1
Tree identification
The section on ‘Trees’ in Chapter 8 of the book acknowledges that while it is not always easy to tell one type of tree from another, it is nevertheless important that surveyors should be familiar with the most common types of tree so that the majority of those in the top half of Table 1, which lists the relative subsidence hazard of full-grown trees, can be identified with a fair degree of confidence.
This section contains illustrations of 14 species of deciduous, broad-leafed trees and a single coniferous type. The top ten listed in Table 1 are all represented. It should be appreciated that each of these species may have several different varieties, the characteristics of which may be quite distinct from each other. Distinguishing one variety of willow or poplar from another of the same species can be a challenge, but this is less important for the surveyor’s purposes than the basic skill of being able to recognise that some variety or other of willow or poplar is present. The surveyor should initially concentrate on separating the most common species from each other. Working towards this will naturally lead to a greater understanding and recognition of the varieties within different species.
Each illustration shows the general appearance when in leaf, the silhouette when the leaves have fallen in winter, the bark and the leaf shape and colour.Where appropriate, buds, flowers and fruits are also shown. Below the general appearance, the likely mature height is given in metres, although some of these heights differ slightly from those given in Table 1.Once mature, trees can live a remarkably long time. Of those illustrated, oaks can live 500 years, common limes and London planes, 400 years and sycamores, up to 350 years. More short-lived trees, such as birch and rowan, pose a different problem as branches or whole trees die and need to be removed before they become hazardous.
The illustrations have been compiled from the pages of The Pocket Guide to the Trees of Britain and Northern Europe by Alan Mitchell and John Wilkinson, an invaluable publication originally from A C Black of London, now part of the Bloomsbury Publishing Group by whom it has since been updated and reprinted. There are many pocket guides available, and one of these should be part of the on-site armoury of every surveyor inspecting dwellings so that the species that are found in the gardens, parks, hedges and town streets but which are unlikely to cause damage can be distinguished from the ones most commonly associated with damage to buildings. At the very least, the following illustrations can be printed and carried by the surveyor.
It is not generally realised that poplars and willows (illustrations 1 and 2) are of the same family, the contrast between the tall, graceful lombardy variety of poplar and the weeping willow being quite striking. They are all fast growing with shallow roots but have a number of different crown shapes. The black poplar is fairly common in the northern towns, being resistant to smoky air. Aspen poplars(not illustrated)have fluttering leaves of the same shape as those of a grey poplar but grow to about only half the height and develop rows and columns of distinctive diamond-shaped indentations in their trunks as they mature.
Not all willows (illustration1)have the long slender leaves of those shown; some have leaves that are broader. However, all varieties produce silky catkins or ‘pussy willows’ in the spring. The bat willow, straight and more upright than others, grows extremely fast, mainly in East Anglia and South East England. As its name implies, this tree provides the wood for cricket bats. Both poplars and willows tolerate heavy pruning, which might prove an option where damage has been caused. During growth, they can cause a persistent deficit the moisture content of clay soil; so when felling is undertaken, care is needed to cope with subsequent heave.
Oaks (illustration 3) comprise one of the largest of all groups. They all produce recognisable acorns, which may ease recognition; but they also have a wide variety of leaf shapes, not all of which would be commonly regarded as typical of the characteristic oak leaf.Some are quite plain with sawtooth edges, for example. Oaks are likely to cause a persistent deficit of moisture in clay soil, which can result in long-term problems from heave if they are cut down.
The harsh, rough upper surface of the leaves of the English elm (illustration 4) differentiates it from other varieties, and it is said to have a unique outline to its crown. The leaves of all elms are, to an extent, asymmetric at the base. Elms are known for dropping branches unexpectedly. A virulent form of the elm disease fungus arrived around 1960, carried by the elmbark beetle, to which English elms readily succumbed, necessitating their removal when affected.Full-sized trees are now rarely encountered across much of the country, although residual pockets remain; for example around Brighton, in Edinburgh and on the Isle of Man. The suckering habit of elms, however, means that even where trees have been lost, shoots continue to spring up – often in hedge-like rows;these can reach heights of 4m or 5mbefore the bark becomes thick enough for the beetle to attack,causing the sapling to succumb once more. Although such restricted growth is unlikely to cause foundation damage, owners should be advised that each successive cycle of regrowth and demise will have implications for maintenance and expenditure over repeating periods of about 15 years, depending on the point in the cycle at which the surveyor is inspecting and when the purchaser takes on responsibility.
Familiar to all schoolboys for its conkers with their spiky green casings in the autumn, the horse chestnut (illustration 5) is easy to recognise by its large leaves and long tresses of white flowers in spring. It has an unfortunate tendency to shed heavy branches in summer but will tolerate heavy pruning.
The London plane (illustration 6) is a hybridthat is common in towns because of its resistance to pollution. There may be some understandable confusion between the names of London planes, sycamores and maples, not helped by the sycamore being known as a plane in Scotland and as the sycamore maple in North America! Planes, sycamores and maples are all in the same family, Acer, so they all have the same characteristic five-pointed leaf shape as the Canadian maple leaf. The distinctive London plane has a burred trunk that copes well with a polluted atmosphere by regularly shedding its surface layer. It has a rich and glossy green leaf and one or two large, fruit-like seed balls on each stalk, which can persist on the tree throughout the winter and disintegrate into individual seeds in the spring. It is regularly subjected to heavy pruning, successfully controlling the height and size of the crown.
The common ash, Figure 7, is often very shallow rooted but grows well in cities. It is the only variety of ash producing coal black buds, an unmistakable identifier during the winter. Unlike the example illustrated, many tend to fork much lower, nearer the ground. An Ash tree will tolerate the crown being pruned and this can be done but still retain a reasonable shape. Self-sown seedlings will often be encountered and clients must be advised to remove them before they become too large to deal with easily.
The Leyland Cypress, Figure 8, has become notorious for its use as a hedging plant where its rapid growth will create a dense screen in only a few seasons but, if left uncontrolled, can quickly become a nuisance, to the extent that legislation has been introduced to allow controls to be implemented. Until recently, the Monterey Cypress, Cupressus Macrocarpa, was the only coniferous tree generally recognised as likely to pose a risk to buildings but experience is suggesting that an accumulated mass of Leyland Cypress in planted hedges, if allowed to grow to any significant size, can also cause a significant degree of moisture deficit in shrinkable clay soils, enough to lead to foundation movement, especially as hedges are often planted immediately adjacent to buildings.
Described in the The Pocket Guide as the worst tree known for lining streets and avenues, the Common Lime, Figure 9, often achieves a great height and can live a long time, but its roots are known to be invasive.It sprouts around the base and is usually infested with greenfly. As anyone who has parked a car near a Lime tree will know, the sticky droplets from the greenfly are a pain to remove. It will, however, tolerate heavy pruning, something regularly imposed by local authorities on the street specimens.
Described as a dull tree in the The Pocket Guide the Sycamore, Figure 10, is part of the Maple family, the Acer species. If is very accommodating, thriving better than most other trees in coastal areas, smoky city air or the high hills. It shades out other trees, while its heavy leaf falls blanket out anything growing underneath. Its little ‘whirly bird’ seeds get carried everywhere and many a surveyor will have found little trees growing happily in the silt of out of the way gutters. It is important that clients should be advised to remove self-sown seedlings promptly, as with those of the ash. Sycamores are tolerant of heavy pruning. Maples are not illustrated here and individual examples of Field Maples, almost indistinguishable from Sycamores at first glance, may occasionally be encountered by surveyors but most of the Maples found in a domestic context will be specimens planted for their decorative leaves, which may be particularly delicately shaped or especially colourful in the autumn.
Beech trees, Figure 11, are large and bear nuts which are partially or wholly enclosed in a cup or husk. The common Beech prefers freely draining soil and is not often encountered on clay so accordingly is only rarely implicated in damage to buildings. In the United Kingdom, it is confined mainly to England. Old trees have a reputation of being unstable, a point to bear in mind if fully grown mature specimens are encountered in gardens. Beech hedging is useful because, although the tree is deciduous, hedges retain their leaves and continue to provide screening during the winter. Incidentally the Beech family of trees includes Sweet Chestnuts and Oaks.
The Rose family embraces a wide variety of trees, including a number of fruit trees such as Apple, Pear,Plum, Almond, Peach, and Cherry,as well as Hawthorn, Cotoneaster, Rowan and Whitebeam. The Wild Cherry, Figure 12, is much planted in gardens, parks and streets and is considered a first class amenity tree. The crown is whorled, i.e. the smaller branches and leaves ring round the centre stem, unusual in a broadleafed tree but like many conifers. It does not take kindly to pruning, as is also the case with other examples of the species. All can have very shallow roots with a risk of damage if planted by the unsuspecting owner in shrinkable clay very close to a dwelling. By no means large trees, both Cherry and Apple trees have been implicated in a surprising number of cases where foundation movement has taken place.
The timber of the Birch family, the trees of which grow further north than any other, is much used for making plywood and all are fast growing. The Silver Birch, Figure 13, is easy to recognise, particularly from its white bark and ‘weeping’ silhouette in winter but, favouring sandy soils, is rarely implicated in damage. It is a tree that requires careful pruning if that should prove to be necessary. Alders are of the same family but prefer damp ground.
Rowans and Whitebeams, Figure 14,are from the same family but Rowans have ‘pinnate’ leaves, a number of leaves on a single stem, rather like those of the Ash, while the latter have the simple leaves shown. Both have the distinctive red berry-like fruit in the autumn. Commonly planted in streets because of its low mature height, not often implicated in damage and, if necessary, will tolerate heavy pruning.
Robinia, Figure15, is also known as the False Acacia or Locust Tree, grows well in towns and cities but is not common north or west of the Midlands. A smaller golden coloured variety called ‘Frisia’ is now a commonly planted tree for small gardens. Like Elms, they are known for dropping branches unexpectedly. The ‘pinnate’ style of the leaf, with its multiple leaflets, is similar to the Ash and Rowan but the shape of the tree is completely different from either, smaller in scale than ash, irregular and angular branched, often with multiple divisions of the trunk and, not least, sharp thorns along the branches.
Trees near dwellings but beyond the distance of the mature height shown on the illustrations can be ignored generally, although in the case of Willows and Poplars the distances given in Box 3 suggest that the distances from these trees should be 40m instead of 25m for the Willow and 35m rather than 26m for the Poplar. Risk of damage increases substantially the closer the tree is to the dwelling and those dwellings with shallow foundations will always be more vulnerable. Two storey dwellings from the 1920s onwards, without basements, are the type of buildings most likely to be affected while extensions, porches, garages, conservatories and bay windows of any era, are even more at risk.
iResource – Tree identification