ice retreated, far fewer plants had survived to advance northwards again. Of those that did, the trees most adaptable to different soils and conditions were able to form dominant colonies with less competition. For the time being, the post-Ice Age botanical landscapes were established — wonderful variety in North American woods and forests, not in the European. And away from parks and gardens, this is still the situation today.

But in compensation, the Europeans have been active for centuries in collecting trees from other parts of the world and bringing home the seeds of unfamiliar species — to Germany, France and Holland but most of all to Britain. From the late seventeenth century onwards a passion for new trees provided commercial impetus for collecting expeditions launched by individuals, botanic gardens or the tree nurseries themselves such as Veitch. As a result, warmed by the Gulf Stream and favoured by a moist maritime climate without extremes of heat or cold, more tree species can be found growing today in Britain and Ireland — and usually growing better — than in any comparable area of the temperate world.

The vast majority of tree species growing in the great British and Irish gardens, both public and private, are therefore exotic species, or cultivars of them. They include by far the widest coverage of American trees in the world outside North America itself, but also trees originally from all other parts of the temperate world. The British Isles have thus come to form over the centuries something of a 'warehouse' of the world's cool-climate trees, and a Mecca for tree-enthusiasts everywhere.

Note that throughout this book, we exclude of course the tropics, where over three-quarters of the world's flowering plants are to be found — but see page 15.

Flowering plants (Angiospermae) began to appear about 120 million years ago. They completely dominated the earth's vegetation in the following 30 million years. There are reckoned to be now over 250,000 species, three quarters of which are tropical. All trees are classified as flowering plants. Even conifers have primitive structures that resemble flowers and serve the same reproductive purpose.

Many garden trees are 'cultivars'. This means in broad terms that they have been first produced by human selection from chance seedlings and have ever after been propagated vegetatively so that their individual characteristics are perpetuated rather than lost again in a genetic 'soup'. They may be grafted, layered, grown from cuttings or replicated from their tissues in a laboratory. It is as if a red-headed Scotsman could be selected and conveniently reproduced, a process now less distant if not yet imminent. A new vegetatively produced plant will furthermore begin flowering at an early age, unlike many seedlings which go through a lengthy juvenile stage. A twig on a flowering tree behaves in exactly the same way whether it remains on its parent or finds itself grafted on to an entirely new set of roots. It cannot know that its circumstances have changed. However, the lifespan of a vegetatively produced plant is likely to be shorter than that of a seedling.

Breeding new flowering trees has been practised for hundreds of years. There is always the prospect that two good trees crossed with each other under controlled conditions may produce an even better plant. It might have superior flowers, more (or less) vigorous growth, sweeter scent or greater resistance to disease. However, the production of spectacular new progeny without careful prior selection of the parents is an unusual occurrence. It does not often result from unmethodical or arbitrary seed collecting.

Within naturally occurring variations, rather than the human-induced, botanists recognise certain subdivisions of the actual species: notably subspecies, varieties and forms. These, representing successively less significant changes from the species' normal characteristics, are normally signalled in a scientific name by the abbreviations (not in italic type) subsp., var. and f. A hybrid between two different species (or less often genera) is indicated by the multiplication sign ×. Cultivars are accorded names with initial capitals, printed within single inverted commas, not in italic type.

Thus nurserymen's selections of the hybrid Black Italian Poplar have names such as Populus × canadensis 'Serotina'; and the shrubby northern populations of the European Bird Cherry are Prunus padus subsp. borealis. It is useful to remember that italic type is reserved for names of naturally occurring plants themselves rather than names of 'man-made' cultivars or of categories used by botanists to indicate relationships. Plant names are far from stable, and the complexities of scientific nomenclature mercifully beyond the scope of this brief introduction.

Trees in a garden give it permanence, depth and vertical interest. Unlike ground-hugging flower beds, which left alone will repeat themselves with only minor changes each year, trees develop character as they grow. Week by week and year by year, trees constantly change — in colour, form, light and shade, in trunk, branches, bark, shoots, foliage and flowers.

Of course, they also provide shelter from extremes of heat, cold and wind. They purify the air, protect and mulch the ground around them, extract soil moisture but reduce waterlogging, conserve warmth, harbour wild life and generally benefit everything living in their immediate vicinity: not just ourselves.

A long time is required for a tree to reach maturity, but not to grow. Trees in fact grow at the same rate as herbaceous garden plants, anything from 10cm to one metre each year according to the species and its age. A tree may stand in one place for 100 years and each year it will stack upon itself the equivalent of a whole new flower bed.

Planting such a phenomenal vegetable deserves more thought and planning than it is usually given. The first decision is the choice of species. With so many to choose from, start by eliminating those groups which you do not want, for instance very large, very small, non-flowering evergreens, conifers or poisonous trees. Those that are tender in the neighbourhood are seldom worth bothering with: sooner or later a cold winter will probably kill them. Conversely some alpine species cannot take too much heat. If your site is acid or alkaline, of course discount trees that like the opposite conditions. Check on the health of other people's trees nearby. See what grows well, then also call to mind all the other species in the same genus. Be positive about what you want from your tree; is it shade, colour, foliage texture, flowers, bark, fruit (decorative or edible), scent, shelter, conservation value (e.g. good for bees) or at what time of the year is it to be at its best?

Finally, having found a tree you like, try to be sure that you will still like it in the future when its size and shape is dramatically changed. Visualize it in 10 years time, or 20 or 50 years if you can. The kind of tree a garden can accommodate obviously depends on the space available, and walls, buildings, paths, roads, neighbours, light and windows must all be considered. Services under and above ground too: some species are adept at seeking out your leaky drains and poor foundations, will bring down your power cables or wreck your television reception. Areas close to doors and windows may not be the place for scented trees that attract biting or stinging insects, or become tenanted by unwanted roosting birds.

Some trees create unpleasant smells, cast too-dense shade, produce irritating seed fluff, drop copious squashy fruit or slippery dead leaves on your paths or sticky honey-dew on your car. Then they may be blown down or struck by lightning. As a rule it is not wise to plant a tree closer to a dwelling than 80% of its own ultimate height. On shrinkable clays the distance should be greater and species that sucker or have spreading roots should be avoided altogether. But the commonest error is simply planting too close to a house for comfort. Drive out of any town to see houses in the process of disappearing in their little thickset, self-made forests: in 100 years they will be invisible, perhaps unreachable.

Changing the view. If the above has not put you off altogether, remember that any large established tree will mightily obstruct a view. If the view is to be retained, then either the tree should not be planted there, or as it grows its lower branches should be progressively cut off to reveal the view again. (This can actually be attractive: enhancing a view by framing it. Blue sky always appears more intense when seen next to green foliage.) True, sometimes the object of planting a tree is positively to obstruct an unpleasant view. The view then becomes the tree itself.

Now consider the lighting. For example, brown autumn leaves look muddy and dull lit from the front, but can be transformed to gold when back-lit. White blossom all but disappears when back-lit but shines out when lit from the front. Trees with large or two-coloured leaves show off their foliage well with side light. The best way to determine a tree's 'best side' is to walk right round an existing specimen on a sunny day and see the different effects for yourself.

Shade is another obvious factor. Observe the path of the sun round your garden and calculate how far and in which direction at different times of day a new tree would cast its shade. Garden flowers below a tree will usually be concentrated on its sunny side, with the tree planted on the shady side of a border.

Planting a tree

The hole. Digging the hole to plant a tree is your only opportunity to cultivate and improve a piece of ground that will not be touched again for a very long time. It is well worth taking extra trouble to do it properly. Your tree will grow much better from the start and for a long time after. Don't attempt the job until the weather and the season are right. Choose an overcast day in the dormant season with no wind and no risk of frost. If something unexpected appears in the hole, such as water, stop. Look again at your design and try somewhere else nearby.

The most important factor is certainly not fertilising (see below) but the size of the hole. The more broken-up congenial soil around the roots, the less resistance to their rapid growth and the more oxygen available to them. This is true however large or small your tree may be. The hole must be substantially larger than the root-ball of your plant. None of the ordinary roots should need bending to fit them in — though excessively long side roots can be cut back to a sensible length. The tree should also be planted on the same day that the hole is dug. Leaving a hole open in sun, rain or frost changes the soil structure and kills organisms in the immediate vicinity, many of which might have been beneficial to the tree. On heavy soils, surface drainage away from the pit may be needed to save the refilled hole from filling with rain-water. Double digging outwards from below the site is often enough to avoid this.

Staking. If a wooden tree stake is to be used, drive it in to the bottom of the empty hole on the windward side in exposed situations. But in a sheltered garden the stake can be placed behind the tree if it will be mainly viewed from one direction. If many trees are planted together it does look better if all the stakes are on the same side. Stakes that have not been pressure-treated with preservative may not last for long and they can carry diseases such as honey fungus. Do not cut the stake to its final length until the tree is planted.

Planting. The tree, which should have remained in its packing, or its pot, in the shade, can now be inspected. Trim off any damaged shoots with sharp secateurs and inspect for signs of disease. Unpack the root ball, even from a so-called biodegradable container, and cut any damaged roots off cleanly. Pull out any roots that are coiled around the rest. Either make them point outwards or cut them off. Replace some of the excavated soil to form a mound in the bottom of the pit. Build this up until the tree, when placed on the top of it, will have its root collar level with the natural surface of the ground. As quickly as possible begin to fill the hole. Shake the tree occasionally and press the soil gently round the roots to expel air. Do not skin the roots or compress the soil by treading or pressing too hard.

Good natural topsoil should go into the hole, without adding peat or any other organic material. This is almost as important, and as disregarded, as digging a big enough hole. The point is that the decomposition of organic material changes the nature of the soil near the roots. It can cause anaerobic conditions by holding excess moisture and restricting soil air movement. It can reduce the supply of oxygen to the roots through its own oxidation as it decomposes; and of nitrogen as the population of microorganisms (which absorb nitrogen) is increased to break down the organic material. Farmyard manure can also encourage diseases and weed seeds. Generally speaking, fertilizers can encourage strong competitive weed growth and promote top growth of the plant. This leads to root/shoot imbalance and then shoot tip death as the tree struggles to redress the equilibrium which is essential for its sustained growth.

So leave the tree well alone without dosing its immediate soil with anything. Only on notably infertile sites may a little slow-acting manure such as bonemeal be acceptable. Otherwise give any feeding as a mulch in a few years time when the tree has established a new root-system.